Tissue-Interfacing Actuating Adhesive Prevents Muscle Atrophy Through Mechanical Stimulation

<b><i><u>Introduction</u></i></b> Hydrogel tissue adhesives serve as an attractive tool kit for surgical interventions, as they can provide mechanical support and hemostatic capacity while sealing the wound site and preventing leakage. While most existing strategies have focused on the biochemical properties of tissue adhesives, much less effort has been devoted to engineering their mechanical functions. One promising function of adhesives is the ability to provide mechanical stimulation to the tissue to promote tissue healing and rehabilitation. It is known that externally imposed mechanical stimuli and physical cues can regulate various biological processes, including cell proliferation, migration, and differentiation, through a process termed mechanotransduction, which converts mechanical signals into changes in cell function. Thus, hydrogel adhesives capable of generating active mechanical stimulation, in addition to the adhesive function, are expected to further promote wound closure, tissue healing and rehabilitation. The objective of this study is to develop a hydrogel adhesive system with a soft actuation mechanism to provide mechanical stimulation directly and locally on the target tissue in a precisely controlled manner. We hypothesized that the active hydrogel adhesive could activate disuse muscles and prevent the development of muscle atrophy.<br/><br/><b><i><u>Materials & Methods</u></i></b> Active hydrogel adhesive was developed by combining a soft robotic actuator based on a shape memory alloy (SMA) spring and an elastomer that ensheathes the SMA, and a tough adhesive that efficiently transmits actuation to the underlying tissue. The actuation of SMA was achieved by applying a voltage (5V). The actuator and tough adhesive were assembled using benzophenone chemistry. The adhesive was then applied to tissue with chitosan and coupling reagents, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and sulfated N-hydroxy-succinimide (NHS). To induce disuse muscle atrophy, hindlimb immobilization was performed for 2 weeks in female C57BL6/J mice with 14-18 weeks of age. One hindlimb was maintained in a knee joint extension and ankle plantar flexion position, and fixed by wrapping the hindlimb with a non-elastic surgical bandage to the foot. Mechanical stimulation was performed every day for 5 minutes at ~20 % of tissue strain and 0.1 Hz.<br/><br/><b><i><u>Results & Discussion</u></i></b> The active hydrogel adhesive successfully achieved robust adhesion to muscle tissue (&gt;500 J/m²), maintained <i>in vivo </i>stability for 2 weeks, and was able to create tissue strain of up to 20%. Real-time high frequency ultrasound imaging revealed that the skeletal muscle underlying the adhesive was substantially deformed along the actuation axis during actuation, demonstrating mechanical stimulation <i>in vivo</i>. Next, the therapeutic effects on disuse muscle atrophy were evaluated. Mice treated with mechanical stimulation while undergoing disuse atrophy had muscle fibers with much larger cross-sectional area compared to untreated mice. The gross size and weight of muscles was also significantly greater in the stimulated mice. Muscle function analysis showed that the muscles of the treated mice generated substantially higher tetanic forces than those of untreated mice, with force levels similar to those of muscles in healthy, active mice. These results indicate that the active adhesive could prevent, or at least delay, the occurrence of disuse muscle atrophy due to immobilization.<br/><br/><b><i><u>Conclusions</u> </i></b>This work reports a hydrogel adhesive system with mechanically active elements, which not only forms robust tissue adhesion, but also generates stimulation on the target tissue for mechanotherapy. Mechanical stimulation was found to attenuate the onset of muscle atrophy, preventing the loss of muscle mass and maintaining muscle force generation. This work demonstrates the therapeutic potential of mechanical stimulation in the context of muscle atrophy and paves the way for the implementation of these stimulations in hydrogel adhesives.