Abstract3D printing offers enormous flexibility in fabrication of polymer objects with complex geometries. However, it is not suitable for fabricating large polymer structures with geometrical features at the sub-micrometer scale. Porous structure at the sub-micrometer scale can render macroscopic objects with unique properties, including similarities with biological interfaces, permeability and extremely large surface area, imperative inter alia for adsorption, separation, sensing or biomedical applications. Here, we introduce a method combining advantages of 3D printing via digital light processing and polymerization-induced phase separation, which enables formation of 3D polymer structures of digitally defined macroscopic geometry with controllable inherent porosity at the sub-micrometer scale. We demonstrate the possibility to create 3D polymer structures of highly complex geometries and spatially controlled pore sizes from 10 nm to 1000 µm. Produced hierarchical polymers combining nanoporosity with micrometer-sized pores demonstrate improved adsorption performance due to better pore accessibility and favored cell adhesion and growth for 3D cell culture due to surface porosity. This method extends the scope of applications of 3D printing to hierarchical inherently porous 3D objects combining structural features ranging from 10 nm up to cm, making them available for a wide variety of applications.

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These gels represent a class of soft materials with unique properties resembling soft biological tissues, such as tendons, ligaments, cartilage, muscles, and skin. [6] Hydrogels, obtained by cross-linking hydrophilic polymer chains in aqueous solutions, [7] possess the intrinsic lowmodulus nature and tissue-like properties, which make them applicable to tissue engineering, [8] optical devices, [9] biomedicine [10] and actuators. [11] In the pursuit of high performance, most research in the field of polymer gels has been focused on the chemical nature and polymer network architectures and their interactions, [12] such as ideal polymer networks, [13] interpenetrating polymer networks, [14] nano/micro composite polymer networks, [10,15] and hierarchically structured polymer networks. [16] The small molecule solvent is the second component of gels and is often considered to be a nonfunctional liquid that impregnates and expands a functional polymer network. Recently, ionic liquids have been used to replace water in hydrogels, [17] resulting in soft materials with long-term stability. [18] Multicomponent solvent systems, in which water is mixed with organic solvents (such as glycerol, [19] ethylene glycol [20] and sorbitol [21] ), were introduced into gel networks to maintain the performance of materials in harsh environments. However, because of the low molecular weight of solvents used and weak interactions with polymer networks, the reported Polymer gels, such as hydrogels, have been widely used in biomedical applications, flexible electronics, and soft machines. Polymer network design and its contribution to the performance of gels has been extensively studied. In this study, the critical influence of the solvent nature on the mechanical properties and performance of soft polymer gels is demonstrated. A polymer gel platform based on poly(ethylene glycol) (PEG) as solvent is reported (PEGgel). Compared to the corresponding hydrogel or ethylene glycol gel, the PEGgel with physically cross-linked poly(hydroxyethyl methacrylate-co-acrylic acid) demonstrates high stretchability and toughness, rapid self-healing, and long-term stability. Depending on the molecular weight and fraction of PEG, the tensile strength of the PEGgels varies from 0.22 to 41.3 MPa, fracture strain from 12% to 4336%, modulus from 0.08 to 352 MPa, and toughness from 2.89 to 56.23 MJ m -3 . Finally, rapid self-healing of the PEGgel is demonstrated and a self-healing pneumatic actuator is fabricated by 3D-printing. The enhanced mechanical properties of the PEGgel system may be extended to other polymer networks (both chemically and physically cross-linked). Such a simple 3D-printable, self-healing, and tough soft material holds promise for broad applications in wearable electronics, soft actuators and robotics.

Reproduction of native tissues in vitro is important as a tool, as it both enables investigation of fundamental biological processes, and drug and toxicity screenings. In order to closely mimic complex tissues in vitro, artificial multicellular systems are created from different cell types in spatially ordered structures or welldefined geometries in a 3D microenvironment. [1-3] These systems can be built from building blocks [4-7] such as cell sheets, [8] cell-laden microgels, [5] cell spheroids, [9] and organoids. [10,11] Precise control of cellular composition and spatial distribution of building blocks within artificial multicellular systems allows for reconstitution of native tissues in their healthy and disease state in vitro. [12] There are a number of methodologies developed for fabrication of complex 3D cell systems in vitro. [3-7,13,14] Directed assembly allows manual positioning or stacking building blocks to form 3D architectures. [15,16] Birey et al. applied this method for fusion of two forebrain organoids in order to mimic the human brain development and demonstrate inter-neuronal migration. [15] The method of directed assembly is, however, manual and not compatible with high throughput. Remote assembly, such as, acoustic node, [14,17] magnetic cell levitation, [13,18] optical tweezers, [19] or laser-guided direct writing, [20] can achieve assembly of cells or spheroids against gravity or viscous forces. Chen et al. demonstrated the assembly of hepatic organoids by an acoustic node technique, and the technique was able to achieve formation of bile canaliculi networks resembling native hepatic tissue. [17] Souza et al. used magnetic cell levitation to manipulate a glioblastoma cell spheroid, and a human astrocyte spheroid in order to create a cell invasion model. [18] These methods depend on sophisticated equipment, paramagnetic media, or introduce a risk of laser-induced cell damage. Another common strategy used to fabricate multicellular architecture is assembly of cell-laden hydrogels or microgels, [5,21,22] or cell seeding on scaffolds. [7,23] However, these biomaterial-based methods failed to provide high cell packing density. The use of artificial scaffolds or gel matrices additionally lead to disadvantages for constructing 3D tissue models due to their influence on cell-cell interactions, autocrine, and paracrine signaling. 3D printing [3] is a promising method for designing and achieving multicellular architectures, but it is relatively slow, not always compatible with high throughput and relies on printable bio-inks for maintaining 3D structure and cell viability. Artificial multicellular systems are gaining importance in the field of tissue engineering and regenerative medicine. Reconstruction of complex tissue architectures in vitro is nevertheless challenging, and methods permitting controllable and high-throughput fabrication of complex multicellular architectures are needed. Here, a facile and high-throughput method is developed based on a tunable droplet-fusion technique, allowing prog...