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Hydrogel tubes made of sodium alginate (SA) have potential applications in drug delivery, soft robots, biomimetic blood vessels, tissue stents, and other fields. However, the continuous preparation of hollow SA hydrogel tubes with good stability and size control remains a huge challenge for chemists, material scientists, and medical practitioners. Inspired by the plant apical growth strategy, a new method named soft cap-guided growth was proposed to produce SA hydrogel tubes. Due to the introduction of inert low gravity substances, such as air and heptane, into the extrusion needle in front of calcium chloride solution to form a soft cap, the SA hydrogel tubes with controllable sizes were fabricated rapidly and continuously without using a template through a negative gravitropism mechanism. The SA hydrogel tubes had good tensile strength, high burst pressure, and good cell compatibility. In addition, hydrogel tubes with complex patterns were conveniently created by controlling the motion path of a soft cap, such as a rotating SA bath or magnetic force. Our research provided a simple innovative technique to steer the growth of hydrogel tubes, which made it possible to mass produce hydrogel tubes with controllable sizes and programmable patterns.

Photocuring 3D printing of hydrogels, with sophisticated, delicate structures and biocompatibility, attracts significant attention by researchers and possesses promising application in the fields of tissue engineering and flexible devices. After years of development, photocuring 3D printing technologies and hydrogel inks make great progress. Herein, the techniques of photocuring 3D printing of hydrogels, including direct ink writing (DIW), stereolithography (SLA), digital light processing (DLP), continuous liquid interface production (CLIP), volumetric additive manufacturing (VAM), and two photon polymerization (TPP) are reviewed. Further, the raw materials for hydrogel inks (photocurable polymers, monomers, photoinitiators, and additives) and applications in tissue engineering and flexible devices are also reviewed. At last, the current challenges and future perspectives of photocuring 3D printing of hydrogels are discussed., (© 2024 Wiley‐VCH GmbH.)

Poly(vinyl alcohol) (PVA)-based hydrogels have attracted great attention and been widely used in biological tissue engineering. With the development of modern medicine, precision medicine requires the customization of medical materials. However, lacking of photocurable functional groups or the performance of rapid phase transition makes PVA-based hydrogels difficult to be customizable molded through photocuring 3D printing technique. In this research, customizable PVA-based hydrogels with high performance through 3D photocurable printing and freezing-thawing (F-T) process are obtained. The ability of 3D-printable is endowed by the introduction of polyvinyl alcohol-styrylpyridine (PVA-SBQ), which can be photo-crosslinked quickly without photoinitiator. Meanwhile, the tunable mechanical properties are achieved by adjusting the mass ratio of PVA-SBQ to PVA, and PVA can offer the physical crosslinking points through freezing-thawing (F-T) process. The hydrogels with high resolution are prepared by digital light procession 3D printing with the mass ratio 1:1 of PVA-SBQ to PVA solution. Attributed to the absence of initiator, and no small molecule residues inside the hydrogels, the hydrogels have good biocompatibility and have the potential to be applicated in the field of biological tissue engineering., (© 2023 Wiley-VCH GmbH.)

Tough hydrogels are prepared from two monomers via photopolymerization of hydroxyethyl acrylate and sol–gel of methyltrimethoxysilane. Constitution and water content could be tuned easily because of the good water solubility of both monomers and two non-interfering polymerization processes. The hydrogels exhibit excellent integrated performance with toughness, high resilience, fast self-recovery, and self-healing. [ABSTRACT FROM AUTHOR]

Hydrogels are widely used in biological dressing, tissue scaffolding, drug delivery, sensors, and other promising applications owing to their water‐rich soft structures, biocompatibility, and adjustable mechanical properties. However, most of the conventional hydrogels are isotropic. The anisotropic structures existed widely in the organizational structure of plants and animals, which played a crucial role in biological systems. In this work, a method of limited domain swelling to prepare anisotropic hydrogels is proposed. Through spatially controlled swelling, the extension direction of hydrogels can be limited by a tailored mold, further achieving anisotropic hydrogels with concentration gradients. The external solution serves as a swelling solution to promote swelling and extension of the hydrogel matrix in a mold which can control the extension direction. Due to the diversity of external solutions, the method can be applied to prepare a variety of stimulus‐responsive polymers. The limited domain swelling method is promising for the construction of anisotropic hydrogels with different structures and properties. [ABSTRACT FROM AUTHOR]

This study reports an effective method for controlling substance‐release sites of hydrogel. Glycidyl methacrylate, which contains two functional groups, namely, double‐bond acrylate and epoxide, is photografted on a hydrogel surface through hydrogen abstraction photopolymerization due to the existence of a hydrogen donor, such as an amine, in the hydrogel matrix. The remaining epoxide group crosslinks the polymer chain of polyglycidyl methacrylate. Substance release of hydrogel is changed due to the altered surface texture of hydrogel. Rate and site‐controlled substance release are achieved by controlling the thickness and site of surface grafting and the extent of epoxide ring opening. This study may provide a novel method for achieving hydrogel function or modified performance of other biomaterials to meet biological activity requirements. A method to surface‐selectively graft a crosslinking layer on the surface of hydrogel through radical and cationic photopolymerization is reported. Rate and site‐controlled substance release of hydrogel are achieved by controlling the thickness and site of surface grafting and the extent of epoxide ring opening. [ABSTRACT FROM AUTHOR]

• Anterior growth is a new method to prepare hydrogel tube rapidly and continuously. • Sizes of hydrogel tubes are adjustable synchronously within a large range. • Anterior growth is able to develop branches on a done tube. • The growth of tubes can realizes designed patterning. • The growth of tubes can pass through complex maze to connect designated points. Hydrogel tubes (HTs) represent a significant facet of hydrogel morphology. Nevertheless, the continuous, rapid, and facile fabrication of hydrogel tubes remains a formidable challenge. In this study, we introduce a novel technique named as Anterior Growth (AG) for the continuous preparation of HTs. This innovative approach employs air bubbles as guidance to direct the growth of polyvinyl alcohol (PVA) HTs crosslinked by copper ions in alkaline solution. Various factors that affect the sizes of HTs from micrometers to millimeters have been explored, including gauge of needles, air bubble volumes, injection rates, and concentrations of NaOH, copper nitrate and PVA. Single HT with various sizes can be easily fabricated by simply adjusting injection rates. The reversibility of PVA-Cu2+ crosslinking makes it possible to produce HTs with multiple branches in a convenient way by generating an orifice out of the HT wall and then repeating the growing process. Inspired by plant thigmotropism, HTs can be guided to grow along designed shapes such as different angles and patterns by combination of air bubble and silicone rubble substrates. Remarkably, this method allows HTs to serve as voyagers navigating intricate flow channels with a projected trajectory, indicating the capabilities of patterning and traversing complex channels. To further application, AG allows other materials to prepare HTs such as sodium alginate or more, showcasing excellent practical value. [ABSTRACT FROM AUTHOR]

Conductive hydrogels have drawn significant attention in the field of stretchable sensors. However, the freezing and drying of water, the introduction of additional ions and ions leakage made the preparation process relatively complicated and significantly limited their long-term stability and safety. In this work, Poly (vinyl alcohol) – based organo-hydrogels were fabricated via photodimerization and freezing-thawing (F-T) cycles of the mixed solution of Polyvinyl alcohol bearing styrylpyridinium group (PVA-SBQ) and Poly (vinyl alcohol) (PVA). The as-prepared PVA-SBQ/PVA organo-hydrogels possessed good stretchability (389%), facture strength (0.39 MPa), conductivity (3.1 *10−4 S/cm at 25 ℃ and 4 *10−5 S/cm at −30 ℃) and temperature tolerance (−30 ℃). And the organo-hydrogels displayed a gauge factor of 1.6 at a strain of 100% and a low degree of hysteresis (5.34% at 100% strain). In addition, the as-prepared organo-hydrogels exhibited good low temperature sensing performance, which could monitor strain at − 30 ℃, making the organo-hydrogel suitable for use in the field of strain sensors. Therefore, it is believed that the PVA-SBQ/PVA organo-hydrogels offer a promising prospect for practical applications. [Display omitted] • PVA organo-hydrogels were achieved by photodimerization and freezing-thawing cycles. • The side group SBQ endowed the organo-hydrogels with photo-dimerizable properties and ionic conductivity, thus there is no need to introduce substances such as salt solutions or small molecule initiators. • The as-prepared organo-hydrogels had good low-temperature sensing performance due to the introduction of glycerol. [ABSTRACT FROM AUTHOR]