Fabrication and characterization of self-folding thermoplastic sheets using unbalanced thermal shrinkage

Progress of shrink polymer micro- and nanomanufacturing

Traditional lithography plays a significant role in the fabrication of micro- and nanostructures. Nevertheless, the fabrication process still suffers from the limitations of manufacturing devices with a high aspect ratio or three-dimensional structure. Recent findings have revealed that shrink polymers attain a certain potential in micro- and nanostructure manufacturing. This technique, denoted as heat-induced shrink lithography, exhibits inherent merits, including an improved fabrication resolution by shrinking, controllable shrinkage behavior, and surface wrinkles, and an efficient fabrication process. These merits unfold new avenues, compensating for the shortcomings of traditional technologies. Manufacturing using shrink polymers is investigated in regard to its mechanism and applications. This review classifies typical applications of shrink polymers in micro- and nanostructures into the size-contraction feature and surface wrinkles. Additionally, corresponding shrinkage mechanisms and models for shrinkage, and wrinkle parameter control are examined. Regarding the size-contraction feature, this paper summarizes the progress on high-aspect-ratio devices, microchannels, self-folding structures, optical antenna arrays, and nanowires. Regarding surface wrinkles, this paper evaluates the development of wearable sensors, electrochemical sensors, energy-conversion technology, cell-alignment structures, and antibacterial surfaces. Finally, the limitations and prospects of shrink lithography are analyzed.

Untethered microgripper-the dexterous hand at microscale

Untethered microgrippers that can navigate in hard-to-reach and unpredictable environments are significantly important for biomedical applications such as targeted drug delivery, micromanipulation, minimally invasive surgery and in vivo biopsy. Compared with the traditional tethered microgrippers, the wireless microgrippers, due to the exceptional characteristics such as miniaturized size, untethered actuation, dexterous and autonomous motion, are projected to be promising microtools in various future applications. In this review, we categorize the untethered microgrippers into five major classes, i.e. microgrippers responsive to thermal, microgrippers actuated by magnetic fields, microgrippers responsive to chemicals, light-driven microgrippers and hybrid actuated microgrippers. Firstly, the actuation mechanisms of these microgrippers are introduced. The challenges faced by these microgrippers are also covered in this part. With that, the fabrication methods of these microgrippers are summarized. Subsequently, the applications of microgrippers are presented. Additionally, we conduct a comparison among different actuation mechanisms to explore the advantages and potential challenges of various types of microgrippers. In the end of this review, conclusions and outlook of the development and potential applications of the microgrippers are discussed.

Controlled shape deformation of bilayer films with tough adhesion between nanocomposite hydrogels and polymer substrates

Shape-shifting materials have received increasing attention owing to their promising applications in soft robotics, biomedical devices, actuators, morphing aircraft and so on. However, their practical applications are limited due to their weak mechanical strength, low interfacial adhesion and complex preparation method. In this paper, bilayer films were synthesized by in situ one-step forming soft and water-swellable nanocomposite hydrogels on the surface of the rigid and nonresponsive poly(ethylene terephthalate) (PET) film without any surface modification. The strong interfacial toughness between the hydrogel layer and the PET layer, the high swelling ability of the soft hydrogel layer, and the high strength of the rigid PET film endowed the bilayer film with excellent self-bending behaviour. The shape deformation of the bilayer films can be controlled by adjusting the geometry parameters of the bilayer film, such as the hydrogel thickness, the aspect ratio and the width of the bilayer film. Moreover, the bilayer film exhibited excellent reversible bidirectional self-bending behaviour. In addition, the mechanisms for driving the shape transformation were discussed. We believe this work will provide a promising and simple strategy to develop novel responsive materials with controlled shape deformation.

Solvent triggered irreversible shape morphism of biopolymer films

We report the controlled reversible and irreversible folding behavior of a biopolymer film simply by tuning the solvent characteristics. Generally, solvent triggered folding of soft membranes or film is achieved by unfolding. Here, we show that this unfolding behavior can be suppressed/delayed or even completely eliminated by altering the intrinsic nature of the solvent. A reversible folding of biopolymer film is observed in response to water, whereas, an irreversible folding is observed in the presence of an aromatic alcohol (AA) solution of different molar concentrations. The folding and unfolding behavior originates from the coupled deformation-diffusion phenomena. Our study indicates that the presence of an AA influences the relaxation behavior of polymer chains, which in turn affects the release of stored strain energy during folding. Controlling the reversibility as well as the actuation time of the biopolymer film by tuning the solvent is explained in detail at the bulk scale by applying appropriate experimental techniques. The underlying mechanism for the observed phenomena is complemented by performing a simulation study for a single polymer chain at the molecular length scale. Due to the solvent-triggered hygromorphic response, biopolymer films exhibit huge potential as sensors, soft robots, drug delivery agents, morphing medical devices and in biomedical applications. We provide experimental evidence for the weight lifting capacity of permanently folded membranes, amounting to ∼200 times their own weight.