Heating-triggered shape actuation is vital for biomedical applications. The likely overheating and subsequent damage of surrounding tissue, however, severely limit its utilization in vivo. Herein, cooling-triggered shapeshifting is achieved by designing dual-network hydrogels that integrate a permanent network for elastic energy storage and a reversible network of hydrophobic crosslinks for "freezing" temporary shapes when heated. Upon cooling to 10 °C, the hydrophobic interactions weaken and allow recovery of the original shape, and thus programmable shape alterations. Further, multiple temporary shapes can be encoded independently at either different temperatures or different times during the isothermal network formation. The ability of these hydrogels to shapeshift at benign conditions may revolutionize biomedical implants and soft robotics.

Various types of clinical complications may emerge when a medical device (catheter, microelectrode, or microneedle patch) is deployed to the human body. It has been widely recognized that mechanical mismatch between an implant and a host tissue is one of the leading factors for adverse effects such as irritation and inflammation, which is evident in orthopedic, [1] neural, [2,3] and reconstructive implants. [4] It is generally desired that an implant material precisely matches Young's modulus of the surrounding tissue that may range between E ≈ 10 2 Pa of supersoft brain and adipose tissues to E ≈ 10 5 Pa of muscle and skin. However, forceful implantation of biomedical devices such as piercing, insertion or twisting impose significant mechanical stresses on the device. [5-8] Therefore a certain rigidity is desired to prevent device failure, minimize tissue damage, and simplify storage and handling procedures. [9,10] This presents an oxymoronic challenge for an ideal implant: designing an adaptive material that readily penetrates tissue, but transitions into a tissue-soft implant upon insertion into the body (Figure 1A). Thermoplastics are one of the most commonly used materials possessing a well-defined transition between hard and soft states with Young's moduli of E hard ≈ 1 GPa and E soft ≈ 1 MPa, respectively (Figure 1B, Table S1, Supporting Information). However, conventional thermoplastic materials have two problems: i) E soft ≈ 1 MPa is significantly higher than soft tissue modulus ranging within 10 2-10 5 Pa and ii) softening (melting or glass) transition temperature of high molecular weight polymers cannot be tuned without altering their chemical composition. It is even more challenging to adjust the mechanical and thermal properties independently of each other. The softness issue alone could be resolved by using different types of bioinspired polymeric gels. [14-17] However, not only it is difficult to prepare gels with a sharp GPa-to-kPa transition (Figure 1C, Table S2, Supporting Information), their solvent fraction instigates uncontrolled leaching inside the body, [13] which causes various health risks. The second problem can be addressed via copolymerization of monomers with different melting temperatures. [18-20] However, this involves chemical composition and biocompatibility variation and yet, copolymer modulus is limited by chain entanglements to >10 5 Pa. [21,22] The ability of living species to transition between rigid and flexible shapes represents one of their survival mechanisms, which has been adopted by various human technologies. Such transition is especially desired in medical devices as rigidity facilitates the implantation process, while flexibility and softness favor biocompatibility with surrounding tissue. Traditional thermoplastics cannot match soft tissue mechanics, while gels leach into the body and alter their properties over time. Here, a single-component system with an unprecedented drop of Young's modulus by up to six orders of magnitude from the GPa to kPa level at a control...

The inability to re-process thermosets hinders their utility and sustainability. An ideal material should combine closed-loop recycling and upcycling capabilities. This trait is realized in polydimethylsiloxane bottlebrush networks using thermoreversible Diels-Alder cycloadditions to enable both reversible disassembly into a polymer melt and on-demand reconfiguration to an elastomer of either lower or higher stiffness. The crosslink density was tuned by loading the functionalized networks with a controlled fraction of dormant crosslinkers and crosslinker scavengers, such as furan-capped bis-maleimide and anthracene, respectively. The resulting modulus variations precisely followed the stoichiometry of activated furan and maleimide moieties, demonstrating the lack of side reactions during reprocessing. The presented circularity concept is independent from the backbone or side chain chemistry, making it potentially applicable to a wide range of brush-like polymers.

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