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Cell and tissue stiffness is an important biomechanical signalling parameter for dynamic biological processes; responsive polymeric materials conferring responsive functionality are therefore appealing for in vivo implants. We have developed thermoresponsive poly(urea-urethane) nanohybrid scaffolds with 'stiffness memory' through a versatile 3D printing-guided thermally induced phase separation (3D-TIPS) technique. 3D-TIPS, a combination of 3D printing with phase separation, allows uniform phase-separation and phase transition of the polymer solution at a large interface of network within the printed sacrificial preform, leading to the creation of full-scale scaffolds with bespoke anatomical complex geometry. A wide range of hyperelastic mechanical properties of the soft elastomer scaffolds with interconnected pores at multi-scale, controlled porosity and crystallinity have been manufactured, not previously achievable via direct printing techniques or phase-separation alone. Semi-crystalline polymeric reverse self-assembly to a ground-stated quasi-random nanophase structure, throughout a hierarchical structure of internal pores, contributes to gradual stiffness relaxation during in vitro cell culture with minimal changes to shape. This 'stiffness memory' provides initial mechanical support to surrounding tissues before gradually softening to a better mechanical match, raising hopes for personalized and biologically responsive soft tissue implants which promote human fibroblast cells growth as model and potential scaffold tissue integration. STATEMENT OF SIGNIFICANCE: Biological processes are dynamic in nature, however current medical implants are often stronger and stiffer than the surrounding tissue, with little adaptability in response to biological and physical stimuli. This work has contributed to the development of a range of thermoresponsive nanohybrid elastomer scaffolds, with tuneable stiffness and hierarchically interconnected porous structure, manufactured by a versatile indirect 3D printing technique. For the first time, stiffness memory of the scaffold was observed to be driven by phase transition and a reverse self-assembly from a semicrystalline phase to a quasi-random nanostructured rubber phase. Early insight into cell response during the stiffness relaxation of the scaffolds in vitro holds promise for personalized biologically responsive soft implants.

This work reports cellular responses to a family of 3D-TIPS thermoresponsive nanohybrid elastomer scaffolds with different stiffness softening both in vitro and in vivo rat models. The results, for the first time, have revealed the effects of initial stiffness and dynamic stiffness softening of the scaffolds on tissue integration, vascularisation and inflammo-responses, without coupling chemical crosslinking processes. The 3D printed, hierarchically interconnected porous structures guide the growth of myofibroblasts, collagen fibres and blood vessels in real 3D scales. In vivo study on those unique smart elastomer scaffolds will help pave the way for personalized and biologically responsive soft tissue implants and implantable devices with better mechanical matches, angiogenesis and tissue integration.

Kirigami technique, a method to reconfigure structures via mechanical approaches, has received much attention in material science, due to its versatile and unconventional structural transformations. The counterparts in the electromagnetic metamaterial field has recently allowed for the tunable control of electromagnetic responses. However, they are limited to global tuning of absorption, chirality, etc., leaving much potential of controlling spatially varying distribution and therefore the optical wavefront unexploited. Here, the authors propose a class of kirigami‐based reconfigurable gradient metasurfaces through which the electromagnetic wavefront can be tuned over continuous‐state ranges by changing the meta‐structures from folded (compact) to unfolded (large surface) configurations. As the proof‐of‐concept, meta‐devices including switchable anomalous refractor and reconfigurable metalens are demonstrated both in simulations and experiments. Moreover, a new paradigm to mitigate chromatic dispersion is also realized by the kirigami‐based reconfigurable metalens, which is able to keep the focal length unchanged over a continuous frequency band by setting metalens with various folding states. Their approach provides a new alternative for designing reconfigurable gradient metasurface with additional mechanical properties and may have potential applications in advanced devices such as reconfigurable optical components and imaging system.

Reconfigurable Gradient Metasurfaces Kirigami, the ancient art of paper cutting, has recently been widely studied in modern science and technology. In article number 2107699, Ke Chen, Yijun Feng, and co‐workers use 3D kirigami configurations to achieve reconfigurable gradient metasurfaces. This concept combines continuous functional variation with structural self‐folding and develops a series of electromagnetic meta‐devices including a reconfigurable anomalous refractor, tunable focusing metalens, and achromatic metalens.

Despite the attention given to the development of novel responsive implants for regenerative medicine applications, the lack of integration with the surrounding tissues and the mismatch with the dynamic mechanobiological nature of native soft tissues remain in the current products. Hierarchical porous membranes based on a poly (urea–urethane) (PUU) nanohybrid have been fabricated by thermally induced phase separation (TIPS) of the polymer solution at different temperatures. Thermoresponsive stiffness softening of the membranes through phase transition from the semicrystalline phase to rubber phase and reverse self‐assembly of the quasi‐random nanophase structure is characterized at body temperature near the melting point of the crystalline domains of soft segments. The effects of the porous structure and stiffness softening on proliferation and differentiation of human bone‐marrow mesenchymal stem cells (hBM‐MSCs) are investigated. The results of immunohistochemistry, histological, ELISA, and qPCR demonstrate that hBM‐MSCs maintain their lineage commitment during stiffness relaxation; chondrogenic differentiation is favored on the soft and porous scaffold, while osteogenic differentiation is more prominent on the initial stiff one. Stiffness relaxation stimulates more osteogenic activity than chondrogenesis, the latter being more influenced by the synergetic coupling effect of softness and porosity.

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