- Open Access
Advances in biomimetic stimuli responsive soft grippers
© The Author(s) 2019
- Received: 15 April 2019
- Accepted: 5 June 2019
- Published: 1 July 2019
A variety of biomimetic stimuli-responsive soft grippers that can be utilized as intelligent actuators, sensors, or biomedical tools have been developed. This review covers stimuli-responsive materials, fabrication methods, and applications of soft grippers. This review specifically describes the current research progress in stimuli-responsive grippers composed of N-isopropylacrylamide hydrogel, thermal and light-responding liquid crystalline and/or pneumatic-driven shape-morphing elastomers. Furthermore, this article provides a brief overview of high-throughput assembly methods, such as photolithography and direct printing approaches, to create stimuli-responsive soft grippers. This review primarily focuses on stimuli-responsive soft gripping robots that can be utilized as tethered/untethered multiscale smart soft actuators, manipulators, or biomedical devices.
- Soft actuators
- Intelligent systems
- Soft robots
Inspired by the shape change of biological systems, such as Rhododendron  and Dionaea muscipula (Venus flytrap) leaves [2, 3], biomimetic shape-morphing soft robots have been extensively proposed by utilizing stimuli-responsive hydrogels, polymer, or their hybrid combination [4–6]. The stimuli responsive materials and their architectures can be transformed into three-dimensional (3D) self-assembled, -curved, or -folded structures in response to external triggers without any manual control . In particular, newly emerged biomimetic stimuli-responsive soft gripping systems have been highlighted because of their promising applications in smart actuators, flexible electronics, biosensors, micro/nanomanipulators, smart medicine, and surgery [7–30].
In engineering biomimetic soft grippers, hydrogels, and polymer are attractive materials for the following reasons: first, polymer or hydrogels exhibit moduli ranges (~ KPa) similar to those of biological tissues and organs ; and second, polymer or hydrogels can swell by several orders of magnitudes in volume in response to external stimuli [32, 33]. This swelling/de-swelling mechanism can yield large deformations that enable self-actuation without any tethered external power sources. Multilayer thin-film fabrication or direct printing is widely used to create biomimetic soft grippers because multilayer structures can achieve spontaneous 3D curving, wrinkling, or folding shapes using different swelling behaviors between layers . Furthermore, diverse engineering methodologies, such as photolithography, 3D printing, direct contact mode molding, or micro- and nano-imprinting, can be utilized to create stimuli-responsive soft grippers for advanced functional biomimetic actuators, drug delivery capsules, tiny biopsy tools, or valves for lab-on-a-chip applications .
A broad discussion of stimuli-responsive materials and applications [35–40] and a variety of shape-changeable soft robotic systems have been reviewed for a more comprehensive analysis on the recent advances in soft robotics [31, 41–45]. This review primarily focuses on the recent progress of stimuli-responsive soft grippers in terms of material designs, fabrication methods, and applications. First, we summarize the stimuli-responsive materials, including N-isopropylacrylamide (NIPAM)-based hydrogels, liquid crystalline networks, and elastomers. Next, we present engineering techniques, such as photolithography and direct printing methods, to create soft gripping devices by organizing the stimuli-responsive materials. We then give an overview of the applications of the stimuli-responsive soft grippers in a number of fields, including soft machines, biological medicine, and surgical tools. Finally, we discuss the current open challenges and possible new fields of interest for these stimuli-responsive soft grippers.
2.1 N-isopropylacrylamide (NIPAM)-based stimuli-responsive hydrogels
Meanwhile, Gracias et al. proposed several shape transformable stimuli responsive grippers using pNIPAM-based hydrogels [9, 10, 13, 63, 64]. They developed a monolayer of pNIPAM-co-acrylic acid (AAc) gripper that could actuate by using a thin crosslinking gradient along the thickness in hinges (Fig. 1b) . However, the thin-monolayer soft gripper did not have enough force to grip an object because of its softness. Accordingly, they developed bilayer geometric grippers composed of stiff (16 MPa) segments polypropylene fumarate (PPF) and thermally responsive low shear modulus (162 kPa) pNIPAM-AAc (Fig. 1c) . They specifically validated that these bilayer grippers possess sufficient strength to excise a cell from tissue clumps. Furthermore, they demonstrated remotely guided soft grippers using magnetic fields by embedding iron oxide (Fe2O3) nanoparticles. Furthermore, they recently reported multistate shape-changing bilayer soft grippers composed of poly (oligo [ethylene glycol] methyl ether methacrylate) (POEGMA) (Fig. 1d) . The hand-shaped grippers were composed of POEGMA-based multi domains that exhibited different LCSTs and volume transition temperature such that these grippers reversibly underwent multistate folding and unfolding according to several heating and cooling cycles.
Besides the hybrid nanoparticles with pNIPAM hydrogels, Ko et al. have developed low voltage driven electro-thermally shape changeable soft grippers composed of the silver nanowire (Ag NW) deposited low-density polyethylene (LDPE) and thermochromic ink deposited polyvinylchloride (PVC) bilayer . Particularly, they proposed a variety of biomimetic color changeable flower- or tendril-shaped self-bending, -rolling, or -twisting soft actuators that demonstrated long-term stability of actuation under more than 10,000 cycles of heating and cooling conditions. Furthermore, their Ag NW percolation network heater could generate sufficient heat to obtain large deformation (curvature up to 2.5 cm−1 at 40 °C) at low voltages compared to other high voltage driven electroactive polymer actuators . These unique mechanical properties coupled with thermally-driven color shifting characteristics of soft electro-thermal actuators (ETA) have opened up another new possible field of intelligent camouflageable soft robots.
2.2 Liquid crystalline material-based stimuli-responsive hydrogels
In addition, 3D printing techniques have garnered significant attention for patterning 3D structures by a direct layer-by-layer process . 3D printing enables the printing of various materials ranging from plastics  to softer gels  and even cell-cultured soft materials . Four-dimensional (4D) printing has recently been highlighted as an innovative new technology [78, 93–96]. The combination of stimuli-responsive materials and the 4D printing technique has offered another new route to design 3D structures that change shapes over time with an appropriate stimulus. Gladman et al. demonstrated gripper morphologies generated by biomimetic 4D printing (Fig. 4c) . They introduced a 4D printing technique using a programmable stimuli-responsive hydrogel composite ink to control the alignment of cellulose fibrils that adjust the anisotropic stiffness and swelling strain. The 4D printed thermally responsive biomimetic flower-shaped grippers were actuated through a temperature change-driven swelling process. Wang et al. also proposed shape-programmable polyester (PE) polymer–paper bilayer composites manufactured by 3D printing (Fig. 4d) . They suggested a thermally responsive 3D-printed gripper that could pick-and-place a ball according to heating and cooling processes.
4.1 Soft actuators (tethered or untethered)
Another feasible method to control the actuation of hydrogel-based soft grippers is using tethered electrical assistance (Fig. 5b) . Palleau et al. described an ionoprinted tethered electrical stimulus-driven soft gripper that rapidly actuates through mechanical strain coupled with electrically directed binding and different shrinkage. An X-shaped soft gripper was organized using sodium polyacrylate gel, which could gently manipulate an object not only in liquid, but also in air. This gripper was suspended with a wire to extract a polydimethylsiloxane target in ethanol and release it in water reversibly. This type of soft gripper exhibits a programmable temporal and spatial shape deformation as a new class of soft actuators. Meanwhile, Yuk et al. developed an optically and sonically camouflageable hydraulic soft transparent gripper that could noninvasively hold and release a live goldfish (Fig. 5c) . They demonstrated its robustness and functionality by operating multiple actuation cycles. They also showed that hydraulic soft grippers could grip objects at a high speed of less than 1 s response time and a high force over 1 N compared to general osmotic hydrogel actuators. In addition, Peng et al. manufactured tethered programmable and complex shape-deformable tough hydrogel grippers by ion dip-dyeing and transfer printing (Fig. 5d) . By selectively printing appropriate patterns on one-dimensional gel strips, a 3D stimuli responsive soft gripping robot was actuated after swelling the ion-patterned domains in water. This swelling-deformable stimulus-responsive ion-printed hydrogels can be used as a tethered soft gripper that grips (folding) a target in water and releases (unfolding) it in ethanol reversibly.
Furthermore, Abdullah et al. proposed a stimuli-responsive shape morphing soft gripper composed of bilayer thin films with no hinges (Fig. 6c) . They demonstrated the fundamental relationship of the bifurcation mismatch strain between layers of hingeless soft grippers and how it generated mechanical shape transformation through thermoresponsive shape morphing. Their demonstration of a finite element model and experimental validation suggested the applicable diverse array of gripping systems. In addition, an untethered soft gripper is valid for miniaturized microscale soft gripping robots. Jia et al. demonstrated a wide range of microscopic object transportations using a soft gripper, called the universal soft robotic microgripper (Fig. 6d) . They proved the feasibility of the gripping functions by several combinations of geometrical interlocking between an untethered soft gripper and an object using shape memory and thermally responsive microgel structures. Furthermore, their untethered soft gripper was developed to conduct a series of pick up, transport, release, and recovery processes with no active visual or force feedback.
4.2 Biological medicine
Miniaturized and untethered stimuli-responsive soft grippers have been proposed as attractive less-invasive surgical biopsy tools (Fig. 7b and d) [9, 10, 21, 83]. Current medical tools and techniques have been developed toward minimally invasive approaches. However, most of the dominant developments of medical instruments and techniques still include various wired or tethered systems that limit access to hard-to-reach areas inside a body. Even untethered biomedical devices, such as diagnostic image scanners  or biopsy tools , are relatively large because of the systematic integration of several functional parts, including batteries, sensors, and cameras, which results in another geometric limitation for scale downsizing. To resolve these restrictions, stimuli-responsive soft micro robots can be utilized as a new class of less-invasive intelligent soft machines. In this respect, Malachowski et al. demonstrated self-folding thermo-magnetically responsive pNIPAM-based grippers that could grasp a clump of cells under flow, which displayed the possibility of grasping mucosal tissue in the gastrointestinal tract (Fig. 7b) . In addition, Breger et al. introduced a high-modulus (16 MPa) and stiff segmented polymer (i.e., PPF) to offer sufficient strength, such that a thermo-magnetically responsive untethered soft gripper could capture and excise cells from a live fibroblast clump (Fig. 7d) . These examples highlight the potential application of shape-changeable untethered small-scale soft robots for intelligent less-invasive surgical biopsies.
In summary, stimuli-responsive soft grippers have significantly attracted attention because of their promising applications in soft robotics, biomimetics, and biomedical engineering. Stimuli-responsive soft robots undergo programmable shape morphing in response to external environmental cues. Stimuli responsive materials, NIPAM-based hydrogels, liquid crystalline soft materials, and elastomers were preferentially considered to implement smart shape-deformable soft robots. Many innovative methodological strategies to construct gripper-shaped soft robots have specifically been developed over the past few decades. Photolithography or 3D printing is widely used in designing 3D shape-morphing soft grippers because of its scalability and manufacturability. A spatially controlled actuation response of approximately 10 × 30 μm in size of a stimuli-responsive NIPAM-based soft actuator was recently created using a two-photon absorption of focused light . The development of this innovative two-photon lithographic strategy suggests the possibilities of the emergence of nanoscale stimuli-responsive soft grippers in the near future.
In addition, one of the most challenging points for stimuli-responsive soft grippers is the improvement of the response speed with the sensitivity feedback. The actuation of stimuli-responsive soft robots generally remains at a level of slow reactant behavior. Abnormal or nonlinear effects, such as sudden buckling or snapping of small-scale soft robots, are rarely explored. Another important challenging point is to develop soft grippers with tunable physical properties to delicately grip objects without damaging the gripper or the object itself. Many mechanical and/or physical modeling recently provided insights for understanding the shape morphing and stiffness of stimuli-responsive soft robots [8, 9, 61, 102, 106]. The shape change prediction by mechanical and/or chemical computational modeling of soft robots can facilitate the manufacture of soft robots possessing the proper softness to avoid target damage.
The precise navigation and transportation of soft grippers into target areas as well as the function of soft grippers is another big challenge, particularly, at deep in vivo locations for clinical drug delivery or biopsy. Since the human body is opaque, vision-based feedback and tracking of miniaturized soft robots are impossible. In order to overcome this limitation, recently, the ultrasound image feedbacks coupled gradient magnetic fields have been shown to be suitable for accurate control of the directionality of untethered soft robots for automated pick-and-place . However, in order to realize autonomous in vivo navigation and transportation model of untethered soft robots, various unexplored magnetic resonance, ultrasound, or even near-infrared radiation guidance systems have to be developed in animal models at the near future. In conclusion, the innovative hybrid of material selections and synthesis, 3D manufacturing methods, and precisely controllable operation systems have to be developed in parallel to realize the multifunctionality, multi-responsible sensitivity, and highly sensitive feedback of new prospective soft gripping robots.
This research was supported by Sookmyung Women’s University Research Grants (1-1803-2009).
CY reviewed, analyzed and wrote the paper. The author read and approved the final manuscript.
The author declares no competing interests.
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