Silicon dioxide/poly(vinyl alcohol) composite hydrogels with high mechanical properties and low swellability

Drug-Loaded and Anisotropic Wood-Derived Hydrogel Periosteum with Super Antibacterial, Anti-Inflammatory, and Osteogenic Activities

Current artificial periostea mainly focus on osteogenic activity but overlook structural and mechanical anisotropy, as well as the importance of antibacterial and anti-inflammatory properties. Here, inspired by the anisotropic structure of wood, the delignified wood (named white wood, WW) with a porous and highly oriented cellulose fiber skeleton was obtained, which was further filled with polyvinyl alcohol (PVA) hydrogel loaded with curcumin (Cur) and phytic acid (PA). The prepared wood-derived hydrogel composite membranes can not only exhibit an obvious anisotropic structure and good mechanical properties but also sustainably release loaded drugs to obtain long-term biological activities. Creatively, PA can effectively improve the bioavailability of Cur; more importantly, Cur and PA play an obvious synergistic effect in antibacterial, anti-inflammatory, and osteogenic activities. Compared with the wood-derived hydrogel composite membranes without drug loading, as well as loaded with Cur or PA only, these loaded with Cur and PA are significantly more conducive to inhibiting the growth of bacteria and inflammatory response and facilitating the adhesion, proliferation, and osteogenic differentiation of bone marrow mesenchymal stem cells. This kind of anisotropic wood-derived hydrogel composite membrane with fantastic antibacterial, anti-inflammatory, and osteogenic activities is expected to be ideal artificial periostea.

An Ultra-Stretchable Polyvinyl Alcohol Hydrogel Based on Tannic Acid Modified Aramid Nanofibers for Use as a Strain Sensor

The mechanical performance is critical for hydrogels that are used as strain sensors. p-Aramid nanofiber (ANF) is preferable as an additive to the reinforce the mechanical performance of a poly(vinyl alcohol) (PVA). However, due to the limited hydrogen bond sites, the preparation of ultra-stretchable, ANF-based hydrogel strain sensor is still a challenge. Herein, we reported an ultra-stretchable PVA hydrogel sensor based on tea stain-inspired ANFs. Due to the presence of numerous phenol groups in the tannic acid (TA) layer, the interaction between PVA and the ANFs was significantly enhanced even though the mass ratio of TA@ANF in the hydrogel was 2.8 wt‰. The tensile breaking modulus of the PVA/TA@ANF/Ag hydrogel sensor was increased from 86 kPa to 326 kPa, and the tensile breaking elongation was increased from 356% to 602%. Meanwhile, the hydrogel became much softer, and no obvious deterioration of the flexibility was observed after repeated use. Moreover, Ag NPs were formed in situ on the surfaces of the ANFs, which imparted the sensor with electrical conductivity. The hydrogel-based strain sensor could be used to detect the joint movements of a finger, an elbow, a wrist, and a knee, respectively. This ultra-stretchable hydrogel described herein was a promising candidate for detecting large-scale motions.

PVA-Based Hydrogels Loaded with Diclofenac for Cartilage Replacement

Polyvinyl alcohol (PVA) hydrogels have been widely studied for cartilage replacement due to their biocompatibility, chemical stability, and ability to be modified such that they approximate natural tissue behavior. Additionally, they may also be used with advantages as local drug delivery systems. However, their properties are not yet the most adequate for such applications. This work aimed to develop new PVA-based hydrogels for this purpose, displaying improved tribomechanical properties with the ability to control the release of diclofenac (DFN). Four types of PVA-based hydrogels were prepared via freeze-thawing: PVA, PVA/PAA (by polyacrylic acid (PAA) addition), PVA/PAA+PEG (by polyethylene glycol (PEG) immersion), and PVA/PAA+PEG+A (by annealing). Their morphology, water uptake, mechanical and rheological properties, wettability, friction coefficient, and drug release behavior were accessed. The irritability of the best-performing material was investigated. The results showed that the PAA addition increased the swelling and drug release amount. PEG immersion led to a more compact structure and significantly improved the material’s tribomechanical performance. The annealing treatment led to the material with the most suitable properties: besides presenting a low friction coefficient, it further enhanced the mechanical properties and ensured a controlled DFN release for at least 3 days. Moreover, it did not reveal irritability potential for biological tissues.

An Artificial PVA-BC Composite That Mimics the Biomechanical Properties and Structure of a Natural Intervertebral Disc

Disc herniation is one of the most ubiquitous healthcare problems in modern cities—severe patients eventually require surgical intervention. However, the existing operations—spinal fusion and artificial disc replacement—alter the biomechanics of the spine, leaving much room for improvement. The appropriateness of polyvinyl alcohol (PVA) for biomedical applications has been recognised due to its high water content, excellent biocompatibility, and versatile mechanical properties. In this study, a newly-designed PVA–bacterial cellulose (PVA-BC) composite was assembled to mimic both the biomechanics and annular structure of natural intervertebral discs (IVDs). PVA-BC composites of various concentrations were fabricated and tested under unconfined compression and compressive creep in order to acquire the values of the normalised compressive stiffness and whole normalised deformation. The normalised compressive stiffness increased considerably with an increasing PVA concentration, spanning from 1.82 (±0.18) to 3.50 (±0.14) MPa, and the whole normalised deformation decreased from 0.25 to 0.13. Formulations of 40% PVA provided the most accurate mimicry of natural human IVDs in normalised whole deformation, and demonstrated higher dimensional stability. The biocompatible results further confirmed that the materials had excellent biocompatibility. The novel bionic structure and formulations of the PVA-BC materials mimicked the biomechanics and structure of natural IVDs, and ensured dimensional stability under prolonged compression, reducing the risk of impingement on the surrounding tissue. The PVA-BC composite is a promising material for third-generation artificial IVDs with integrated construction.

Versatile Bilayer Hydrogel for Wound Dressing through PET-RAFT Polymerization

Multifunctional hydrogel-based wound dressings have been explored for decades due to their huge potential in multifaceted medical intervention to wound healing. However, it is usually not easy to fabricate a single hydrogel with all of the desirable functions at one time. Herein, a bilayer model with an outer layer for hydrogel wound dressing was proposed. The inner layer (H_m-PN_n) was a hybrid hydrogel prepared by N-isopropylacrylamide and chitosan-N-2-hydroxypropyl trimethylammonium chloride (HACC), and the outer layer (PVA_o-PAm_p) was prepared by polyvinyl alcohols and acrylamide. The two hydrogel layers of the bilayer model were covalently connected with excellent interfacial strength by photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. The outer layer exposed to the ambient environment exhibited good stretchability and toughness, while the inner-layer hydrogel adhered to the skin exhibited excellent softness, antibacterial activity, thermoresponsivity, and biocompatibility. In particular, the inner layer of a hydrogel demonstrated excellent antibacterial capability toward both Staphylococcus aureus as Gram-positive bacteria and Escherichia coli as Gram-negative bacteria. Cell cytotoxicity showed that the cell viability of all H_m-PN_n layer hydrogels exceeds 80%, confirming that the hydrogels bear excellent biocompatibility. In vivo experimental results indicated that the H_m-PN_n/PVA_o-PAm_p bilayer hydrogel has a significant effect on the acceleration of wound healing, which was demonstrated in a full-thickness skin defect model showing improved collagen disposition and granulation tissue thickness. With these results, the established multifunctional bilayer hydrogel exhibits potential as an excellent wound dressing for wound healing applications, especially for open and infected traumas.

Self-Healing, Self-Adhesive and Stable Organohydrogel-Based Stretchable Oxygen Sensor with High Performance at Room Temperature

With the advent of the 5G era and the rise of the Internet of Things, various sensors have received unprecedented attention, especially wearable and stretchable sensors in the healthcare field. Here, a stretchable, self-healable, self-adhesive, and room-temperature oxygen sensor with excellent repeatability, a full concentration detection range (0-100%), low theoretical limit of detection (5.7 ppm), high sensitivity (0.2%/ppm), good linearity, excellent temperature, and humidity tolerances is fabricated by using polyacrylamide-chitosan (PAM-CS) double network (DN) organohydrogel as a novel transducing material. The PAM-CS DN organohydrogel is transformed from the PAM-CS composite hydrogel using a facile soaking and solvent replacement strategy. Compared with the pristine hydrogel, the DN organohydrogel displays greatly enhanced mechanical strength, moisture retention, freezing resistance, and sensitivity to oxygen. Notably, applying the tensile strain improves both the sensitivity and response speed of the organohydrogel-based oxygen sensor. Furthermore, the response to the same concentration of oxygen before and after self-healing is basically the same. Importantly, we propose an electrochemical reaction mechanism to explain the positive current shift of the oxygen sensor and corroborate this sensing mechanism through rationally designed experiments. The organohydrogel oxygen sensor is used to monitor human respiration in real-time, verifying the feasibility of its practical application. This work provides ideas for fabricating more stretchable, self-healable, self-adhesive, and high-performance gas sensors using ion-conducting organohydrogels.

PVA-Based Hydrogels: Promising Candidates for Articular Cartilage Repair

The complex, gradient physiological structure of articular cartilage is a severe hindrance of its self-repair, leaving the clinical treatment of cartilage defects a demanding issue to be addressed. Currently applied tissue engineering treatments and traditional non-tissue engineering treatments have different limitations, for example, cell dedifferentiation, immune rejection, and prosthesis-related complications. Thus, studies have been focusing on seeking promising candidates for novel cartilage repair methods. Polyvinyl alcohol (PVA) hydrogels with excellent biocompatibility and tunable material properties have become the alternatives. For pure PVA hydrogels, the mechanical strength and lubricity are not capable of replacing articular cartilage until proper modifications are done. This paper summarizes the research progress in PVA hydrogels, including the preparation, modification, and cartilage-repair-aimed biomimetic improvements. Design guidance of PVA hydrogels is put forward as assistance to functional hydrogel preparation. Finally, the prospects and main obstacles of PVA hydrogels are discussed.

Advances in versatile anti-swelling polymer hydrogels

Swelling is ubiquitous for traditional as-prepared hydrogels, but is unfavorable in many situations, especially biomedical applications, such as tissue engineering, internal wound closure, soft actuating and bioelectronics, and so forth. As the swelling of a hydrogel usually leads to a volume expansion, which not only deteriorates the mechanical property of the hydrogel but can bring about undesirable oppression on the surrounding tissues when applied in vivo. In contrast, anti-swelling hydrogels hardly alter their volume when applied in aqueous environment, therefore reserving the original mechanical performance and size-stability and facilitating their potential application. In the past decade, with the development of advanced hydrogels, quite a number of anti-swelling hydrogels with versatile functions have been developed by researchers to meet the practical applications well, through integrating anti-swelling property with certain performance or functionality, such as high strength, self-healing, injectability, adhesiveness, antiseptics, etc. However, there has not been a general summary with regard to these hydrogels. To promote the construction of anti-swelling hydrogels with desirable functionalities in the future, this review generalizes and analyzes the tactics employed so far in the design and manufacture of anti-swelling hydrogels, starting from the viewpoint of classical swelling theories. The review will provide a relatively comprehensive understanding of anti-swelling hydrogels and clues to researchers interested in this kind of materials to develop more advanced ones suitable for practical application.

Freeze-Thawing-Induced Macroporous Catechol Hydrogels with Shape Recovery and Sponge-like Properties

Catechol-containing hydrogels have been exploited in biomedical fields due to their adhesive and cohesive properties, hemostatic abilities, and biocompatibility. Catechol moieties can be oxidized to o-catecholquinone, a chemically active intermediate, in the presence of oxygen to act as an electrophile to form catechol-catechol or catechol-amine/thiol adducts. To date, catechol cross-linking chemistry to fabricate hydrogels has been mostly performed at room temperature. Herein, we report large increases in catechol cross-linking reaction kinetics by the freeze-thawing process. The formation of ice crystals during freezing steps spatially condenses catechol-containing polymers into nearly frozen (yet unfrozen) regions, resulting in decreases in the polymeric chain distances. This environment allows great increases in catechol cross-linking kinetics, a phenomenon that can also occur during thawing steps. The increased cross-linking rate and spatial condensation in the cryogels provide unique wall and pore structures, which result in elastic, spongelike hydrogels. The moduli of the cryogels prepared by glycol-chitosan-catechol (g-chitosan-c) were improved by 3-6-fold compared to room temperature-cured conventional hydrogels, and the degree of improvement increased depending on the freezing time and the number of freeze-thawing cycles. Unlike typical cell encapsulations before cross-linking, which have often been a source of cytotoxicity, the macroporosity of cryogels allows nontoxic cell seeding with ease. This research offers a new way to utilize catechol cross-linking chemistry by freeze-thawing processes to simultaneously regulate mechanical strength and porous structures in catechol-containing hydrogels.

Gelatin Hydrogels Reinforced by Absorbable Nanoparticles and Fibrils Cured In Situ by Visible Light for Tissue Adhesive Applications

Most gelatin hydrogels used in regenerative medicine applications today are fabricated by photocrosslinking due to the convenience and speed of this method. However, in most cases photoinitiators are used, which require UV light, which, in turn, can cause cell and tissue damage, or using functionalized gelatin. Recently, ruthenium (II) tris-bipyridyl chloride has been studied as an initiator that can induce dityrosine bond formation using visible light. In addition, continuous fibrils and small particles are often used to reinforce composite materials. Therefore, this study investigated the visible-light-induced photocrosslinking of native gelatin molecules via dityrosine bonds formation as well as gel reinforcement by collagen fibrils and mesoporous bioactive glass (MBG) particles. The results show that collagen and MBG exerted a synergistic effect on maintaining gel integrity with a dental LED curing light when the irradiation time was shortened to 30 s. Without the two reinforcing components, the gel could not form a geometric shape stable gel even when the exposure time was 120 s. The shear strength increased by 62% with the collagen and MBG compared with the blank control. Furthermore, our results demonstrate that the addition of collagen and MBG enhanced gel stability in an artificial saliva solution. These results demonstrate the considerable advantages of using tyrosine-containing biomolecules, and using a dental LED curing light for the crosslinking of hydrogels in terms of their suitability and feasibility for use as bioadhesives in confined clinical working space, such as the oral cavity, and in application as in situ-crosslinked injectable hydrogels.

Tough and Low Friction Polyvinyl Alcohol Hydrogels Loaded with Anti-inflammatories for Cartilage Replacement

The development of new materials that mimic cartilage and its function is an unmet need that will allow replacing the damaged parts of the joints, instead of the whole joint. Polyvinyl alcohol (PVA) hydrogels have raised special interest for this application due to their biocompatibility, high swelling capacity and chemical stability. In this work, the effect of post-processing treatments (annealing, high hydrostatic pressure (HHP) and gamma-radiation) on the performance of PVA gels obtained by cast-drying was investigated and, their ability to be used as delivery vehicles of the anti-inflammatories diclofenac or ketorolac was evaluated. HHP damaged the hydrogels, breaking some bonds in the polymeric matrix, and therefore led to poor mechanical and tribological properties. The remaining treatments, in general, improved the performance of the materials, increasing their crystallinity. Annealing at 150 °C generated the best mechanical and tribological results: higher resistance to compressive and tensile loads, lower friction coefficients and ability to support higher loads in sliding movement. This material was loaded with the anti-inflammatories, both without and with vitamin E (Vit.E) or Vit.E + cetalkonium chloride (CKC). Vit.E + CKC helped to control the release of the drugs which occurred in 24 h. The material did not induce irritability or cytotoxicity and, therefore, shows high potential to be used in cartilage replacement with a therapeutic effect in the immediate postoperative period.

Long-Shelf-Life Polymer Electrolyte Based on Tetraethylammonium Hydroxide for Flexible Zinc-Air Batteries

Flexible zinc-air batteries (ZABs) have been considered as one of the most outstanding energy storage devices for flexible and portable electronics because of their superior energy density and environmental friendliness. As the "blood" of flexible ZABs, electrolytes play a significant role in determining their performance, such as discharge working time, cycling property, and shelf life. Herein, a novel polymer electrolyte based on quaternary ammonium hydroxides is first applied in flexible zinc-air batteries. Tetraethylammonium hydroxide (TEAOH) is innovatively used as the ionic conductor with poly(vinyl alcohol) (PVA) as the polymer host in the polymer electrolyte and exhibits a good water retention capability, resulting in not only a good shelf life but also a good working life of the flexible zinc-air batteries. The fabricated polymer electrolyte maintains its high ionic conductivity of 30 mS cm^-1 even after 2 weeks. In addition, the as-assembled zinc-air batteries based on the TEAOH-PVA electrolyte exhibit excellent discharge performance and cycling life compared to those based on the commonly used KOH-PVA electrolyte, and no notable degradation is observed after 2 weeks. Furthermore, flexible TEAOH-PVA-based zinc-air batteries can power a light-emitting diode (LED) electronic watch, a mobile phone, and an LED screen, indicating the very large potential of the high-performance zinc-air batteries that are safe, cost-effective, and remarkably flexible.