Natural organic structures form via a growth mode in which nutrients are absorbed, transported, and integrated. In contrast, synthetic architectures are constructed through fundamentally different methods, such as assembling, molding, cutting, and printing. Here, we report a photoinduced strategy for regulating the localized growth of microstructures from the surface of a swollen dynamic substrate, by coupling photolysis, photopolymerization, and transesterification together. Photolysis is used to generate dissociable ionic groups to enhance the swelling ability that drives nutrient solutions containing polymerizable components into the irradiated region, photopolymerization converts polymerizable components into polymers, and transesterification incorporates newly formed polymers into the original network structure. Such light-regulated growth is spatially controllable and dose-dependent and allows fine modulation of the size, composition, and mechanical properties of the grown structures. We also demonstrate the application of this process in the preparation of microstructures on a surface and the restoration of large-scale surface damage.

This review discusses the currently available 3D printing approaches, design concepts, and materials that are used to obtain programmable hydrogel actuators. These polymer materials can undergo complex, predetermined types of motion and thereby imitate adaptive natural actuators with anisotropic, hierarchical substructures. 3D printing techniques allow replicating these complex shapes with immense design flexibility. While 3D printing of thermoplastic polymers has become a mainstream technique in rapid prototyping, additive manufacturing of softer polymers including polymer hydrogels is still challenging. To avoid deliquescence of printed hydrogel structures, the polymer inks used for hydrogel manufacture need to be sheer-thinning and thixotropic, with fast recovery rates of the high viscosity state. This is achieved by adding polymer or particle-based viscosity modifiers. Further stabilization of the interfaces of the printed voxels, e.g., by UV cross-linking, is often also required to obtain materials with useful mechanical properties. Here state-of-the-art techniques used to 3D print stimulus responsive, programmable polymer hydrogels, and hydrogel actuators, as well as ink formulation and post-printing strategies used to obtain materials with structural integrity are reviewed.

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Physical entanglement of polymer chains is an interaction that is believed to be too weak to build polymer networks for hydrogelation.

Synergy between biomaterial surfaces and cells is known to be important due to the direct and inevitable interactions that mediate cell behavior. Thus, the design of biomimetic surfaces with proper topography and chemistry is crucial for optimization of cellular responses. Herein, we report surface topography mimicking ability of chitosan (CH) biopolymer and its promising application as a platform for osteoblast cell culture. CH is frequently used in bone tissue engineering applications. For this reason, anisotropic bone surface was chosen to demonstrate its surface mimicking skill. Initially, bone surface topography is replicated by using soft lithography and polydimethylsiloxane (PDMS) molds. Subsequently, solvent casting by CH is performed on the replicated molds, and then polymer membranes with bone surface topography are obtained. To prepare nanocomposite, graphene oxide (GO) is blended into CH membranes to enhance biocompatibility. It is observed that CH and CH/GO nanocomposite membranes are both eligible to mimic anisotropic bone surface. Considering the surface of bone tissue, hydroxyapatite (HA) modification is also conducted using ultraviolet/ozone method. Following that, human osteoblasts are chosen to evaluate the cell responses on mimicked surfaces. The results indicate that surface mimicking has a positive impact on osteoblast viability and morphology.