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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.

Osteocytes are capable of resorbing and replacing bone local to the lacunar-canalicular system (LCS remodeling). However, the impacts of these processes on perilacunar bone quality are not understood. It is well established that aging is associated with reduced whole-bone fracture resistance, reduced osteocyte viability, and truncated LCS geometries, but it remains unclear if aging changes perilacunar bone quality. In this study, we employed atomic force microscopy (AFM) to quantify sub-micrometer gradations from 2D maps surrounding osteocyte lacunae in young (5 mo) and aged (22 mo) female mice. AFM-mapped lacunae were also imaged with confocal laser scanning microscopy to determine which osteocytes had recently deposited bone as determined by the presence of fluorochrome labels. These assays allowed us to quantify gradations in nanoscale mechanical properties of bone-forming/non-bone-forming osteocytes in young and aged mice. This study reports for the first time that there are sub-micrometer gradations in modulus surrounding lacunae and that these gradations are dependent upon recent osteocyte bone formation. Perilacunar bone adjacent to bone-forming osteocytes demonstrated lower peak and bulk modulus values when compared to bone near non-bone-forming osteocytes from the same mouse. Bone-forming osteocytes also showed increased perilacunar modulus variability. Age reduced lacunar size but did not significant effect modulus gradation or variability. In general, lacunar morphology was not a strong predictor of modulus gradation patterns. These findings support the idea that lacunar-canalicular remodeling activity changes the material properties of surrounding bone tissue on a sub-micrometer scale. Therefore, conditions that affect osteocyte health have the potential to impact bone quality.