Microrobots Based on Smart Materials with Their Manufacturing Methods and Applications

Sun, Jiawei; Cai, Shuxiang; Yang, Wenguang; Leng, Huiwen; Ge, Zhixing; Liu, Tangying

doi:10.3390/inventions9030067

Open AccessReview

Microrobots Based on Smart Materials with Their Manufacturing Methods and Applications

by

Jiawei Sun

¹,

Shuxiang Cai

^1,*,

Wenguang Yang

¹

,

Huiwen Leng

¹,

Zhixing Ge

²

and

Tangying Liu

^1,*

¹

School of Electromechanical and Automotive Engineering, Yantai University, Yantai 264005, China

²

State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China

^*

Authors to whom correspondence should be addressed.

Inventions 2024, 9(3), 67; https://doi.org/10.3390/inventions9030067

Submission received: 9 May 2024 / Revised: 31 May 2024 / Accepted: 12 June 2024 / Published: 14 June 2024

(This article belongs to the Section Inventions and Innovation in Biotechnology and Materials)

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Abstract

:

In recent years, the field of microrobots has exploded, yielding many exciting new functions and applications, from object grasping and release to in vivo drug transport. Smart responsive materials have had a profound impact on the field of microrobots and have given them unique functions and structures. We analyze three aspects of microrobots, in which the future development of microrobots requires more efforts to be invested, and in which smart materials play a significant role in the development of microrobots. These three aspects are smart materials for building microrobots, manufacturing methods, and the functions and applications they achieve. In this review, we discuss the deformation mechanism of materials in response to external stimuli, starting from smart materials, and discuss fabrication methods to realize microrobots, laying the theoretical foundation for future smart material-based microrobots to realize their intelligence and programmability.

Keywords:

smart material; microrobots; manufacturing method; biomedical applications

1. Introduction

With the continuous development of robotics, the application of robots in the industrial field, medical field, and education field is becoming more and more extensive [1,2,3,4]. For example, esophageal capsule endoscopy is used to obtain images of the esophagus to diagnose disease [5]. Further examples include the use of vascular interventional surgery robots in medicine [6]. In the last decade of the 20th century, due to the advent of microcontrollers and the emergence of micro-mechanical systems on silicon, microrobots were born, although many of them do not use silicon as a mechanical component other than in sensors. The earliest research and conceptual designs for such small robots were conducted in the early 1970s in classification studies for U.S. intelligence agencies. Applications envisioned at the time included prisoner of war rescue assistance and electronic interdiction missions. The underlying miniaturization support technology had not yet been fully developed, so progress in prototype development was not immediately available from this early computational and conceptual design. Microrobots are built on large-scale integrated circuits that require the integration of numerous sensors and other actuators, which greatly limits their miniaturization. In nature, there are many intelligent and small creatures, such as flycatchers, earthworms, etc., that rely on simple systems to perform specific functions. Inspired by creatures in nature, we can learn from them how to design microrobots that respond to environmental stimuli and have different response properties in different environments.

Microrobots require the integration of numerous sensors and integrated circuits, making it difficult to miniaturize them. Therefore, microrobots controlled at the macroscopic scale using integrated circuits and programming languages cannot be translated to the microscopic scale. We need a new approach to solve this problem from a completely new perspective, with smart materials and reproducible geometries for programmable functions of new microrobots [7]. Smart materials play a decisive role in programmable microrobots and are the key to realizing smart microrobots. Smart materials for manufacturing microrobots have been increasingly researched and explored [8,9,10]. Smart materials respond in a useful, reliable, and repeatable manner when subjected to changes in stimuli in the environment [11]. Smart materials are used in microrobots because of their information recognition and accumulation functions, response functions, self-diagnostic capabilities, self-adaptive capabilities, and self-healing capabilities [12,13]. The development of smart materials combined with advanced manufacturing technologies has enabled the creation of microrobots with unique functions. Inspired by smart materials, a novel approach to microrobot design has been applied, and we need to understand the response mechanisms of smart materials and plan their systematic distribution to realize microrobots that can respond to environmental stimuli. Continued advances in the field of materials science have provided a vast array of materials for the development of microrobots. Exploiting the differences in the response of these materials to environmental stimuli is important for designing and implementing multifunctional microrobots. The environmental stimulus–response properties and repeatable deformability of smart materials provide a new idea for the design and application of microrobots. Thus far, depending on the external stimulus–response, smart materials applied to microrobots are light responsive [14,15,16,17], temperature responsive [18,19,20,21], humidity responsive [22], magnetic responsive [23,24,25], and pH responsive.

Smart materials are the raw material fundamental for building microrobots, but it is still a challenge to realize the process from a quantitative to a qualitative change in materials. To date, many manufacturing methods have been widely used in the manufacture of microrobots, for example, the flip-mold method [20,26,27], direct ink writing techniques [28,29,30], fused deposition printing [19], lithography [31,32,33], projection micro-stereolithography [18], and two-photon polymerization [34]. We need to consider the materials suitable for each manufacturing method as well as the manufacturing precision and principles, which have a huge impact on the realization of multifunctional microrobots. The use of smart materials to build microrobots is a new perspective to solve their miniaturization, and with continuous research, they are bound to have a wide range of applications in the future. However, the research in this direction is still in the basic stage and remains in the laboratory, and there is still much room for progress toward industrialization. In this direction, materials are the key to achieving this breakthrough, and the exploration of new materials with excellent physicochemical properties yet responsive to environmental stimuli is a boost to the design of microrobots. Based on the existing foundation, we need to fully understand the response mechanism of existing smart materials and their physicochemical properties, which can be of great help for the construction of new materials and the fabrication of microrobots.

In this review, we take a critical look at the foundations of future microrobot development from the perspective of smart materials. The existing challenges require exploration and experimentation in materials and manufacturing methods for microrobots. To guide the development of microrobots, we illustrate the manufacturing methods and functions of the frontier of microrobots based on smart materials. In the context of robotics, these aspects provide the technological basis for the use of smart materials in the manufacture of microrobots. Microrobots are driven by combining external stimuli with their pre-programmed structures [17,21]. We trigger pre-programmed planning of the structure and molecular arrangement of the material by selecting materials with specific stimuli and deformations under the stimulus source [20]. Material pre-programming plays a crucial role in realizing the functionality of microrobots.

2. Smart Materials That Respond to Different Stimuli

Smart materials are capable of responding to changes in multiple stimuli in the environment [35,36]. Different paradigms of microrobots can be achieved by planning multiple stimulus responses of the material [37]. Smart materials have low hardness properties in physical aspects, and flexible microrobots are widely manufactured and studied [38]. Using external stimulus–response as the entry point, this review describes smart materials that respond to light, temperature, humidity, magnetism, electricity, and pH. These materials have different forms of molecular structural transitions in response to different stimuli. The characteristics of some smart materials are summarized in Table 1.

2.1. Light-Driven Materials

Analyzed from a microscopic perspective, light-sensitive smart materials have various deformation mechanisms under light, such as their internal molecular structure transformation, the photothermal effect of the material, and the ability of cross-linked molecules to accommodate water molecules. Microrobots are commonly built from a variety of materials as a system, so double-layered structures are one of their widely used robot forms. One layer is the active layer, which realizes the response to external stimuli; the other layer is the passive layer, which does not respond to external stimuli and is used as a support or a limit for the deformation of the microrobots.

Liquid crystal elastomers are liquid crystal polymers (LCPs) that are cross-linked and have elastic properties in the isotropic or liquid crystal state. Their internal molecular structure is changed under the stimulation of light, which in turn leads to a change in the arrangement order of liquid crystal elements, resulting in macroscopic deformation of the material [39]. The deformation mechanism of liquid crystal elastomers is divided into two types, one is the transition of the molecular structure of the material between trans and cis isomerization under light; the other is the photothermal effect of the material. The photothermal effect is the transformation of light into heat by a material that causes it to change stress [40]. Unlike solid and liquid structures, LCPs are polymers with a certain sequence of internal molecules. They have the anisotropy of liquid crystals and the elasticity of a loosely cross-linked polymer. The compounds 4-Methoxybenzoic acid 4-(6-acryloyl ox-acetyloxy)phenyl ester and 4[4[6-Acryloxyhex-1-yl)oxyphenyl] carboxy benzonitrile are two monomers used in the synthesis of LCPs. LCP networks allow for three-dimensional deformation in a single film with a uniformly distributed chemical composition. LCP networks in a single film with uniformly distributed chemical composition can achieve three-dimensional deformation [16]. Figure 1A presents the bending deformation of this LCP under different light.

A hydrogel is a three-dimensional network structure formed by the polymerization of monomers that can hold water molecules. Changes in the water molecules in a hydrogel can cause dramatic changes in its volume resulting in deformation. The material synthesized from N-Isopropyl acrylamide(NIPAM) and poly(ethylene glycol) diacrylate has similar properties to hydrogel and is used to make microrobots [41]. NIPAM and poly(ethylene glycol) diacrylate deformation are presented in Figure 1B. Furthermore, the bilayer structure formed by the combination of NIPAM and gold nanorods has a unique shape change. Coating gold nanorods onto a hydrogel layer made of NIPAM can achieve a bilayer structure. Gold nanorods will limit the expansion and deformation of the hydrogel layer. This deformation restriction creates stresses between the materials to achieve the deformation of the double-layer structure [42].

The compound 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester can be used to synthesize liquid crystal elastomers with other materials. The macroscopic deformation of the material is caused by the inhomogeneous volume change resulting from the molecular structure change under light [39]. Pre-programming of the molecular structure of a material is an important way to achieve the planned deformation of the material. The liquid crystal elastomer synthesized from 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester was deformed in light as presented in Figure 1C. The composites composed of 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl and dispersed red have light-responsive properties. The dispersed red has a photo-thermal conversion effect to change the molecular orientation of the material [43]. Graphene oxide (GO) is a non-traditional form of soft material with hydrophilic and photothermal effects. The difference in thermal expansion coefficients between GO and azobenzene-doped networks was used to implement bilayer actuators. The fine and complex biomimetic behavior of the actuator can be achieved by the predetermined molecular arrangement in the material microchannels and the mismatch of the coefficient of thermal expansion (CTE) between the bilayers [44]. The deformation of GO and azobenzene under light is presented in Figure 1D.

The double-layer structure is the most widely used form of structure for microrobots. However, the double-layer structure can affect the performance of the robot due to unstable connections. Therefore, a microrobot with a single-layer structure would avoid this problem. For example, a monolayer structure is made using iron tetroxide and GO. The gradient distribution of iron tetroxide in GO hinders the diffusion of water molecules. The photothermal effect of GO is the change in water in the material to produce macroscopic deformation [45]. Under certain humidity conditions, the deformation of iron tetroxide and GO under light is presented in Figure 1E. Poly(N-isopropylacrylamide) (PNIPAM) hydrogels have rapid water absorption and water loss properties in water under light [38]. The deformation of PNIPAM hydrogel in an aqueous environment is presented in Figure 1F.

Graphene is a new material with a single layer of two-dimensional honeycomb lattice structure formed by the stacking of carbon atoms. Under the light, the different thermal expansion coefficients of graphene and polydimethylsiloxane (PDMS) produce macroscopic deformation of the material [46]. Figure 1G presents the deformation results of the graphene and PDMS composites. Materials possessing a large CTE produce large volume changes as the active layer. Conversely, materials with a small CTE are used to limit the local deformation of the material.

Figure 1. Properties of the material under a light. (A) 4-Methoxybenzoic acid 4-(6-acryloyl ox-acetyloxy)phenyl ester, 4[4[6-Acryloxyhex-1-yl)oxyphenyl]carboxybenzonitrile and other materials synthesized by the deformation of LCP materials. Reproduced from Reference [16] with permission from Springer Nature. (B) Deformation of hydrogels synthesized by N-Isopropylacrylamide and poly(ethylene glycol) diacrylate in the presence of light. Reproduced from Reference [41] with permission from Elsevier. (C) 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl deformation of composite structures with other materials. Reproduced from Reference [39] with permission from Springer Nature. (D) Deformation of composite materials such as GO. Reproduced from Reference [44] with permission from the American Chemical Society. (E) Deformation of GO composites with ferric tetroxide. Reproduced from Reference [45] with permission from Elsevier BV. (F) Deformation of PNIPAM hydrogel. Reproduced from Reference [38] with permission from MDPI (Basel, Switzerland). (G) Structural deformation of graphene and PDMS composites. Reproduced from Reference [46] with permission from the American Chemical Society.

Table 1. The lists of smart materials and their deformation mechanisms.

Power	Materials	Deformation Mechanisms	Manufacturing Method	Application Function	Reference
Light	4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester	Molecular arrangement change	Lithography	Grab objects and roll	[39,43]
	polydimethylsiloxane, Graphene	Coefficient of thermal expansion	Die-turning method	Bionic fish, tracked robot	[46,47]
	Ink, polyethylene terephthalate, acrylic acid	Multilayer construction	Die-turning method	Crawling robot	[48]
	Titanate nanosheet, poly(N-isopropyl acrylamide), gold nanoparticles	Material electrostatic repulsion	Lithography	Earthworm crawling robot	[49]
	N-isopropyl acrylamide	Change in water content	Projective micro stereo lithography; Two-photon polymerization	Bionic flower, microgripper, double arm micro driver, bionic jellyfish	[41,50,51]
	polyethylene	Coefficient of thermal expansion	Die-turning method	capture	[52]
	Fe₃O₄, graphene oxide	Differences in absorbency	Die-turning method	tongs	[45]
Temperature	Graphene, polyethylene	Differences in thermal stress	Die-turning method	Bionic flower	[53]
	poly(N-isopropyl acrylamide), poly(acrylic acid-co-acrylamide)	Thermal response expansion	Die-turning method	Bionic flower, grab	[26]
	poly (D,L-lactide-co-trimethylene carbonate), poly (trimethylene carbonate), Fe₃O₄	Molecular arrangement change	Direct ink writing	Bionic flower	[54]
	Polylactic acid, Polycaprolactone	Molecular chain change	Direct ink writing	Bionic flower	[29]
	N-Isopropylacrylamide, graphene oxide	Thermal response expansion	Electrochemical reduction; Die-turning method	Bionic flower, grab action	[20,55]
Humidity	graphene oxide, poly(methyl methacrylate)	Different water solubility	Die-turning method	tongs	[56]
	cellulose nanofibers, carbon nanotubes, graphene oxide	Different water absorption	Die-turning method	Gripper, humidity switch	[57]
Magnetism	Polydimethylsiloxane, NdFeB	Magnetic force action	Die-turning method; Direct ink writing	Swimming robot, bionic turtle, microgripper	[28,58,59]
	Ecoflex, NdFeB	Magnetic force action	Die-turning method	Swimming robot	[60,61]
PH	acrylic acid, N-isopropylacrylamide	Electrostatic stress difference	Direct laser writing	Bionic flower, bionic fish	[34,62]
	chitosan and, carboxymethylcellulose	Differences in expansion	Die-turning method	capture	[63]

2.2. Temperature-Driven Materials

Hydrogels are widely used as temperature-responsive materials in the fabrication of microrobots. The magnitude of solubility at different temperatures is the essence of hydrogel deformation. In addition, shape memory polymers are capable of switching between different shapes in response to temperature. At different temperatures, the internal molecular arrangement of the shape memory polymer changes as a macroscopic shape switch.

The hydrogels synthesized from poly(oligo ethylene glycol methyl ether methacrylate (Mn = 500)-bis(2-methacryloyl) oxyethylene disulfide) and poly(acrylamide-N, N′-bis(acryloyl) cystamine) synthesized hydrogels deformed at a higher temperature as shown in Figure 2A. Under temperature variation, poly(oligoethylene methyl ether methacrylate (Mn = 500)-bis(2-methacryloyl)oxyethylene disulfide) has a higher temperature stability than poly(acrylamide-N, N′-bis(acryloyl) cystamine) which has a significant thermal expansion response. The different volume changes within the material are manifested as macroscopic deformations [31]. Diethylene glycol methyl ether methacrylate and poly(ethylene glycol) methyl ether methacrylate were used to synthesize hydrogels. The hydrogels can be controlled by varying the length of the glycol side chains at a low critical solubility temperature (LCST) and exhibit a reversible volume shift near the critical solubility temperature [33]. The hydrogels synthesized from Diethylene glycol methyl ether methacrylate and poly(ethylene glycol) methyl ether methacrylate deformed at the temperatures presented in Figure 2B. Methacrylate, for example, is a shape memory polymer with a temporary shape and a permanent shape. The change in the internal molecular arrangement of the shape memory polymer is manifested as a shape switch under temperature change [18]. Figure 2C illustrates the shape switching of methacrylate at a higher temperature. The hydrogel synthesized from NIPAM, acrylic acid (AA), and sodium dodecyl sulfate (SDS) deforms at a higher temperature as presented in Figure 2D. When the temperature is higher than the critical solution temperature, the hydrophobic effect of isopropyl dominates and the distance between molecules decreases, leading to shrinkage of the material. When the temperature is lower than the critical dissolution temperature, the polymer absorbs water and swells [64]. Graphene and polyethylene (PE) are used to make temperature-responsive actuators. Graphene has a small CTE compared to PE, and exploiting the difference in its CTE is the key to planning material deformation.

The material can be transformed into a three-dimensional structure at a higher temperature by different pre-stressing [53]. The deformation of the material synthesized from graphene and PE under pre-stress is shown in Figure 2E. A mixture of graphene oxide-poly(N-isopropyl acrylamide) hydrogel and perylene bisimide-functionalized hyperbranched polyethyleneimine blend deforms at a higher temperature as presented in Figure 2F. Due to temperature shrinkage, PNIPAM can control its deformation by temperature [65]. Printed by TangoBlack+, VeroClear’s polymer has shape memory and achieves shape transformation at a certain temperature [66]. Bisphenol A ethoxylate diacrylate (BPADA), glycidyl methacrylate (GMA), and n-butyl acrylate (BA) can be synthesized as a shape memory polymer. The transformation from one shape to another is achieved by the action of temperature [67].

Figure 2. Temperature-sensitive materials: (A) poly(oligo ethylene glycol methyl ether methacrylate (Mn = 500)-bis(2-methacryloyl)oxyethylene disulfide) and poly(acrylamide-N, N′-bis(acryloyl)cystamine) complex hydrogel deformation. Reproduced from Reference [31] with permission from the American Chemical Society. (B) Hydrogel deformation of Diethylene glycol methyl ether methacrylate and poly(ethylene glycol) methyl ether methacrylate composite. Reproduced from Reference [33] with permission from Wiley-VCH Verlag. (C) Shape memory polymers with methacrylate compounded with other materials. Reproduced from Reference [18] with permission from Springer Nature. (D) Hydrogel deformation of NIPAM and AA, SDS composite. Reproduced from Reference [64] with permission from Elsevier. (E) Deformation of graphene and polyethylene (PE) composites. Reproduced from Reference [53] with permission from Springer Nature. (F) Deformation of graphene oxide-poly(N-isopropyl acrylamide) hydrogel and perylene bisimide-functionalized hyperbranched polyethyleneimine (PBI-HPEI) composites. Reproduced from Reference [65] with permission from Wiley-VCH Verlag.

2.3. Moisture-Driven Materials

Moisture-sensitive materials use the water uptake and water loss of the material to deform the material. Similar to photoresponsive hydrogels, the water absorption and loss of the material lead to volume expansion and contraction to produce macroscopic deformation. Cellulose nanofibers and carbon nanotubes as well as GO can be used to prepare humidity-responsive actuators. The excellent moisture sensitivity of cellulose nanofibers and GO can deform the material. Hydrophobic carbon nanotubes facilitate the rapid desorption of water molecules and thus promote material deformation. The unique porous structure of carbon nanotubes with numerous nanochannels accelerates the water exchange rate [57]. The deformation of the material made from the composite of cellulose nanofibers, carbon nanotubes, and GO is presented in Figure 3A. The GO and poly(methyl methacrylate) (PMMA) were gradient assembled to develop robust moisture- and photo-responsive actuators. In a humid environment, GO adsorbs water molecules to deform the material. Conversely, under light, the photothermal effect of GO causes GO to desorb water while leading to the expansion of PMMA [56]. The deformation of a crawling centipede made of GO in a humid environment is presented in Figure 3B [68]. PDMS is insensitive to moisture, and it and GO are fabricated as bilayer-structured drivers [69].

Polyvinyl alcohol is a water-soluble organic compound. A novel humidity-responsive nanofiber actuator can be realized using electrostatic spinning fabrication methods. In the presence of humidity, the material shrinks differently along the thickness direction. Uneven shrinkage and deformation of the actuator can be achieved by planning the shrinkage gradient in the thickness direction [70]. The deformation of polyvinyl alcohol is presented in Figure 3C. Poly(propylene glycol) dimethacrylate is a hydrophobic material. The difference in volume change between these two materials is used to produce macroscopic deformations, as presented in Figure 3D. Using digital light processing techniques, it is possible to create a bilayer structure with a certain shape and material distribution [71].

Figure 3. Deformation of humidity-responsive materials. (A) Deformation of cellulose nanofiber, carbon nanotube, and graphene oxide composites. Reproduced from Reference [57] with permission from the American Chemical Society. (B) Deformation of graphene oxide materials. Reproduced from Reference [68] with permission from Wiley-Blackwell. (C) Deformation of polyvinyl alcohol materials. Reproduced from Reference [70] with permission from the American Chemical Society. (D) Deformation of poly(ethylene glycol) diacrylate, poly(propylene glycol) dimethacrylate composites. Reproduced from Reference [71] with permission from the American Chemical Society.

2.4. Materials for Magnetic Drive

Magnetically sensitive materials are capable of deformation, tumbling, and movement in response to magnetic fields. The magnetic field as a stimulus source does not require direct contact with the material, and the distribution of the magnetic field can be planned. Magnetically sensitive materials are a composite of elemental iron or iron oxides, which produce shape changes in response to a magnetic field. Iron oxide, ferric tetroxide, and NdFeB are widely used iron-containing compounds.

By uniformly dispersing iron oxide nanoparticles into the hydrogel solution and then planning the distribution of iron oxide nanoparticles in the hydrogel under the action of an external magnetic field, the planned deformation of the material can be achieved under the stimulation of the external magnetic field [32]. Jellyfish-like microrobots made of elastomer and NdFeB can use the interaction with fluid to achieve the task of feeding on objects [60,61]. A new programmable magnetic response composite designed from polydimethylsiloxane and NdFeB enables asymmetric multi-mode drive [59]. Its deformation under a magnetic field is presented in Figure 4A.

The materials synthesized from ferrous tetroxide and acrylate have a different magnetic response deformation from the former. At room temperature, the material does not deform in a constant magnetic field. When the magnetic field is alternating, the iron tetroxide deforms the material by increasing its temperature and causing a decrease in its modulus. The material can use this feature to fix a temporary shape [36]. NdFeB particles are embedded in PDMS, which acts as a source of stimulation to deform the material. PDMS is characterized by heat resistance, water resistance, and low surface tension. The material composed of PDMS and NdFeB particles has a good magnetic field response performance [28]. Figure 4B,C present the deformation of PDMS with NdFeB material under a magnetic field.

Figure 4. Magnetically driven materials. (A,C) PDMS and NdFeB composites are deformed under a magnetic field. Reproduced from Reference [58] with permission from Wiley. Reproduced from Reference [59] with permission from the American Chemical Society. (B) PDMS and NdFeB composites are deformed under a magnetic field. Reproduced from Reference [28] with permission from the American Chemical Society.

2.5. pH-Driven Materials

pH-responsive materials can change their internal network structure to produce macroscopic deformation, or their internal molecular clusters can generate electrostatic stresses to induce deformation. Microrobots stimulated by pH are dependent on acidic and alkaline conditions, which greatly limits their application areas. Hydrogels are common materials used for pH actuation.

Polyacrylamide can be used to synthesize hydrogel-like materials. Polyacrylamide is compounded with DNA to produce hydrogel-driven materials, which can lead to changes in the molecular cross-link density under changes in pH, resulting in changes in the water content of the hydrogel and deformation of the material [72]. The deformation of the material made of polyacrylamide and DNA under different pH is presented in Figure 5A. The composites synthesized by calcium alginate hydrogel and chitosan have different variations for different pH values. Calcium alginate can exist in acidic conditions and dissolve in alkaline conditions to release the drug [73]. AA and NIPAM were used to fabricate a pH-responsive hydrogel bionic fish microrobot. The AA changes the network structure of the material under the influence of pH so that the material will have different expansion rates in different parts. The deformation of different parts of the material can be achieved by planning the expansion of the material [62]. The materials synthesized with AA and NIPAM were deformed at different pH values as shown in Figure 5C.

2.6. Materials for Electric Drive

On the one hand, the electrically driven smart material uses the heat generated by the voltage to cause it to expand and deform. On the other hand, the material generates electrostatic stresses in response to the voltage causing its macroscopic deformation. The materials Poly(3,4-ethylene-dioxythiophene)/poly(styrene sulfonate) and Ti3C2Tx change the electrostatic force between their interiors under the action of voltage, leading to changes in the interlayer support structure and cause macroscopic deformation of the material [74]. Ti3C2Tx is a metal carbide with a metal-like electrical conductivity. The materials synthesized from NiTi alloy and carbon nanotubes with PDMS are deformed under voltage as presented in Figure 5B. The NiTi alloy acts as a current transmitter and the carbon nanotubes are equivalent to a resistor. In the energized state, the temperature of the resistor increases to deform the material [75]. The compounds 4,4′-bis(6-hydroxyhexyloxy)biphenyl and 4-(6-Hydroxy hexyloxy) cinnamic acid (6HCA) can synthesize an LCP. Ni-Cr alloy has a conductive effect. Embedding Ni-Cr alloy into the LCP can transfer heat and deform the LCP [76]. The material deformation synthesized by LCP and Ni-Cr alloy is shown in Figure 5D.

The deformation of the composite material synthesized from polypropylene and cellulose fibers is presented in Figure 5E. Under the action of voltage, polypropylene expands by heat while cellulose fibers lose water and contract, resulting in the deformation of the actuator [77]. A liquid crystal network is a material in which molecules are arranged in an anisotropic manner. When heated above the phase change temperature or cooled below the phase change temperature, the liquid crystal network causes the material to shrink or expand. The liquid crystal network was synthesized from 4,4′-Dihydroxybiphenyl and p-coumaric acid. The electrical stimulation response can be achieved by embedding nickel-chromium alloy as a resistive wire in the liquid crystal network. When power is applied, Joule heating causes the liquid crystal network to contract. After the power is turned off, the liquid crystal network cools down under the action of air and recovers its shape [76]. The compound 1,4-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene is capable of synthesizing an elastomer. Carbon black has the effect of conducting electricity. The distribution of carbon black on elastomers enables the conductive heating of the material to generate stresses that can deform the material [78]. The deformation of the elastomer with carbon black is shown in Figure 5F.

Figure 5. Electricity and pH corresponding smart materials. (A) Hydrogel deformation synthesized by polyacrylamide. Reproduced from Reference [72] with permission from Wiley-VCH Verlag. (B) Deformation of NiTi alloy, carbon nanotubes, and PDMS composites. Reproduced from Reference [75] with permission from Wiley-VCH Verlag. (C) Acrylic acid (AA), NIPAM synthesized composite deformation. Reproduced from Reference [62] with permission from the American Chemical Society. (D) Deformation of 4,4′-bis(6-hydroxyhexyloxy)biphenyl and 6HCA composites. Reproduced from Reference [76] with permission from Wiley-Blackwell. (E) Deformation of polypropylene, cellulose fibers composites under voltage. Reproduced from Reference [77] with permission American Chemical Society. (F) Deformation of elastomeric materials synthesized by 1,4-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene. Reproduced from Reference [78] with permission from the American Chemical Society.

2.7. Hybrid Stimulus-Driven Material

Some composites are capable of responding to multiple stimuli and multiple stimuli allow for more complex planning and programming for advanced actions and applications. NIPAM is a material with a hydrophilic amide group and a hydrophobic isopropyl group and has an LCST. NIPAM is mainly used to make temperature-sensitive polymer gels. Poly(ethylene glycol) diacrylate is not responsive to temperature and can be used with iron oxides to make materials that are responsive to magnetic fields. Combining hydrogels made from NIPAM with poly(ethylene glycol) diacrylate and iron oxides enables the production of multi-responsive materials that are responsive to temperature and magnetic fields [79]. Polypyrrole (PPy) is a material that can respond to humidity, light, and temperature. PPy can absorb water, causing its volume to swell. PPy has excellent light-absorbing properties, and it can convert light energy into heat energy, which expands the volume of the material. Similarly, the temperature can directly cause the volume expansion of PPy. Polyethylene glycol terephthalate (PET) is a material that is very unresponsive to humidity, light, and temperature. The compounding of PPy with PET allows the production of multi-stimulated bilayer structures [80]. NdFeB and PDMS are compounded into a material whose CTE is different from that of polytetrafluoroethylene, causing the material to bend and deform. The material compounded with NdFeB and PDMS has an excellent magnetic response and can control its deformation by planning the magnetic field. MXene has an excellent electrical conductivity and is embedded in the material to transmit current [81]. The thermal effect of the electric current can deform the material.

3. Manufacturing Method

Unlike ordinary materials, smart materials are mostly soft materials and therefore require processing methods that are adapted to their characteristics. The existing material processing methods are mainly divided into additive manufacturing and subtractive manufacturing. Additive manufacturing, which can also be called three-dimensional (3D) printing, mainly refers to the accumulation of materials layer by layer under the action of 3D printers to eventually form the object to be printed. Subtractive manufacturing is the process of removing and trimming a piece of material to achieve a final shape and is mainly used for hard materials such as steel. Because many soft materials can be polymerized and formed under light, curing materials using light is an important method for processing soft materials, such as photolithography and two-photon polymerization. This paper provides a brief introduction and summary of the manufacturing methods involved in the paper. The main methods include flip-mold [20,26,27], direct ink writing technology [29,30], fused deposition modeling [19], lithography [31,32,33], projection micro-stereolithography [18], two-photon polymerization [34,82].

3.1. Flip-Mold Method

The mold-flipping method uses a mold of a certain shape to create the material into the shape of the mold. Figure 6A presents the process of manufacturing methods using PDMS as a mold. A mold was made in advance with PDMS, the light-curing solution was added to the mold, and the solution was polymerized and formed under UV light. Then, another light-curing solution was added to the mold and the solution polymerized under UV light [83]. This allowed the production of bilayer structures made of two different materials. The curing method will vary for different materials, the common ones being UV curing and heat curing. For objects with complex shapes or uncertain shapes, a simple mold can be made to cure the material before further processing. Figure 6B presents the combination of two glass cover sheets and a quadrilateral box to form a simple mold. The material is injected between the glass plates piece by piece for polymerization, creating a bilayer quadrilateral, which is further sheared into the planned shape [84]. The flip-mold method is simple and feasible, requiring only a simple or complex mold to be made to cure the material, and it does not require sophisticated and complex instruments. However, the accuracy achieved by the flip-mold method is also relatively low and not suitable for the fabrication of high-precision microrobots.

3.2. Direct Ink Writing (DIW)

DIW simply means that the ink is squeezed out of the deposition nozzle under pressure [85]. This technology typically requires a printer and an air pump system. DIW requires inks with specific rheological properties. Important rheological parameters of inks include apparent viscosity, yield stress under shear and compression, and viscoelastic properties [86]. The ink needs to be able to heal quickly after extrusion, i.e., the ink flows at a lower viscosity under shear but quickly recovers its mechanical properties after the shear force is removed [87]. The viscosity of the ink has a huge impact on printing. When the ink viscosity is too low, it cannot maintain the shape of the material; when the ink viscosity is too high, it cannot be extruded from the nozzle, resulting in an irregular structure of the printed material. Thus, choosing the right ink viscosity is an important aspect to ensure a good print structure [88]. Figure 6D shows the process of printing a tubular structure with a two-nozzle printer. The two-nozzle printer prints the two materials in alternating layers, accumulating them layer by layer and eventually printing them as a tubular structure [30]. Figure 6E is a schematic diagram of DIW. The ink is placed in a container above the nozzle and the ink is squeezed out of the nozzle under pressure. By moving the position of the nozzle, it is possible to print pre-planned 3D structures. The size of the nozzle, the pressure applied, and the speed of movement of the nozzle all affect the printed microrobot structure [54]. An ink contains a polydimethylsiloxane matrix. The addition of fumed silica nanoparticles to the ink allows the ink to have the shear thinning and shear yielding behavior required for DIW. This ensures that the composite ink can be smoothly extruded from the nozzle by external air pressure [58].

3.3. Fused Deposition Modeling (FDM)

FDM belongs to the category of 3D printing, which is mainly applied to thermoplastic polymers. During the printing process, the filament is melted and printed on a bed (i.e., printing platform) and cured after printing [89]. During the printing process, the filament experiences temperatures higher than its melting temperature while being stretched.

Figure 6C is a schematic diagram of the FDM. Shown on the left is a light-responsive shape memory material, and in the center is a 3D printing device for FDM. The print material is heated and melted at the nozzle and thus extruded from the nozzle. The motor controls the movement of the nozzle to print the object. The right side of the image displays the deformation of the printed cube in the light [90]. Polylactic acid is one of the most widely used shape memory polymers in amateur FDM printers. Flat surfaces are created by printing layers with the same pattern using Polylactic acid and stacking layers with different patterns. We can print planes of the desired deformation features on top of each other and program the desired shape transformation into the final multi-layer planar structure [19].

Figure 6. Schematic diagram of the manufacturing method. (A) Schematic diagram of the flip-mold method using PDMS. Reproduced from Reference [83] with permission from Elsevier. (B) Schematic diagram of the flip-mold method using two glass cover plates. Reproduced from Reference [84] with permission from John Wiley and Sons Inc. (C) Schematic diagram of FDM. Reproduced from Reference [90] with permission from Wiley-Blackwell. (D) Schematic of direct inking using two nozzles. Reproduced from Reference [30] with permission from the American Chemical Society. (E) Schematic of direct inking using a single nozzle. Reproduced from Reference [54] with permission from Elsevier BV.

3.4. Photolithography

Photolithography is the polymerization of materials under the action of light using a certain pattern of masks’ underexposure. Due to the use of a certain shape of mask, the lithographed objects have the same shape in height. The stacking of different shapes can be achieved with different masks and used to create objects with complex shapes.

Figure 7A presents the lithography process with four different masks. The reaction solution is first deposited on a sliding plate coated with a sacrificial layer. UV light is irradiated through Mask 1 to polymerize some of the material. UV polymerization by sequential addition of materials and replacement of masks with different shapes enables the planning of shapes [33]. The overlay of the sacrificial layer facilitates the removal of the final formed material. Figure 7B presents a reaction chamber made of tape and glass sheets. The solution is injected into the reaction chamber and a mask of a certain shape is placed at a certain distance above the solution, enabling selective cross-linking of the solution in the light region, and then the bilayer structure is prepared by injecting another solution for photopolymerization [72]. The shape of the mask can be designed in CAD with different geometric parameters, such as the shape, size, and thickness of the monolayer hydrogel, which can be achieved at different UV light irradiation intensities [31]. A polymer gripper is prepared by a sequential lithography process that uses continuous lithography to mold the material [91].

3.5. Projection Micro-Stereolithography (PμSL)

Stereolithography (SLA) is one of the most commonly used methods for printing shape memory polymers with very high precision and can employ a range of materials [92]. PμSL is an improved method based on stereolithography, which is a high-precision additive manufacturing technology based on the principle of surface projection lithography. The core technology of PμSL is digital light processing technology [93]. Digital light processing is linked to dynamic mask lithography, where the data for each layer of the structure is given as a black-and-white image. These images are exposed through thousands of individually adjustable digital micromirror devices [94]. Digital light processing technology can customize planned patterns based on device exposure and has a wide range of applications in printing polymers [95,96,97]. Figure 7C is a schematic diagram of the PμSL technique. The computer-aided design model is first cut into a series of closely spaced horizontal two-dimensional digital images, which are then transferred to a digital microdisplay where ultraviolet light generated by a light-emitting diode display is spatially modulated with the pattern of the corresponding two-dimensional images and irradiated onto the surface of the light-cured polymer solution. Once the material in the exposed area is cured to form a layer, the substrate on which the fabricated structure is based is lowered through a translation phase and then the next image is projected, allowing the new layer to polymerize on top of the previous layer. This process is repeated until the entire structure is fabricated [18]. The difference between PμSL and ordinary lithography is the difference in their masks. While ordinary lithography utilizes a fixed-shape mask, PμSL uses a dynamic mask generated by a computer. Figure 7E presents the curing of the hydrogel with the digital light processing technique. The left side of the image indicates the light pattern cured in the middle of the material, and the right side indicates the light pattern cured at both ends of the material. Finally, the cured graphics are globally cured in the new material in the light mode [71].

3.6. Two-Photon Polymerization (TPP)

Two-photon polymerization has a wide range of applications in microrobot manufacturing [98]. Femtosecond laser-based TPP enables high-resolution 3D shapes. TPP can be applied not only in optics, chemistry, physics, biomedical engineering, and microfluidics but also in the fabrication of microrobots [99]. TPP requires materials that are not prone to single-photon absorption (i.e., transparency) at laser wavelengths before two-photon absorption can be used. A photoresist is a material used for TPP. A photoresist is generally a mixture of monomers, cross-linkers, photoinitiators, and other materials [100]. A schematic diagram of TPP is presented in Figure 7D. The femtosecond laser beam has enough laser pulse energy to irradiate the material. Only at the focal point of the laser pulse can the two-photon absorption of the material occur, leading to the polymerization of the material. The energy of the laser at the non-focal position is not sufficient to cause two-photon absorption of the material [101]. The position of the laser focus can be controlled using a computer, allowing the material to polymerize where it is needed to create a microrobot. The unpolymerized material can be rinsed off. Because TPP is made using femtosecond laser aggregation, the focal point of the laser is very small and can reach nanometer precision. Thus, it is said that TPP can realize high-precision object production [102]. Laser threshold power and polymer linewidth are two key parameters to evaluate the accuracy of TPP processing. Different laser power aggregates produce materials with different precision. For different materials, we must choose the right laser power to ensure the processing accuracy of TPP [41].

Figure 7. Schematic diagram of the manufacturing method. (A) Lithography schematic. Reproduced from Reference [33] with permission from Wiley-VCH Verlag. (B) Lithography schematic. Reproduced from Reference [72] with permission from Wiley-VCH Verlag. (C) Projection micro-stereolithography. Reproduced from Reference [50] with permission from the Institute of Physics Publishing. (D) Two-photon polymerization. Reproduced from Reference [34] with permission from Elsevier. (E) Digital light processing technology. Reproduced from Reference [71] with permission from the American Chemical Society.

4. Functions and Biomedical Applications

Microrobots are capable of complex manipulation tasks that are still essentially syntheses of simple movements such as bending, rotating, rolling, etc. To conduct research on microrobots we need to go from simple to complex, starting with simple actions and then using the simple actions to make combinations of complex actions. Only when its functional movement is realized can it be applied in a real environment. Research with a large amount of real-life experimental data eventually reaches maturity for use in specialized fields, such as biomedicine. Microrobots made from smart materials that can intelligently perform basic functions and actions are used in environments with stringent requirements. However, the miniaturization of microrobots can achieve functions that cannot be achieved by ordinary robots. It has great potential in the medical field. Currently, most microrobots made of smart materials are soft structures that cannot withstand large loads. Microrobots made of smart materials can take simple actions based on environmental stimuli, such as realizing rolling, crawling, and grasping. Some microrobots are modeled after plants and animals that exist in nature and mimic their movements to create microrobots, tentatively called bionic plant and animal microrobots. Then, some microrobots can transport objects and realize changes in the position of the objects through the movements mentioned above.

4.1. Functions

4.1.1. Grabbing Action Function

Microrobots made of smart materials are available in cross-shaped, jellyfish-shaped, tweezer-shaped, and many other types. The microrobot is made to deform and clamp objects under external stimuli, and when the environmental stimuli disappear, the deformation of the microrobot is restored and the clamping of the objects is released.

By combining a moisture-active/alkyl-inert GO layer with an alkyl-active/moisture-inert polydimethylsiloxane layer, a dual-response bilayer brake can be fabricated. The actuator is integrated with four microfluidic channels, three thin tubes for the hexane channel, and a thick tube for the humidity channel, where the actuator can be bent to clamp and release objects by adjusting the hexane and humidity stimuli [69]. In addition, a combination of hydrogel and elastomer is used to create a soft microrobot that can bend and straighten in response to changes in light. An actuator is a claw-like object. The hydrogel dehydrates under the light, the claw is open. Upon turning off the light, the hydrogel shrinks to grasp the object [103]. A four-finger gripper with an electrothermal drive is prepared by combining a magnetic NdFeB/polydimethylsiloxane composite layer, MXene film, and PTFE tape. When a voltage is applied to the gripper, each finger is deformed and bent inward to grasp the object, and after the voltage is turned off, the fingers return to their original shape and release the grasp of the object [81]. A robust conductive composite thin-film humidity driver was prepared using one-dimensional cellulose nanofibers and carbon nanotubes and GO. The excellent moisture sensitivity of cellulose nanofibers and GO, as well as hydrophobic carbon nanotubes that facilitate rapid water desorption into the actuator, provide excellent actuation. The water vapor inside the tube allows for the controlled gripping and releasing of objects by the gripper [57].

A two-armed type of hydrogel microcantilever is shown in Figure 8A. The actuator can respond quickly in water under near-infrared light and is capable of repeated deformation. The distance between the two arms of the driver can be controlled by controlling the focus laser and the power of the laser [41]. Figure 8B shows a simple gripper made using light-driven materials. The bending of the gripper can be realized under the action of light, and the gripper can grip objects larger than its weight [52]. Figure 8C also shows a double-arm actuator made of light-driven material. The actuator grips objects by light drive, has a fast response time, and has an adhesive force on the object when gripping it. The figure shows a double-arm drive to clamp different objects, driven in near-infrared light [48]. Figure 8D represents a single-arm actuator and a two-arm actuator gripping an object. Above that is a single-arm actuator, in the state of current off, the actuator remains straight; under the action of current, the single-arm actuator can achieve bending and hooking objects. Below the picture is a double arm actuator, in the absence of current, the actuator remains straight; under the action of current, the double arms bend inward to clamp the object and achieve movement of the object [75]. Figure 8E shows a cross-shaped manipulator made of hydrogel. The manipulator can bend in the water to achieve the wrapping of objects. The temperature of the water can be controlled to achieve the grasp and release of objects, and these deformations can be cycled many times [20]. Figure 8F shows a Y-shaped gripper made using smart materials. The Y-shaped grip consists of an arm and two fingers. The near-infrared light drive enables the actions of grasping, raising, and lowering objects. The actuator is capable of grasping objects 150 times its size [104].

4.1.2. Scrolling and Crawling

Microrobots made of smart materials can roll and crawl in response to environmental stimuli. Some animals use rolling and crawling to move forward, such as millipedes, which crawl through their several feet. Figure 9A shows a fabricated sheet structure crawling microrobot. The microrobot can creep and crawl under the action of near-infrared light, and the robot can move forward by controlling the switch of light [105]. An electrically driven actuator can be formed using a double layer of liquid crystal elastomer and carbon black. Joule heating of the carbon black can create uneven stresses in the actuator, causing the actuator to roll forward. Figure 9B shows a cylindrical electric driver made from liquid crystal elastomer and carbon black. Under the action of the current, the drive rolls forward by turning into an approximately vertical ellipse, the current is turned off, and the drive stops moving [78]. Figure 9C shows a cylinder-type robot made with several blades according to certain rules. The ends of this microrobot can be bent by the action of light. The microrobots are deformed by shining light on certain areas. Rational planning of deformation positions allows the microrobot to advance sequentially [43]. Figure 9D shows a cone with several magnetic responses on a piece of sheet material. The cone is slightly deformed by the action of the magnetic field so that the whole advances. The microrobot can move forward on a flat surface, can carry a certain weight of objects forward, and can also climb over surfaces with gullies [106].

4.1.3. Transporting Objects

There are many examples of transporting objects by a robot in life. Examples include cargo transport robots for industrial applications and food delivery robots for the restaurant industry. They have a variety of functions such as intelligent navigation, autonomous obstacle avoidance, data saving, and item storage. Microrobots made of smart materials cannot be so smart, but they also have their advantages, such as operating in a small space.

Figure 10A shows a cross-shaped microrobot that is driven under a magnetic field. By controlling the size and position distribution of the magnetic field, the robot’s function of grasping objects can be realized, and the change in the magnetic field is used to make the microrobot jump over obstacles and release objects [32]. A model diagram of a drug transport microrobot is shown in Figure 10B. The microrobot enables targeted drug delivery in mice, and it enables object grasping. This microrobot is driven by using the different pH values of various locations in the mouse [107]. Figure 10C shows a cross-shaped smart lifter made using nanofibers for object transportation. The lifter can be raised or lowered by adding ethanol to the solution [70]. Figure 10D shows a microrobot made by a photoresist. The microrobot grasps the object when exposed to proton stimulation; moving to the other side by the action of the magnetic field, the object is released as the composition of the solution changes [108].

4.2. Biomedical Applications

4.2.1. Target Capture

Microrobots used in biomedical applications inevitably come into contact with cells, and localizing and grasping cells or other objects is a function that microrobots should have. Figure 11A shows a microgripper made of hydrogel, wherein Figure 11A(i) indicates that the microgripper is able to move between black objects under the control of a magnetic field. Figure 11A(ii) represents the ability of the microgripper to clamp and excise a portion of a fibroblast cell mass from the fibroblast cell mass, and the bending contraction of the microgripper is realized under the change in temperature. Figure 11A(iii) represents a state diagram of the microgripper when it is holding the cell, where the dotted line is added to visualize the gripper [91]. Figure 11B shows a series of images of the process of grasping and releasing an object by a microgripper composed of hydrogel. The microgripper is in a stretched state above a certain temperature, and as the microgripper approaches the object, it wraps around the object and grabs it as the temperature decreases, and then transports it to a certain location to release the object. The hydrogel is biodegradable and improves safety in medicine. The movement of the microrobot is controlled by a magnetic field [31].

Figure 11C demonstrates the manipulation of the microgripper in an isolated human fallopian tube tissue section. Figure 11C(a) represents the microgripper reaching the bifurcation point after which it keeps bending and contracting with the change in temperature. Figure 11C(b) represents the microgripper being guided away from the bifurcation point as it gradually moves away. The microgripper enables drug release as well as performing other tasks. The microgripper mainly consists of silicon oxide, silicon dioxide, and paraffin wax. The working principle is described as follows: at different temperatures, the clamp is subjected to different stresses. At low temperatures, paraffin wax has a large stiffness, so that the clamp is in a stretched state; with the temperature increase, the paraffin wax stiffness decreases, the clamp cannot resist the role of residual stress, and clamp bending contraction occurs [21]. Figure 11D shows the shape of a microrobot carrying microbeads capable of stepping out of the number eight under the control of a magnetic field. It is able to accurately reach every location point, and the circle changes from red to blue where it passes. This is a necessary function for a microrobot to realize biomedical applications, and the microrobot fabricated in this paper is able to realize the above functions. The microrobot is composed of hydrogel and alginate microparticles, and near-infrared light can cause the microrobot to open and close, with a magnetic field driving its movement [109].

Hyperthermia is a method of treating tumors by differentiating the heat sensitivity of tumor cells from normal cells in order to kill the tumor cells. Figure 11E shows the application of microrobots made of hydrogel in the human body. The double-layer structure of the hydrogel is deformed by its magneto-thermal effect and wraps around the cancer cells, releasing heat to reach a certain temperature to kill the cancer cells. Remote operation using magnetic control is more convenient than manual operation and can reach places that manual operation cannot [110]. A schematic diagram of the operation of the micro clamp is shown in Figure 11F. The micro clamp operates in a vascularized environment. It is capable of clamping objects in blood vessels, such as blood clots, for endovascular treatment. The microrobot is jointly controlled by a direct current (DC) magnetic field, which controls the movement of the microgripper, and an alternating magnetic field, which controls the bending and contraction of the microgripper, and the DC magnetic field guides the microgripper to a specified position. An alternating magnetic field can cause the microgripper to generate heat, increasing its own temperature, which in turn affects the water content of the hydrogel, causing it to deform in order to capture the object [111].

Figure 11. Microrobots targeted grasping of objects or cells. (A) (i) Movement of microrobots. (ii) Fibroblast clusters grasped by microgrippers. (iii) Diagram of the state of a fibroblast cluster as it is grasped by a microgripper. Reproduced from Reference [91] with permission from the American Chemical Society. (B) A microrobot grasps and releases objects. Reproduced from Reference [31] with permission from the American Chemical Society. (C) (a) Deformation of microrobots with changes in temperature. (b) Motion paths of microrobots. Reproduced from Reference [21] with permission from the American Chemical Society. (D) Motion trajectories of microrobots. Reproduced from Reference [109] with permission from Wiley-Blackwell. (E) Microrobots locate and kill cancer cells. Reproduced from Reference [110] with permission from Elsevier Ltd. (F) The micro clamp grasps the clot in the vessel. Reproduced from Reference [111] with permission from the American Chemical Society.

4.2.2. Vascular Therapy

Blood vessels play an important role in the body for blood transportation. Congestion in the blood vessels will inevitably cause an obstruction of blood flow, which in turn affects the safety of human life. Microrobots have potential applications in vascularization and impurity cleanup. Vessels are vital to the body as they are the channels through which blood travels, but minimally invasive treatments in blood vessels have a number of listed challenges. A schematic of the motion of the microrobot in the vascular model is shown in Figure 12A. It is able to realize targeted drug delivery under the control of spatial magnetic field, and the robot is highly adaptive. It can change its own shape according to blood vessels and remove blood clots effectively. The microrobot consists of magnetorheological fluid, which is capable of shape and stiffness changes and multimodal multifunctional conversion because of its high viscosity [112]. The procedure of TACE using a microrobot is shown in Figure 12B. The microrobot is released from the catheter and guided under a magnetic field under real-time imaging to various locations in the blood supply vessels of the tumor. The microrobot blocks the blood vessels supplying blood, causing an embolism to kill tumor cells, and the microrobot can naturally pass within a few weeks. The microrobot consists of a hydrogel and magnetic nanoparticles that are capable of treating tumors in the blood vessels, targeting and delivering therapies [113].

A schematic diagram of the microrobot for on-demand drug delivery is shown in Figure 12C. A spiral microrobot swims towards the target area under the action of a magnetic field, and high-intensity ultrasound is used to cause the microrobot to release the drug molecules. The figure below represents the release of a drug under the action of ultrasound, where its molecular bonds are broken to release the drug. Self-rolling helical microrobots provide a controlled drug delivery system that is magnetically manipulated and provides efficient drug release. The microrobot uses a single layer of self-folding technology that allows for rapid shape changes and manufacturing simplicity under stimuli. It uses magnetic navigation and the ultrasound-stimulated release of noncovalently bound anticancer drugs. Ultrasound stimulates rapid drug release. The microrobot consists of a soft and hard structure, with the soft layer curling and rolling to release internal stresses and the hard layer consisting of wrinkling and folding. This structure allows for rapid, reproducible production as well as the ability to control the morphology and chemistry of the structure. In contrast, external stimulation has a low rate of drug release and a long stimulation period [114].

4.2.3. Transportation of Drugs

Microrobots are able to reach locations that are difficult to reach by normal methods due to their small size and flexibility. The targeted transportation of drugs by microrobots is one of their major biomedical applications. The targeted drug delivery experiment of the microrobot in an in vitro gastric model is shown in Figure 13A. The position of the microrobot was moved by controlling the magnetic field, and after reaching the specified position, the microrobot was able to adsorb on the surface of the cellular tissue and release the drug, which was detached under the control of temperature. The microrobot consists of polyethyleneglycol diacrylate, PNIPAM, which is driven by a magnetic field and temperature-controlled shrinkage and deformation [115]. Figure 13B shows the application of a microrobot to mimic the structure of the human stomach in an environment that is humid and has crisscrossing channels. The microrobot consists of a piece of sheet material and has a number of magnetically responsive cones, which are slightly deformed by the action of a magnetic field to make the whole move forward; the microrobot is capable of carrying a certain amount of weight to move forward on flat surfaces, and it can also crawl over surfaces with gullies. The microrobot is mainly composed of PDMS and ferromagnetic particles, with magnet rods placed at the bottom to form tapered feet induced by a magnetic field. Because of its multi-legged structure, it is able to move and walk in wet and dry environments and deform under the action of magnetic fields. This is a preliminary experimental process for the realization of biomedical applications of microrobots [106].

A model of a micro-fish made with a pH-responsive hydrogel is shown in Figure 13C. This micro-fish model enables the grasping, moving, and releasing of particles in artificial blood vessels, and the release of drugs for the treatment of Hela cells through the change in solution pH [62]. Figure 13D shows a microrobot being magnetically driven to translate and rotate in a humid environment in the stomach. The microrobot is able to cross gullies and raised areas, and the microrobot is driven by near-infrared light heating to enable it to perform the gripping of goods, and then the magnetic field is used to transport the goods out of the area. Flexible grip and adaptability facilitate its clinical application in cell manipulation, targeted therapy, and minimally invasive surgery. The microrobot consists of NIPAM, Laponite nanoclay, and NdFeB magnetic particles. It has a good processability, biocompatibility, and prints soft microrobots with different structures. The movement of microrobots has been validated to overcome physical barriers in human stomach models to accomplish the active transportation of goods, and the combined control of magnetic fields and temperature enables them to reduce drug leakage during transportation. These factors promote the use of microrobots in biomedical applications [116].

A precisely targeted drug delivery microrobot is shown in Figure 13E. It is a biocompatible and hydrolyzable PEGDA-based drug delivery helical microrobot capable of encapsulating the anticancer drug Adriamycin. Among them, near-infrared light is used as an external stimulus to carry out precise delivery to cancer cell lesions through a magnetic field, which can realize magnetic particles and rapid separation and extraction. If magnetic particles are not quickly isolated and extracted, then they can lead to membrane integrity problems and the apoptosis of normal cells [117].

4.2.4. Cellular Transport

Figure 14A demonstrates the targeting of a magnetically guided microrobot. It is a two-dimensional targeting experiment, where magnetically driven microrobots target the delivery of stem cells. As shown in the figure, it is divided into control, loading, and targeting zones. Initially, the control and loading zones contain stem cells, which are then guided by a magnetic field to the targeting zone. In that experiment, only a few microrobots could not deliver stem cells. It indicated a feasibility test to achieve articular cartilage repair, which is biocompatible and biologically explanatory, with a surface coated with magnetic nanoparticles for magnetic actuation. The ability to perform targeted transport tasks has great potential for application in articular cartilage regeneration [118]. Magnetic micro-propellers capturing, transporting, and releasing fertilized eggs from cattle and rats are the beginning of the realization of a non-invasive, magnetic micro-motor-assisted zygote intrafallopian transfer (ZIFT) application. Figure 14B represents a microrobot capturing, transporting, and releasing a single fertilized egg in a resilient and reversible manner under the control of a magnetic field. The spiral design is ideal for simply pushing cellular cargo forward, and the spiral shape wraps the cargo in the center to prevent injury. Cargo release is reversible. Changing the direction of the magnetic field allows the microrobot to move in different directions [119].

5. Future Perspectives and Conclusions

In this review, we start with materials and describe three main aspects: smart materials, manufacturing methods, and applications. We describe the deformation mechanism of smart materials and some common smart materials. They are briefly classified and introduced according to the stimulus source. The main stimulation sources of smart materials are light, temperature, magnetism, humidity, electricity, pH, etc. Light as a stimulus source does not require direct contact with the material and allows for remote control. However, light cannot penetrate certain materials; for example, when applied in the human environment, light cannot drive microrobots through skin and muscles. In contrast, both the magnetic field and temperature can be driven through the material, and the magnetic field can be planned and laid out to achieve a certain shape distribution of the field. Magnetic fields are an excellent source of stimulation for application in real environments. For temperature, the material response has a strict limit to temperature, certain materials can only respond at a certain temperature, but the actual application temperature may conflict with the material response temperature.

Smart materials are diverse, but the principles by which they respond to certain stimuli can be broadly classified into several categories. Hydrogel-like materials change their water content by varying the cross-link density of the polymer network resulting in a change in the volume of the material. Light, temperature, and humidity can all affect the moisture content of the material. Light can change the cross-link density of a material or cause it to lose water through the photothermal effect of the material. Temperature uses the critical dissolution temperature of a material to make it lose or absorb water at a certain temperature. Shape-memory polymers change their internal molecular structure to achieve different shape transitions. Temperature is the most common source of stimulation for shape memory polymers. Liquid crystal elastomers deform the material by changing its internal molecular arrangement. Furthermore, the material can react to certain ions, which leads to a change in the stress within the material and the stress deforms the material. The deformation of smart materials is mainly attributed to changes in the internal molecular structure of the material, expansion of the volume of the material by heat and light, and exposure of the material to electrostatic or magnetic forces. Different materials have different molecular structures, and under the stimulation of the external environment, some internal molecular groups or molecular structures of the materials will change and lead to a series of other changes. Simple programming of the molecular structure of materials has been implemented in several papers to achieve a plan on how the internal structure of material molecules will change and thus lead to the deformation of the microrobot on a macroscopic scale. Although some simple programming is possible, it is still very difficult to achieve the same functionality as a robot controlled by a computer.

This review briefly introduces several manufacturing methods commonly used for microrobots. Different fabrication methods have different fabrication accuracies, for example, TPP is a fabrication method with a high accuracy. Different manufacturing methods are selected according to the characteristics of the microrobot to be made. Microrobots made with smart materials can perform certain functions, such as grasping objects, rolling, crawling, and imitating the movements of plants and animals in nature. Microrobots made of smart materials have great potential for targeted drug delivery and have been experimentally analyzed by many researchers. Microrobots made of smart materials have huge advantages in various fields because of their small size and diverse stimulus sources.

Microrobots made of smart materials have unique advantages in drug delivery, and their application in medicine has great potential. It can release the drug at a specific location and the microrobot can degrade itself in the human body, reducing the difficulty of drug delivery. However, many of the smart materials used in making microrobots are toxic and cannot be used in living organisms; otherwise, the life safety of the organisms will be endangered. Furthermore, some microrobots do not degrade in the human body, and it is a difficult task for them to be expelled from the body. In terms of sensing, smart materials can sense external stimuli and produce changes, but the accuracy of smart materials for sensing changes in external stimuli needs to be improved. In terms of control, it is possible to control the movement and deformation of the smart material through a variety of stimuli, but there are limitations to the precision of these controls. For example, when controlling the deformation of the material, the planned degree of deformation cannot be reached. The sensing properties of the smart material also limit its control. In terms of durability, smart materials are able to withstand multiple deformations, but as the number of times increases there will be a decrease in deformation accuracy. In conclusion, the performance of smart materials is crucial for microrobots, and to a certain extent determines the application prospects and fields of microrobots.

In recent years, microrobots built with smart materials are still in the basic stage and have limited research areas. In the coming years, the development of smart materials is an important factor driving the progress of microrobots. Also, advances in microrobots require a combination of robotics, manufacturing methods, physical chemistry, and other fields. To promote the development of microrobots, researchers should start from the characteristics of microrobots, i.e., how to better achieve miniaturization and intelligence, and reasonably design the framework and structure of microrobots. Although microrobots based on smart materials are currently rarely used in commercial applications, their development in the future is immeasurable.

Funding

This research was funded by National Natural Science Foundation of China (Project No. 62273289), and The Youth Innovation Science and Technology Support Program of Shandong Province (Project No. 2022KJ274).

Data Availability Statement

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

Nomenclature

3D	Three-dimensional
6HCA	4-(6-Hydroxy hexyloxy) cinnamic acid
AA	Acrylic acid
CAD	Computer aided design
CTE	Coefficient of thermal expansion
DC	Direct current
DIW	Direct ink writing
FDM	Fused deposition modeling
GO	Graphene oxide
LCP	Liquid crystal polymers
LCST	Low critical solubility temperature
NIPAM	N-isopropyl acrylamide
PDMS	Polydimethylsiloxane
PE	Polyethylene
PEGDA	Poly(ethylene glycol) diacrylate
PET	Polyethylene glycol terephthalate
PH	Potential of hydrogen
PMMA	Poly(methyl methacrylate)
PNIPAM	Poly(N-isopropylacrylamide)
PPy	Polypyrrole
PμSL	Projection micro-stereolithography
SDS	Sodium dodecyl sulfate
SLA	Stereolithography
TPP	Two-photon polymerization
UV	Ultraviolet
ZIFT	Zygote intrafallopian transfer

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Figure 8. Schematic diagram of the grasping function of the microrobot. (A) Light-driven two-armed microrobot. Reproduced from Reference [41] with permission from Elsevier. (B) Light-actuated sheet gripper. Reproduced from Reference [52] with permission from Wiley-VCH Verlag. (C) Light-actuated dual-arm driver. Reproduced from Reference [48] with permission from Wiley-VCH Verlag. (D) Single-arm and dual-arm actuators. Reproduced from Reference [75] with permission from Wiley-VCH Verlag. (E) Temperature-controlled cross-shaped manipulator. Reproduced from Reference [20] with permission from the American Chemical Society. (F) Light-driven Y-sex microrobot. Reproduced from Reference [104] with permission from John Wiley and Sons Inc.

Figure 9. Rolling and crawling of the microrobot. (A) Creeping crawl of a flaky microrobot. Reproduced from Reference [105] with permission from Elsevier Ltd. (B) Rolling of a microrobot under an electric current. Reproduced from Reference [78] with permission from the American Chemical Society. (C) Rolling of a microrobot under illumination. Reproduced from Reference [43] with permission from Wiley-Blackwell. (D) Crawling of a microrobot under a magnetic field. Reproduced from Reference [106] with permission from Springer Nature.

Figure 10. Microrobot implementing transport function. (A) Movement of the microrobot under a magnetic field. Reproduced from Reference [32] with permission from Wiley-VCH Verlag. (B) Transport model of the drug robot. Reproduced from Reference [107] with permission from Springer Nature. (C) Cross-shaped microrobots ascending and descending in solution. Reproduced from Reference [70] with permission from the American Chemical Society. (D) Microrobots enable the transport of objects. Reproduced from Reference [108] with permission from Chemistry of Materials.

Figure 12. Microrobots removing impurities or treatments in blood vessels. (A) Schematic of the movement of the microrobot in the vascular model. Reproduced from Reference [112] with permission from the American Chemical Society. (B) The process of TACE performed by a microrobot. Reproduced from Reference [113] with permission from the American Association for the Advancement of Science. (C) Schematic of microrobot on-demand drug delivery. Reproduced from Reference [114] with permission from Elsevier.

Figure 13. Drug transportation by microrobot. (A) Targeted drug delivery experiments with a microrobot in an in vitro gastric model. Reproduced from Reference [115] with permission from Wiley-Blackwell. (B) A microrobot that mimics the structure of the human stomach. Reproduced from Reference [106] with permission from Springer Nature. (C) The micro-fish model of drug transport. Reproduced from Reference [62] with permission from the American Chemical Society. (D) A microrobot for transporting drugs in a gastric model. Reproduced from Reference [116] with permission from Elsevier Ltd. (E) Precision-targeted microrobots for drug delivery. Reproduced from Reference [117] with permission from the American Chemical Society.

Figure 14. Microrobot for cell transportation. (A) Magnetically driven microrobots for targeted delivery of stem cells. Reproduced from Reference [118] with permission from John Wiley and Sons Ltd. (B) Microrobots capture and transport fertilized eggs. Reproduced from Reference [119] with permission from Wiley-VCH Verlag.

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Sun, J.; Cai, S.; Yang, W.; Leng, H.; Ge, Z.; Liu, T. Microrobots Based on Smart Materials with Their Manufacturing Methods and Applications. Inventions 2024, 9, 67. https://doi.org/10.3390/inventions9030067

AMA Style

Sun J, Cai S, Yang W, Leng H, Ge Z, Liu T. Microrobots Based on Smart Materials with Their Manufacturing Methods and Applications. Inventions. 2024; 9(3):67. https://doi.org/10.3390/inventions9030067

Chicago/Turabian Style

Sun, Jiawei, Shuxiang Cai, Wenguang Yang, Huiwen Leng, Zhixing Ge, and Tangying Liu. 2024. "Microrobots Based on Smart Materials with Their Manufacturing Methods and Applications" Inventions 9, no. 3: 67. https://doi.org/10.3390/inventions9030067

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Microrobots Based on Smart Materials with Their Manufacturing Methods and Applications

Abstract

1. Introduction

2. Smart Materials That Respond to Different Stimuli

2.1. Light-Driven Materials

2.2. Temperature-Driven Materials

2.3. Moisture-Driven Materials

2.4. Materials for Magnetic Drive

2.5. pH-Driven Materials

2.6. Materials for Electric Drive

2.7. Hybrid Stimulus-Driven Material

3. Manufacturing Method

3.1. Flip-Mold Method

3.2. Direct Ink Writing (DIW)

3.3. Fused Deposition Modeling (FDM)

3.4. Photolithography

3.5. Projection Micro-Stereolithography (PμSL)

3.6. Two-Photon Polymerization (TPP)

4. Functions and Biomedical Applications

4.1. Functions

4.1.1. Grabbing Action Function

4.1.2. Scrolling and Crawling

4.1.3. Transporting Objects

4.2. Biomedical Applications

4.2.1. Target Capture

4.2.2. Vascular Therapy

4.2.3. Transportation of Drugs

4.2.4. Cellular Transport

5. Future Perspectives and Conclusions

Funding

Data Availability Statement

Conflicts of Interest

Nomenclature

References

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