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A major challenge in 3D extrusion bioprinting is the limited number of bioink that fulfills the opposing requirements for printability with requisite rheological properties and for functionality with desirable microenvironment. Here, this limitation is addressed by developing a generalizable strategy for formulating a cell-laden microgel-based biphasic (MB) bioink. The MB bioink comprises two components, that is, microgels in closepacked condition providing excellent rheological properties for extrusion bioprinting, and a hydrogel precursor that forms a second polymer network to integrate the microgels together, providing post-printing structural stability. This strategy enables the effective printing of a range of hydrogels into complex structures with high shape fidelity. The MB bioink offers great mechanical tunability without compromising printability, and hyperelasticity with superb cyclic compression and stretch endurance. Moreover, the microgels and hydrogel precursor of the MB bioink can encapsulate different types of cells, together creating a heterogeneous cellular microenvironment at microscale. It is successfully demonstrated that hepatocytes and endothelial cells with spatial cell patterning by using MB bioink induce the cellular reorganization and vascularization, leading to enhanced hepatic functions. The proposed MB bioink expands the palette of available bioinks and opens numerous opportunities for the biomedical applications such as tissue engineering and soft robotics.

4D printing has attracted tremendous interest because of its potential applications in smart devices, biomedical and tissue engineering. However, conventional shape memory polymers suffer from the single permanent shape and recovery direction, the flexibility of 4D printing is significantly limited. Besides, the cross‐linked networks of photocuring 3D‐printed objects cannot be reprocessed or repaired. To address these issues, the dynamic thiocarbamate bonds are introduced into the photocurable methacrylate to prepare reprocessable and self‐healable 4D printing polythiourethane (4DP‐PTU) with Young's modulus of 1.2 GPa and tensile strength of 61.9 MPa. The printed objects can be easily repaired by reprinting on the damaged surface. The shape memorized 4DP‐PTU features high shape fixity and shape recovery, and reconfigurable permanent shape brought by the solid‐state plasticity. A dual‐mode triggered alarm is obtained by the incorporation of carbon nanotubes to demonstrate the potential application in smart alarms for warning of laser exposure or fire case. Moreover, the surface wettability and cell adhesion performance of 4DP‐PTU with excellent biocompatibility can be facilely adjusted through the exchange reaction with sulfhydryl compounds. Accordingly, 4DP‐PTU may show vast potential applications in the field of robotics, smart alarm, bio‐implants and in solving the environmental challenges of 3D‐printed products.

However, it is often challenging to print large free-form tissue structures owing to the inadequate structural integrity and mechanical stability of softhydrogel-based bioink. [4] For the past decade, considerable effort has been made to address this particular challenge. For example, Kang and co-workers proposed a hydrogel reinforcing strategy by coprinting cell-laden hydrogel bioinks with synthetic polycaprolactone (PCL) polymer that serves as a supporting framework. [5] Although this strategy allowed the fabrication of humanscale bone and cartilage tissue constructs, the mechanically robust PCL fibers often impeded the maturation of soft tissue such as the heart and liver. Alternatively, the embedded 3D bioprinting strategy has gained increasing popularity for constructing complex freeform structures. [6] In this approach, a suspension medium is utilized to support the deposition of bioinks in 3D space before crosslinking. The suspension medium undergoes rapid fluidization at yield stress and then solidification in the absence of stress due to its unique shear-thinning and self-healing properties. [7] The printed structures can easily be removed from the suspension medium by gently washing the suspension medium or raising the temperature. As an example of this strategy, Lee and co-workers printed a functional ventricle and full-size human heart model into a gelatin microparticles-based suspension medium by using a freeform reversible embedding of suspended hydrogels, also termed the FRESH technique. [8] Creating functional tissues and organs in vitro on demand is a major goal in biofabrication, but the ability to replicate the external geometry of specific organs and their internal structures such as blood vessels simultaneously remains one of the greatest impediments. Here, this limitation is addressed by developing a generalizable bioprinting strategy of sequential printing in a reversible ink template (SPIRIT). It is demonstrated that this microgel-based biphasic (MB) bioink can be used as both an excellent bioink and a suspension medium that supports embedded 3D printing due to its shear-thinning and self-healing behavior. When encapsulating human-induced pluripotent stem cells, the MB bioink is 3D printed to generate cardiac tissues and organoids by extensive stem cell proliferation and cardiac differentiation. By incorporating MB bioink, the SPIRIT strategy enables the effective printing of a ventricle model with a perfusable vascular network, which is not possible to fabricate using extant 3D printing strategies. This SPIRIT technique offers an unparalleled bioprinting capability to replicate the complex organ geometry and internal structure in a faster manner, which will accelerate the biofabrication and therapeutic applications of tissue and organ constructs.