This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

Soft Actuators for Small-Scale Robotics.

Novel poly(N-isopropylacrylamide)-clay (PNIPAM-clay) nanocomposite (NC) hydrogels with both excellent responsive bending and elastic properties are developed as temperature-controlled manipulators. The PNIPAM-clay NC structure provides the hydrogel with excellent mechanical property, and the thermoresponsive bending property of the PNIPAM-clay NC hydrogel is achieved by designing an asymmetrical distribution of nanoclays across the hydrogel thickness. The hydrogel is simply fabricated by a two-step photo polymerization. The thermoresponsive bending property of the PNIPAM-clay NC hydrogel is resulted from the unequal forces generated by the thermoinduced asynchronous shrinkage of hydrogel layers with different clay contents. The thermoresponsive bending direction and degree of the PNIPAM-clay NC hydrogel can be adjusted by controlling the thickness ratio of the hydrogel layers with different clay contents. The prepared PNIPAM-clay NC hydrogels exhibit rapid, reversible, and repeatable thermoresponsive bending/unbending characteristics upon heating and cooling. The proposed PNIPAM-clay NC hydrogels with excellent responsive bending property are demonstrated as temperature-controlled manipulators for various applications including encapsulation, capture, and transportation of targeted objects. They are highly attractive material candidates for stimuli-responsive “smart” soft robots in myriad fields such as manipulators, grippers, and cantilever sensors.

Poly(N-isopropylacrylamide)-Clay Nanocomposite Hydrogels with Responsive Bending Property as Temperature-Controlled Manipulators

In a manner reminiscent of macroscale bending and folding techniques such as origami, the out-of-plane assembly of lithographically micro- and nanopatterned thin films, can be used to fabricate three-dimensional (3D) micro- and nanostructured devices. These 3D devices, including microelectronic circuits, sensors, antennas, metamaterials, robotic, and biomimetic constructs, enable new functionalities and are challenging to fabricate by other methods. In this article, we summarize important features of this set of techniques and the devices assembled thereof, with a focus on functional constructs that have been formed by bending, folding, or buckling. At small size scales, manipulation using manual or even wired probes face daunting practical challenges in terms of cost, scalability, and high-throughput manufacturability; hence we emphasize techniques that manipulate strain in thin films so that they can spontaneously assemble into programmed 3D geometries without the need for any wires or probes.

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Origami MEMS and NEMS

Stimuli-responsive actuation behavior of materials is highly interesting for a variety of applications including controlled drug release, tissue engineering, encapsulation of cells, biomedicine, microfabrication, and micromanipulation. [ 1‐17 ] Particularly interesting kind of actuating materials are the polymers. [ 18 ] Among all examples of stimuli-responsive polymeric actuators, thermoresponsive ones deserve a particular attention [ 19 ] because temperature is the most suitable stimulus for working with living object-cells withstand changes of temperature between 4 and 37 °C. Thermally triggered actuating polymeric thin fi lms generally have a layered or patterned structure where one of the layers is thermoresponsive polymers showing temperature dependent reversible swelling and deswelling in water. Poly( N -isopropylacrylamide) (PNIPAM) is a well-known thermoresponsive polymer showing temperature dependent phase separation in water (lower critical solution temperature; LCST = 32 °C), which is widely used for design of thermoresponsive actuators. [ 15,20 ] The crosslinked PNIPAM swells and deswells in water below and above LCST, respectively. Deformation of thermoresponsive fi lms crosslinked in dry/non-swollen state takes place at lower temperatures (below LCST) and derolling happens above LCST when the polymer is dehydrated in a bilayer structure. The actuation time of such actuators depend upon dimensions and increases from several seconds for very thin objects to many minutes for centimeter thick ones. Actuators based on synthetic materials with a combination of fast response and large displacements are rare. In general, fast actuation requires either small size due to enhanced fl uid transport or elastic instabilities such as snap buckling and explosive fracture as shown by analysis on naturally occurring movements in plants and fungi. [ 21 ] The synthetic actuators with fast response make use of porous materials which increase the rate of diffusion of solvent and facilitates fast interaction with the material. Asymmetric polyaniline fi lms made by fl ash welding of corresponding nanofi ber mats show reversible actuation in ≈20 s in presence of camphorsulfonic acid and sodium hydroxide for a 2.5 cm long sample. The casting of aqueous dispersion of

Unusual and Superfast Temperature-Triggered Actuators

Transformer Hydrogels: A Review

We discovered a new and unexpected effect of reversible actuation of ultrathin semicrystalline polymer films. The principle was demonstrated on the example of thin polycaprolactone-gelatin bilayer films. These films are unfolded at room temperature, fold at temperature above polycaprolactone melting point, and unfold again at room temperature. The actuation is based on reversible switching of the structure of the hydrophobic polymer (polycaprolactone) upon melting and crystallization. We hypothesize that the origin of this unexpected behavior is the orientation of polycaprolactone chains parallel to the surface of the film, which is retained even after melting and crystallization of the polymer or the "crystallization memory effect". In this way, the crystallization generates a directed force, which causes bending of the film. We used this effect for the design of new generation of fully biodegradable thermoresponsive polymeric actuators, which are highly desirable for bionano-technological applications such as reversible encapsulation of cells and design of swimmers.

Reversible thermosensitive biodegradable polymeric actuators based on confined crystallization.

Self-folding films are a unique kind of thin film. They are able to deform in response to a change in environmental conditions or internal stress and form complex 3D structures. They are very promising candidates for the design of bioscaffolds, which resemble different kinds of biological tissues. In this paper, a very simple and cheap approach for the fabrication of fully biodegradable and biocompatible self-rolled tubes is reported. The tubes' folding can be triggered by temperature. A bilayer approach is used, where one component is active and another one is passive. The passive one can be any biocompatible, biodegradable, hydrophobic polymer. Gelatin is used as an active component: it allows the design of (i) self-folding polymer films, which fold at room temperature (22 °C) and irreversibly unfold at 37 °C, and (ii) films, which are unfolded at room temperature (22 °C), but irreversibly fold at 37 °C. The possibilities of encapsulation of neural stem cells are also demonstrated using self-folded tubes.

Biodegradable Self-Folding Polymer Films with Controlled Thermo-Triggered Folding

4D Biofabrication: 3D Cell Patterning Using Shape-Changing Films

and pores synthesis 6, 7, 8 Previous study of gas permeation through 4,4’-terphenylthiol (TPT) CNM showed only helium and water vapour permeation above the limit of detection 3 . The permeation of water vapour was nonlinear against its pressure and 1000 stronger than permeation of helium despite their similar kinetic diameters. This anomalous behavior was explained by cooperative movement and clustering of water molecules due to hydrogen bonding 9 . However, the exact mechanisms remained unknown. Here, we demonstrate that the character of permeation is defined by adsorption of gas species. We performed gas permeation measurements through TPT CNM different and found that all measured experienced an which correlated with their kinetic diameters. Furthermore, we identified that entropy loss during and is the of nonlinear of water. Our results also demonstrated that adsorption plays a major role in of all gases, not just

Vladislav Stroganov

Papers

Reversible thermosensitive biodegradable polymeric actuators based on confined crystallization.

Biodegradable Self-Folding Polymer Films with Controlled Thermo-Triggered Folding

4D Biofabrication: 3D Cell Patterning Using Shape-Changing Films

Gas transport mechanisms through molecularly thin membranes