Janus Soft Actuators with On-Off Switchable Behaviors for Controllable Manipulation Driven by Oil

Janus structure hydrogels: recent advances in synthetic strategies, biomedical microstructure and (bio)applications

Janus structure hydrogels (JSHs) are novel materials. Their primary fabrication methods and various applications have been widely reported. JSHs are primarily composed of Janus particles (JNPs) and polysaccharide components. They exhibit two distinct physical or chemical properties, generating intriguing characteristics due to their asymmetric structure. Normally, one side (adhesive interface) is predominantly constituted of polysaccharide components, primarily serving excellent adhesion. On the other side (functional surface), they integrate diverse functionalities, concurrently performing a plethora of synergistic functions. In the biomedical field, JSHs are widely applied in anti-adhesion, drug delivery, wound healing, and other areas. It also exhibits functions in seawater desalination and motion sensing. Thus, JSHs hold broad prospects for applications, and they possess significant research value in nanotechnology, environmental science, healthcare, and other fields. Additionally, this article proposes the challenges and future work facing these fields.

Biomimetic Janus photonic soft actuator with structural color self-reporting

Soft actuators with variable signal/color play an important role in the fields of targeted locomotion, artificial phototropism, drug screening, cargo transportation, and interactive sensing. The ability to achieve rapid response, large curvature, wide bending angle, and full-color display continues to be an unresolved challenge for artificial actuating materials. Inspired by the angle-dependent structural color of broad-tailed hummingbird and the Janus wettability of the lotus leaf, a Janus photonic soft actuator (JPSA) was fabricated by integrating an underwater super-oleophilic copper micro-nano array and oil-phobic inverse opal through a Laplace channel. The JPSA exhibits unidirectional permeability to underwater oil droplets. Attractively, with the combination of a swellable super-oleophilic surface and photonic crystals, JPSAs were endowed with oil-controlled reversible bending behavior with self-reporting angle-dependent color indication. We described for the first time the directional actuating mechanism induced by underwater oil unidirectional penetration and revealed the corresponding actuating kinetics and the inner-stress distribution/transfer by using structural color. As an extension of such theory, a rapid responsive JPSA with a wide bending angle and full-color self-reporting is further fabricated. This work provides an efficient strategy for oil directional transportation and separation in aqueous media and inspires the fabrication of a soft actuator/sensor with structural color self-reporting.

Smart Bionic Surfaces with Switchable Wettability and Applications

In order to satisfy the needs of different applications and more complex intelligent devices, smart control of surface wettability will be necessary and desirable, which gradually become a hot spot and focus in the field of interface wetting. Herein, we review interfacial wetting states related to switchable wettability on superwettable materials, including several classical wetting models and liquid adhesive behaviors based on the surface of natural creatures with special wettability. This review mainly focuses on the recent developments of the smart surfaces with switchable wettability and the corresponding regulatory mechanisms under external stimuli, which is mainly governed by the transformation of surface chemical composition and geometrical structures. Among that, various external stimuli such as physical stimulation (temperature, light, electric, magnetic, mechanical stress), chemical stimulation (pH, ion, solvent) and dual or multi-triggered stimulation have been sought out to realize the regulation of surface wettability. Moreover, we also summarize the applications of smart surfaces in different fields, such as oil/water separation, programmable transportation, anti-biofouling, detection and delivery, smart soft robotic etc. Furthermore, current limitations and future perspective in the development of smart wetting surfaces are also given. This review aims to offer deep insights into the recent developments and responsive mechanisms in smart biomimetic surfaces with switchable wettability under external various stimuli, so as to provide a guidance for the design of smart surfaces and expand the scope of both fundamental research and practical applications.

Graphene Oxide-Based Stimuli-Responsive Platforms for Biomedical Applications

Graphene is a two-dimensional sp2 hybridized carbon material that has attracted tremendous attention for its stimuli-responsive applications, owing to its high surface area and excellent electrical, optical, thermal, and mechanical properties. The physicochemical properties of graphene can be tuned by surface functionalization. The biomedical field pays special attention to stimuli-responsive materials due to their responsive abilities under different conditions. Stimuli-responsive materials exhibit great potential in changing their behavior upon exposure to external or internal factors, such as pH, light, electric field, magnetic field, and temperature. Graphene-based materials, particularly graphene oxide (GO), have been widely used in stimuli-responsive applications due to their superior biocompatibility compared to other forms of graphene. GO has been commonly utilized in tissue engineering, bioimaging, biosensing, cancer therapy, and drug delivery. GO-based stimuli-responsive platforms for wound healing applications have not yet been fully explored. This review describes the effects of different stimuli-responsive factors, such as pH, light, temperature, and magnetic and electric fields on GO-based materials and their applications. The wound healing applications of GO-based materials is extensively discussed with cancer therapy and drug delivery.

Precisely Controlling the Output Force of Hydrogel Actuator Based on Thermodynamic Nonequilibrium Temporary Deformation

Self-shaping hydrogel actuators have promising applications in various fields. However, one hydrogel actuator can generally access only one specifically predefined deformation and output force, which are determined by its thermodynamic equilibrium swelling state under external stimuli. Here, we present a simple yet versatile strategy for precisely programming the output force/energy of dual-gradient hydrogel actuators. The strategy is based on thermodynamic nonequilibrium snapping deformations occurring during the recovery process of predeformed dual-gradient hydrogel actuators in low-temperature water. The output force/energy of such thermodynamic nonequilibrium snapping deformation is highly associated with predeformation conditions of the hydrogel actuators, which increases with the increase of the predeformation temperature or time. In consequence, just by adjusting the predeformation conditions of the dual-gradient hydrogel actuators, their output force, energy, and power can be modulated precisely and continuously during the snapping deformation. The as-prepared hydrogel actuators can not only be used as smart lifters and grippers with ultrahigh accuracy of weight identification but also act as smart switches in the timing circuits with precisely adjustable operating time, paving the way for the design of a new generation of actuation materials.

A controllable oil-triggered actuator with aligned microchannel design for implementing precise deformation

Soft actuators with the integration of facile preparation, rapid actuation rate, sophisticated motions, and precise control over deformation for application in complex devices are still highly desirable. Inspired by the aligned structures of moisture responsive pineal scales, an oil-triggered Janus actuator composed of a smooth hydrophobic surface and a superhydrophobic surface with aligned microchannels by simple laser etching was fabricated successfully, which can deform into various desirable shapes and recover to the original shape when triggered by oil and ethanol molecules. The aligned microchannel design causes different oil spread distances in the longitudinal and transverse directions, resulting in orientation-controlled bending and twisting with large-scale displacement. By changing the orientations of the patterned microchannels, one-dimensional folding deformation, twisting, rolling curling and object-inspired architectures can be facilely programmed. The reversible oil-triggered actuator will inspire more attractive applications such as in vivo surgery, biomimetic devices, energy harvesting systems and soft robotics.

A Focus on Soft Actuation

The present editorial paper analyzes the hundred recent research works on soft actuation to understand the current main research focus in the light of the grand challenges in the field. Two characteristic paper types were obtained: one focuses on soft actuator design, manufacturing and demonstration, while another includes in addition the development of functional materials. Although vast majority of the works showcased soft actuation, evaluation of its robustness by multi-cyclic actuation was reported in less than 50% of the works, while only 10% described successful actuation for more than 1000 cycles. It is suggested that broadening the research focus to include investigation of mechanisms underlying the degradation of soft functional material performance in real cyclic actuation conditions, along with application of artificial intelligence methods for prediction of muscle behavior, may allow overcoming the reliability issues and developing robust soft-material actuators. The outcomes of the present work might be applicable to the entire soft robotics domain.

Gradient Assembly of Polymer Nanospheres and Graphene Oxide Sheets for Dual-Responsive Soft Actuators

Bimorph actuators hold great promise for developing soft robots. However, poor interlayer adhesion between different materials always threatens their stability for long-term usage. In this paper, instead of using a bilayer structure, we reported the gradient assembly of graphene oxide (GO) sheets and polymer nanospheres for developing robust moisture and light dual-responsive actuators. The distribution gradient of poly(methyl methacrylate) (PMMA) nanospheres along the normal direction of a GO paper leads to an asymmetric structure. The front side that mainly consists of GO is quite sensitive to water molecules, which swells upon exposure to moisture, whereas the back side that is rich in PMMA nanospheres expands obviously due to the photothermal effect. The distinct properties of the two sides endow the composite paper with moisture and light dual-responsiveness. Moreover, since GO has been used as a host material, the composite paper shows a moisture-triggered self-healing property, which permits front-to-front and front-to-back healing. The self-healed paper can maintain similar responsive property and reasonable mechanical strength to the pristine one. As a proof of concept, a dual-responsive gripper actuator and a scorpion robot have been fabricated for light and moisture cooperative manipulation. The gradient assembly strategy may open up a new way for developing robust multiresponsive actuators beyond bilayer structures.

Reversibly switching water droplets wettability on hierarchical structured Cu2S mesh for efficient oil/water separation

Surfaces with reversible wettability have broad applications but remain challenging since the switching process is usually energy intensive and complex. In this paper, a pyramid shaped Cu2S film with hierarchical micro/nanostructures is formed on a commercial copper mesh. This film is formed by a spontaneous redox sulfuration reaction and results in a roughened surface, which enables reversible wetting transition between superhydrophilicity to superhydrophobicity. This switching occurs by simple processes such as alternately storing in air or using an ethanol solution treatment and yields cyclic wettability switching for many cycles. This convenient wetting transition behavior, as well as strong stability and efficient oil/water separation with efficiency exceeding 98%, renders it as a potentially useful mesh material for switchable surfaces.