We present here a synthetic strategy for the preparation of melt-processable shape-memory hydrogels with self-healing ability. The supramolecular hydrogel with a water content of 60−80 wt % consists of poly(acrylic acid) chains containing 20−50 mol % crystallizable n-octadecyl acrylate (C18A) segments together with surfactant micelles. The key of our approach to render the hydrogel melt-processable is the absence of chemical cross-links and the presence of surfactant micelles. At temperatures above the melting temperature T m of the crystalline domains of alkyl side chains, the hydrogel liquefies due to the presence of surfactant micelles effective for solubilizing the hydrophobic C18A segments. At this stage, it can easily be shaped into any desired form by pouring into molds. Cooling below T m and removing the surfactant from the gel network results in a hydrogel of any permanent shape with a particularly high compressive strength of 90 MPa and a Young's modulus of 26 MPa. If the hydrogel was damaged on purpose e.g. by cutting into two pieces, the extraordinary mechanical properties can completely be recovered via temperature-induced healing process. The hydrogel also exhibits a complete shape fixity ratio and a shape recovery ratio of 97 ± 2%.

A novel way for the production of shape memory hydrogels containing crystalline domains is described. Hydrogels were prepared by micellar copolymerization of acrylic acid with the hydrophobic comonomer n-octadecyl acrylate (C18) in an aqueous NaCl solution of sodium dodecyl sulfate (SDS). The presence of NaCl causes the SDS micelles to grow and thus enables solubilization of large amounts (16% w/v) of C18 in the micellar solution. DSC measurements show that the swollen hydrogels, possessing 61−84% water, melt and crystallize with a change in temperature. Independent of the hydrophobe level between 20 and 50 mol %, the melting and crystallization temperatures of the hydrogels are 48 ± 2 and 43 ± 2 °C, respectively. The hydrogels exhibit 3 orders of magnitude change in the elastic modulus when the temperature changes between below and above the melting temperature of the crystalline domains. The blocky structure of the network chains formed by micellar polymerization is responsible for the drastic change in their mechanical properties and significant shape memory effect.

Supramolecular semicrystalline hydrogels are soft functional materials consisting of water-swollen hydrophilic polymer chains interconnected by hydrophobic segments forming lamellar crystals. Although such hydrogels with high crystallinity are mechanically strong, with elastic moduli and tensile strength of 80−300 MPa and 4−7 MPa, respectively, they are brittle and rupture at a stretch of less than 20% without yielding. Here, we report that the incorporation of a small amount of a weak hydrophobe into semicrystalline hydrogels significantly increases their toughness and stretchability without losing their high modulus and high strength. We design a highly entangled physical network based on poly(N,N-dimethylacrylamide) (PDMA) chains containing n-octadecyl acrylate (C18A) and lauryl methacrylate (C12M) segments with side chain lengths of 18 and 12 carbons, respectively. By including 0.1−0.4 mol % C12M into the PDMA backbone containing 30 mol % C18A segments, we were able to create more ordered and thinner lamellar crystals with a layered structure. Simultaneously, a brittle-to-ductile transition was observed due to the appearance of necking behavior leading to 10-fold increase of toughness. The significant toughness improvement upon incorporation of C12M into the semicrystalline hydrogels could be explained with the appearance of active tie molecules under external force interconnecting the lamellar clusters. The hydrogels also exhibit reversible tensile deformation induced by heating above the melting temperature of crystalline domains.

One of the most fascinating challenges in recent years has been to produce mechanically robust and tough polymers with smart functions such as self-healing and shape-memory behavior. Here, we report a simple and versatile strategy for the preparation of a highly tough and highly stretchable interconnected interpenetrating polymer network (c-IPN) based on butyl rubber (IIR) and poly(n-octadecyl acrylate) (PC18A) with thermally induced healing and shape-memory functions. Solventfree UV polymerization of n-octadecyl acrylate (C18A) at 30 ± 2 °C in the presence of IIR leads to IIR/PC18A c-IPNs with seaisland or co-continuous morphologies depending on their IIR contents. The lamellar crystals with a melting temperature T m of 51−52 °C formed by side-by-side packed octadecyl (C18) side chains are responsible for more than 99% of effective cross-links in c-IPNs, the rest being hydrophobic associations and chemical cross-links. The c-IPNs exhibit varying stiffness (9−34 MPa), stretchability (72−740%), and a significantly higher toughness (1.9− 12 MJ•m −3 ) than their components, which can be tuned by changing the IIR/PC18A weight ratio. The properties of c-IPNs could also be tuned by incorporating a second, noncrystallizable hydrophobic monomer, namely, lauryl methacrylate (C12M), in the melt mixture. We show that the lamellar clusters acting as sacrificial bonds break at the yield point by dissipation of energy, while the ductile amorphous continuous phase keeps the structure together, leading to the toughness improvement of c-IPNs. They exhibit a two-step healing process with >90% healing efficiency with respect to the modulus and a complete shape-recovery ratio induced by heating above T m of alkyl crystals. The temperature-induced healing occurs via a quick step where C18 bridges form between the damaged surfaces followed by a slow step controlled by the interdiffusion of C18A segments in the bulk. We also show that the strategy developed here is suitable for a variety of rubbers and n-alkyl (meth)acrylates of various side-chain lengths.

This research highlights the use of poly(acrylic acid) (PAAc) cryogels as a pH oscillator in oscillatory bromate-sulfite-ferrocyanide reactions. The cryogels were prepared from frozen aqueous solutions of acrylic acid monomer and N,N 0 -methylenebis(acrylamide) crosslinker at À18C. Fast responsive macroporous PAAc cryogels were obtained at or below 4 w/v % initial monomer concentration. SEM images of PAAc cryogel networks exhibited a heterogeneous morphology consisting of pores of sizes 10 1 lm, typical for macroporous networks formed by the cryogelation technique. PAAc cryogels coupled with the bromate oscillator oscillate between swollen and collapsed states, during which a cyclic three-fold change in the gel volume was observed. The results show that, due to the fast response rate of cryogels, macroscopic gel samples can be used as a pH oscillator which would provide generation of significant amount of mechanical energy.

Recent studies on three-dimensional (3D) bioprinting of cell-laden gelatin methacryloyl (GelMA) hydrogels have provided promising outcomes for tissue engineering applications. However, the reliance on the use of photo-induced gelation processes for the bioprinting of GelMA and the lack of an alternative crosslinking process remain major challenges for the fabrication of cell-laden structures. Here, we present a novel crosslinking approach to form cell-laden GelMA hydrogel constructs through 3D embedded bioprinting without using any external irradiation that could drastically affect cell viability and functionality. This approach consists of a one-step type of crosslinking via bisulfite-initiated radical polymerization, which is combined with embedded bioprinting technology to improve the structural complexity of printed structures. By this means, complex-shaped hydrogel bio-structures with cell viability higher than 90 % were successfully printed within a support bath including sodium bisulfite. This study offers an important alternative to other photo-induced gelation processes to improve the bio-fabrication of GelMA hydrogel with high cell viability.

Cryogel-based scaffolds have attracted great attention in tissue engineering due to their interconnected macroporous structures. However, three-dimensional (3D) printing of cryogels with a high degree of precision and complexity is a challenge, since the synthesis of cryogels occurs under cryogenic conditions. In this study, we demonstrated the fabrication of cryogel-based scaffolds for the first time by using an embedded printing technique. A photo-cross-linkable gelatin methacryloyl (GelMA)-based ink composition, including alginate and photoinitiator, was printed into a nanoclay-based support bath. The layer-by-layer extruded ink was held in complex and overhanging structures with the help of pre-cross-linking of alginate with Ca 2+ present in the support bath. The printed 3D structures in the support bath were frozen, and then GelMA was cross-linked at a subzero temperature under UV light. The printed and cross-linked structures were successfully recovered from the support bath with an integrated shape complexity. SEM images showed the formation of a 3D printed scaffold where porous GelMA cryogel was integrated between the cross-linked alginate hydrogels. In addition, they showed excellent shape recovery under uniaxial compression cycles of up to 80% strain. In vitro studies showed that the human fibroblast cells attached to the 3D printed scaffold and displayed spread morphology with a high proliferation rate. The results revealed that the embedded 3D printing technique enables the fabrication of cytocompatible cryogel based scaffolds with desired morphology and mechanical behavior using photo-cross-linkable bioink composition. The properties of the cryogels can be modified by varying the GelMA concentration, whereby various shapes of scaffolds can be fabricated to meet the specific requirements of tissue engineering applications.

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