Ultrafast 4D printing (<30 s) of responsive polymers is reported. Visible-light-triggered polymerization of commercial monomers defines digitally stress distribution in a 2D polymer film. Releasing the stress after the printing converts the structure into 3D. An additional dimension can be incorporated by choosing the printing precursors. The process overcomes the speed limiting steps of typical 3D (4D) printing.

Inspired by the assembly of Lego toys, hydrogel building blocks with heterogeneous responsiveness are assembled utilizing macroscopic supramolecular recognition as the adhesion force. The Lego hydrogel provides 3D transformation upon pH variation. After disassembly of the building blocks by changing the oxidation state, they can be re-assembled into a completely new shape.

printing is, however, limited by various factors, with printing speed and material versatility being the most decisive. From the standpoint of printing speed, layerwise printing by digital light processing (DLP) offers significant inherent advantage over point-wise printing by most other methods such as fused deposition modeling (FDM) and stereolithography (SLA). [20] Further innovations on DLP such as continuous building in the z-dimension allow achieving a printing speed far exceeding any other methods. [21][22][23] Recent attempt on direct forming 3D object in a volumetric fashion forgoes even the layer-wise printing, but the technique has yet to evolve into the mainstream. [24,25] Generally, DLP employs light curable liquid resins. Upon digital light exposure, the resin cross-links and forms a thermoset polymer that ensures its separation from the surrounding liquid precursors. This rapid liquid-solid separation is the key enabling characteristic for DLP. Although cross-linking ensures such, the resulting thermoset polymers cannot be reprocessed. This prohibits its broader utility in situations that require further processing of the printed materials. In principle, this limitation can be overcome if the DLP processing can be extended to reprocessable thermoplastic polymers. Although light curable thermoplastic polymers are known, meeting the unique DLP requirement for rapid separation between the liquid monomer and the un-crosslinked polymer is not straightforward because of their intrinsic good miscibility. Hereafter, we describe our successful attempt in DLP printing of a thermoplastic polymer and lay out key enabling factors for such a process. We further illustrate that the water dissolvable nature of the resulting polymer is advantageous in making various multifunctional 3D structures. We find that the oxygen naturally present in ambient air can inhibit the light curing process. This effect can be utilized to achieve rapid open-air printing without the complex interfacial engineering typically required for fast DLP printing methods. [21][22][23] We hypothesize that two key factors should be carefully considered for DLP printing of thermoplastic polymers (Figure 1a). Herein, two concurrent and competing processes occur: 1) the polymerization and 2) the diffusion/dissolution of the polymer into the surrounding uncured liquid monomer. Rapid liquidsolid separation is possible only if the first process dominates 3D printing has witnessed a new era in which highly complexed customized products become reality. Realizing its ultimate potential requires simultaneous attainment of both printing speed and product versatility. Among various printing techniques, digital light processing (DLP) stands out in its high speed but is limited to intractable light curable thermosets. Thermoplastic polymers, despite their reprocessibility that allows more options for further manipulation, are restricted to intrinsically slow printing methods such as fused deposition modeling. Extending DLP to thermoplastics is highly desirabl...

Controlling stresses in materials presents many unusual opportunities for their engineering applications. The potential for current approaches is severely limited by the intrinsic tie between the stress and the geometric shape. Here, we report a material concept that allows stress management in a highly efficient digital manner while decoupling the stress and the geometric shape. This is realized in a dynamic covalent shape memory polymer network, for which the elastic shape memory sets the baseline stress level and maintains the geometric shape while the plasticity enabled by the dynamic bond exchange allows stress tuning. With a digital gray scale photothermal mechanism, any arbitrarily defined stress distribution can be created in a free-standing polymer film. The naturally invisible stresses can be further visualized as mechanical colors under polarized light, revealing its potential for encoding hidden information. Our approach expands the technological potential in many areas for which stresses are relevant.

Advanced multifunctional devices increasingly rely on challenging complex shapes for their functions. 3D printing offers a solution but is often limited by the fabrication speed and/or material diversity. 4D printing based on digitally controlled 2D-to-3D transformation is advantageous in speed, but the accessible shapes are limited and integration of multiple materials is difficult. We report herein a concept that significantly extends the technological scope by combining 4D printing with modular assembly. Specifically, 4D photo-printed structures based on dynamically crosslinked polymers can be assembled in a modular fashion by interfacial bond exchange. Complex 3D objects with tailorable multiple materials can consequently be produced. This allows the fabrication of sophisticated shape-memory devices including a 3D Miura-patterned structure with zero Poisson's ratio and a Kresling-patterned cylindrical structure with superior mechanical stability. Our approach extends the possibilities for the future development of multifunctional devices with seamless integration of material, structure, and function.

Poly(ethylene glycol) (PEG) cryogels with aligned porous structures were prepared by unidirectional freezing and subsequently cryopolymerization. The aqueous reaction precursor of poly(ethylene glycol) diacrylate (PEGDA) and the initiator was first unidirectionally frozen using liquid nitrogen or other cooling agents such as frozen ethyl alcohol and frozen acetic ether, followed by radical polymerization at subzero temperature. The morphology of the obtained cryogels was observed by scanning electron microscopy (SEM) and confocal laser scanning microscope (CLSM). The pore structure of the obtained cryogels was the replica of the unidirectional ice crystals formed during the unidirectional freezing. Microtubular pores aligned along the freezing direction were observed in both dry and swollen states. The obtained cryogels showed anisotropic compressive strength according to the pore directions. The effect of the PEGDA molecular weight and the freezing temperature on morphology of the orientationstructured cryogels were also studied. The pore diameter could be adjusted from 10 to 50 mm.

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Shape memory polymers (SMP) with 3D geometries and tunable shape-shifting behavior can open up new opportunities in intelligent devices. Achieving both simultaneously is difficult for conventional approaches. 4D printing allows fabrication of complex 3D SMP geometries that can change shapes (i.e., the fourth dimension is time), but tuning the shape memory response is challenging because of the printing constraints.Here, we report a material and process concept that allows digital light fabrication of SMP with fine control of not only the geometries but also the shape memory characteristics, within a printing time of 30 s. Digital light modulation allows spatio-temporal tuning of the material properties including shape memory transition temperature, rubbery modulus, and maximum elongation (up to 250%). Consequently, the process allows producing multiple-SMP within a single material construct using the same printing precursor. We demonstrate that this unique attribute is beneficial in constructing unusual shape-shifting 3D nano-photonic and electronic devices. The simplicity and versatility of our approach facilitates its future expansion into a wide range of geometrically complex devices with advanced functions.