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Implantable bioelectronics, integrated directly within the body, represent a potent biomedical solution for monitoring and treating a range of medical conditions, including chronic diseases, neural disorders, and cardiac conditions, through personalized medical interventions. Nevertheless, contemporary implantable bioelectronics rely heavily on rigid materials (e.g., inorganic materials and metals), leading to inflammatory responses and tissue damage due to a mechanical mismatch with biological tissues. Recently, soft electronics with mechanical properties comparable to those of biological tissues have been introduced to alleviate fatal immune responses and improve tissue conformity. Despite their myriad advantages, substantial challenges persist in surgical handling and precise positioning due to their high compliance. To surmount these obstacles, softening implantable bioelectronics has garnered significant attention as it embraces the benefits of both rigid and soft bioelectronics. These devices are rigid for easy standalone implantation, transitioning to a soft state in vivo in response to environmental stimuli, which effectively overcomes functional/biological problems inherent in the static mechanical properties of conventional implants. This article reviews recent research and development in softening materials and designs for implantable bioelectronics. Examples featuring tissue-penetrating and conformal softening devices highlight the promising potential of these approaches in biomedical applications. A concluding section delves into current challenges and outlines future directions for softening implantable device technologies, underscoring their pivotal role in propelling the evolution of next-generation bioelectronics.

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Rotational nanogenerators with flexible triboelectric layers have wide applications and high reliability. However, flexible materials cause a severe reduction in contact force and thus triboelectric output power. Unlike previous works devising complex auxiliary structures to solve this issue, this paper focuses on improving the contact material mechanics and proposes a stiffness modulation method. By introducing fine patterns to the contacting rotor-stator pairs, the effective elastic modulus was regulated from approximately 103 to 105 MPa, and the output voltage was modulated from approximately 24.39% to 375.87% compared to the non-patterned rotor-stator pairs, corresponding to a maximal a 14 times increase in output power. A maximal power density of 18.75 W/m2 was achieved on 10 MΩ resistance at 9.6 Hz, which is even beyond the power density of most rigid triboelectric interfaces. Moreover, high reliability could be maintained when the volume ratio of the horizontal patterns exceeded a threshold value of 33.5% as the stator and 63.6% as the rotor for a 0.5 mm linewidth. These results prove the efficacy of the stiffness modulation method for jointly achieving high output power and high reliability in flexible rotational triboelectric nanogenerators.

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The implementation of complex, high-precision optical devices or systems, which have vital applications in the aerospace, medical, and military fields, requires the ability to reliably manipulate and assemble optical elements. However, this is a challenging task as these optical elements require contamination-free and damage-free manipulation and come in a variety of sizes and shapes. Here, a smart, contact-adaptive adhesive based on magnetic actuation is developed to address this challenge. Specifically, the surface bio-inspired adhesives made of fluororubber facilitate contamination-free and damage-free adhesion. The stiffness modulation of packaged magnetorheological grease based on the magnetorheological effect endows the smart adhesive with a high conformability to the optical elements in the soft state, a high grip force in the stiff state, and the ability to quickly release the optical elements in the recovered soft state. The smart adhesive provides a versatile solution for reliably and quickly manipulating and assembling multiscale optical elements with planar or complex 3D shapes without causing surface contamination or damage. These extraordinary capabilities are demonstrated by the manipulation and assembly of various optical elements, such as convex/concave/ball lenses and extremely complex-shaped light guide plates. The proposed smart adhesive is a promising candidate for conventional optical element manipulation technologies.

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Within the field of robotics, stiffness tuning technologies have potential for a variety of applications-perhaps most notably for robotic grasping. Many stiffness tuning grippers have been developed that can grasp fragile or irregularly shaped objects without causing damage and while still accommodating large loads. In addition to limiting gripper deformation when lifting an object, increasing gripper stiffness after contact formation improves load sharing at the interface and enhances adhesion. In this study, we present a novel stiffness and adhesion tuning gripper, enabled by the thermally induced phase change of a thermoplastic composite material embedded within a silicone contact pad. The gripper operates by bringing the pad into contact with an object while in its heated, soft state, and then allowing the pad to cool and stiffen to form a strong adhesive bond before lifting the object. Pull-off tests conducted using the gripper show that transitioning from a soft to stiff state during grasping enables up to 6 × increase in adhesion strength. Additionally, a finite element model is developed to simulate the behavior of the gripper. Finally, pick-and-place demonstrations are performed, which highlight the gripper's ability to delicately grasp objects of various shapes, sizes, and weights.

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Currently soft robots primarily rely on pneumatics and geometrical asymmetry to achieve locomotion, which limits their working range, versatility, and other untethered functionalities. In this paper, we introduce a novel approach to achieve locomotion for soft robots through dynamically tunable friction to address these challenges, which is achieved by subsurface stiffness modulation (SSM) of a stimuli-responsive component within composite structures. To demonstrate this, we design and fabricate an elastomeric pad made of polydimethylsiloxane (PDMS), which is embedded with a spiral channel filled with a low melting point alloy (LMPA). Once the LMPA strip is melted upon Joule heating, the compliance of the composite structure increases and the friction between the composite surface and the opposing surface increases. A series of experiments and finite element analysis (FEA) have been performed to characterize the frictional behavior of these composite pads and elucidate the underlying physics dominating the tunable friction. We also demonstrate that when these composite structures are properly integrated into soft crawling robots inspired by inchworms and earthworms, the differences in friction of the two ends of these robots through SSM can potentially be used to generate translational locomotion for untethered crawling robots.

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Gecko-inspired dry adhesion has attracted much attention for many applications such as soft grippers and wall-climbing robots, which, however, demonstrate stable adhesion on flat surfaces and small adhesion on nonflat surfaces. In practice, geckos' capability of walking upside down on both flat and nonflat surfaces comes from the combined action of adhesive structures for passive adhesion and toe muscles for stiffness modulation. Inspired by this behavior, this study proposes a hierarchal adhesive structure for high and switchable adhesion on nonflat surfaces. The three-layer adhesive consists of a mushroom-shaped structure top layer, stiffness modulation thermoplastic polyurethane (middle layer), and an electrothermal film (bottom layer) that mimics the epidermal adhesive structures, toe muscles, and electromyographic signals, respectively. Through the tunable structural stiffness controlled by adjusting the voltage, the adhesive force can be increased by 1 or 2 orders of magnitude compared to the conventional adhesive structures and further used for attachment and detachment functions. The gecko-inspired soft gripper is successfully tested as a pick-up and drop-down system for transporting a surface with different features, which has great application potential in industrial lines and daily life.

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Polyacrylamide (PAA) hydrogels are now widely used in mechanobiology because the well-defined available protocols allow a robust and reproducible control of substrate stiffness within a physiological range. However, several assays require hydrogels inside traditional plastic substrates and the current methods remain relatively tedious. Here, we present a simple and direct fabrication technique that successfully attaches PAA hydrogels inside polystyrene multiwell plates and Petri dishes of different sizes. It permits a control of the Young's modulus of the gels, within the desired range for mechanobiology. Some critical steps, that had to be overcome to guarantee protein conjugation and cell attachment, are detailed, as they differ from the standardized preparation on glass substrates. To validate our process, we demonstrated that HepG2 and 3T3L1 cell lines as well as primary hepatocytes seeded on PAA gels of different stiffnesses in plastics showed a mechanical response identical to the cells cultured on traditional gels.

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Aluminum nitride offers unique material advantages for the realization of ultrahigh-frequency acoustic devices attributed to its high ratio of stiffness to density, compatibility with harsh environments, and superior thermal properties. Although, to date, aluminum nitride thin films have been widely investigated regarding their electrical and mechanical characteristics under alternating small signal excitation, their ultrathin nature under large bias may also provide novel and useful properties. Here, we present a comprehensive investigation of electric field stiffening effect in c-oriented aluminum nitride piezoelectric thin films. By analyzing resonance characteristics in a 2.5 GHz aluminum nitride-based film bulk acoustic resonator, we demonstrate an up to 10% linear variation in the equivalent stiffness of aluminum nitride piezoelectric thin films when an electric field was applied from -150 to 150 MV/m along the c-axis. Moreover, for the first time, an atomic interaction mechanism is proposed to reveal the nature of electric field stiffening effect, suggesting that the nonlinear variation of the interatomic force induced by electric field modulation is the intrinsic reason for this phenomenon in aluminum nitride piezoelectric thin films. Our work provides vital experimental data and effective theoretical foundation for electric field stiffening effect in aluminum nitride piezoelectric thin films, indicating the huge potential in tunable ultrahigh-frequency microwave devices.

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Soft pneumatic actuators and motor-based mechanisms being concomitant with the cumbersome appendages have many challenges to making the independent robotic system with compact and lightweight configuration. Meanwhile, shape memory actuators have shown a promising alternative solution in many engineering applications ranging from artificial muscle to aerospace industry. However, one of the main limitations of such systems is their inherent softness resulting in a small actuation force, which prevents them from more effective applications. This issue can be solved by combining shape memory actuators and the mechanism of stiffness modulation. As a first, this study describes a shape memory alloy-based soft gripper composed of three identical fingers with variable stiffness for adaptive grasping in low stiffness state and effective holding in high stiffness state. Each finger with two hinges is fabricated through integrating soft composite actuator with stiffness changeable material where each hinge can approximately achieve a 55-fold changeable stiffness independently. Besides, each finger with two hinges can actively achieve multiple postures by both selectively changing the stiffness of hinges and actuating the relevant SMA wire. Based on these principles, the gripper is applicable for grasping objects with deformable shapes and varying shapes with a large range of weight where its maximum grasping force is increased to ∼10 times through integrating with the stiffness changeable mechanism. The final demonstration shows that the finger with desired shape-retained configurations enables the gripper to successfully pick up a frustum-shaped object.

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When designing micro-scale tactile probes, a design trade-off must be made between the stiffness and flexibility of the probing element. The probe must be flexible enough to ensure sensitive parts are not damaged during contact, but it must be stiff enough to overcome attractive surface forces, ensure it is not excessively fragile, easily damaged or sensitive to inertial loads. To address the need for a probing element that is both flexible and stiff, a novel micro-scale tactile probe has been designed and tested that makes use of an active suspension structure. The suspension structure is used to modulate the probe stiffness as required to ensure optimal stiffness conditions for each phase of the measurement process. In this paper, a novel control system is presented that monitors and controls stiffness, allowing two probe stiffness values ("stiff" and "flexible") to be defined and switched between. During switching, the stylus tip undergoes a displacement of approximately 18 µm, however, the control system is able ensure a consistent flexible mode tip deflection to within 12 nm in the vertical axis. The overall uncertainty for three-dimensional displacement measurements using the probing system is estimated to be 58 nm, which demonstrates the potential of this innovative variable stiffness micro-scale probe system.