3D Printing of Self-Healing Materials

This review article presents a comprehensive overview of the latest advances in the field of 3D printable structures with self-healing properties. Three-dimensional printing (3DP) is a versatile technology that enables the rapid manufacturing of complex geometric structures with precision and functionality not previously attainable. However, the application of 3DP technology is still limited by the availability of materials with customizable properties specifically designed for additive manufacturing. The addition of self-healing properties within 3D printed objects is of high interest as it can improve the performance and lifespan of structural components, and even enable the mimicking of living tissues for biomedical applications, such as organs printing. The review will discuss and analyze the most relevant results reported in recent years in the development of self-healing polymeric materials that can be processed via 3D printing. After introducing the chemical and physical self-healing mechanism that can be exploited, the literature review here reported will focus in particular on printability and repairing performances. At last, actual perspective and possible development field will be critically discussed.

Preparation of PEG/P(U-AM-ChCl) composite hydrogels using ternary DES light polymerization and their properties

Deep eutectic solvents (DES) were prepared using urea (U) and acrylamide (AM) as hydrogen bond donors (HBD) and choline chloride (ChCl) as hydrogen bond acceptor (HBA), and polyethylene glycol (PEG) was selected as a filler and uniformly dispersed in DES to prepare PEG/P(U-AM-ChCl) composite hydrogels by light polymerization. The composite hydrogels were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The effects of the content of PEG on the swelling properties, mechanical properties and fatigue resistance of the composite hydrogels were investigated. The results showed that the compressive strength and fatigue strength of the composite hydrogels were gradually enhanced with the increase of the PEG content in the composite hydrogels, in which the maximum compressive strength of the hydrogels with 1 wt% PEG added was increased by 1.86 times. The composite hydrogel had excellent swelling properties, and the equilibrium swelling degree of the hydrogel with 1 wt% PEG added reached 10.15. Meanwhile, the PEG/P(U-AM-ChCl) composite hydrogel had excellent self-healing properties, and the self-healing rate of the composite hydrogel with a PFG content of 1 wt% could reach 91.93% after 48 hours of healing. This study provides a convenient and efficient method to prepare composite hydrogels with superior swelling properties and self-healing properties.

An injectable and self-healing cellulose nanofiber-reinforced alginate hydrogel for bone repair

Biomedical materials are in high demand for transplantation in cases of diseased or damaged bone tissue. Hydrogels are potential candidates for bone defect repair; however, traditional hydrogels lack the necessary strength and multiple functions. Herein, we effectively synthesized a cellulose nanofiber (CNF)-reinforced oxidized alginate (OSA)/gelatin (Gel) semi-interpenetrating network hydrogel through a facile one-step approach without a cross-linker by using the synergistic effects of dynamic imine bonds and hydrogen bonds. The OSA/Gel/CNF sample showed a notable compressive modulus (up to 361.3 KPa). The gelation time (~150 s) ensured excellent injectability. Self-healing exhibited a high efficiency of up to 92 %, which would enable minimally invasive, dynamic adjustments and personalized therapies. Furthermore, the OSA/Gel/CNF hydrogel showed excellent biomineralization (Ca/P ratio ~ 1.69) and enhanced preosteoblast cell (MC3T3-E1) viability (over 96 %), proliferation, and osteogenic differentiation. Thus, this multifunctional hydrogel has promising potential for using in the bone tissue repairs.

Click Step-Growth Polymerization and E/Z Stereochemistry Using Nucleophilic Thiol-yne/-ene Reactions: Applying Old Concepts for Practical Sustainable (Bio)Materials

Polymer sustainability is synonymous with "bioderived polymers" and the zeitgeist of "using renewable feedstocks". However, this sentiment does not adequately encompass the requirements of sustainability in polymers. In addition to recycling considerations and mechanical performance, following green chemistry principles also needs to be maximized to improve the sustainability of polymer synthesis. The synthetic cost (i.e., maximizing atom economy, reducing chemical hazards, and lowering energy requirements) of producing polymers should be viewed as equally important to the monomer source (biomass vs petrol platform chemicals). Therefore, combining the use of renewable feedstocks with efficient syntheses and green chemistry principles is imperative to delivering truly sustainable polymers. The high efficiency, atom economy, and single reaction trajectories that define click chemistry reactions position them as ideal chemical approaches to synthesize polymers in a sustainable manner while simultaneously expanding the structural scope of accessible polymers from sustainably sourced chemicals.Click step-growth polymerization using the thiol-yne Michael addition, a reaction first reported over a century ago, has emerged as an extremely mild and atom-efficient pathway to yield high-performance polymers with controllable E/Z stereochemistry along the polymer backbone. Building on studies of aromatic thiol-yne polymers, around 10 years ago our group began investigating the thiol-yne reaction for the stereocontrolled synthesis of alkene-containing aliphatic polyesters. Our early studies established a convenient path to high-molecular-weight (>100 kDa) E-rich or Z-rich step-growth polymers by judiciously changing the catalyst and/or reaction solvent. This method has since been adapted to synthesize fast-degrading polyesters, high-performance polyamides, and resilient hydrogel biomaterials. Across several systems, we have observed dramatic differences in material properties among polymers with different alkene stereochemistry.We have also explored the analogous thiol-ene Michael reaction to create high-performance poly(ester-urethanes) with precise E/Z stereochemistry. In contrast to the stereoselective thiol-yne polymerization, here the use of monomers with predefined E/Z (geometric) isomerism (arising from either alkenes or the planar rigidity of ring units) affords polymers with total control over stereochemistry. This advancement has enabled the synthesis of tough, degradable materials that are derived from sustainable monomer feedstocks. Employing isomers of sugar-derived isohexides, bicyclic rigid-rings possessing geometric isomerism, led to degradable polymers with fundamentally opposing mechanical behavior (i.e., plastic vs elastic) simply by adjusting the stereochemistry of the isohexide.In this Account, we feature our investigation of thiol-yne/-ene click step-growth polymers and efforts to establish structure-property relationships toward degradable materials with practical mechanical performance in the context of sustainable polymers and/or biomaterials. We have paid attention to installing and controlling geometric isomerism by using these click reactions, an overarching objective of our work in this research area. The exquisite control of geometric isomerism that is possible within polymer backbones, as enabled by convenient click chemistry reactions, showcases a powerful approach to creating multipurpose degradable polymers.

Mechanical properties of an interpenetrating network poly(vinyl alcohol)/alginate hydrogel with hierarchical fibrous structures

Bioinspired hierarchical fibrous structures were constructed in an interpenetrating poly(vinyl alcohol, PVA)/alginate hydrogel network to improve its mechanical properties. The interpenetrating hydrogel network with hierarchical fibrous structures was prepared by combining the confined drying method and freeze-thaw method. First, Ca2+ cross-linked alginate formed a nano-micro hierarchical fibrous structure via the confined drying method. Then, PVA that was uniformly distributed among the Ca2+-alginate chains was cross-linked by hydrogen bonding via the freeze-thaw method, further dividing the hierarchical fibers into finer fibers. The results of a tensile test demonstrated that both the tensile stress and fracture energy improved by more than double after the introduction of 2 wt% PVA, achieving a combination of high strength (∼12.9 MPa), high toughness (∼13.2 MJ m-3) and large strain (∼161.4%). Cyclic tensile tests showed that a hysteresis loop existed on the loading-unloading curves of the hydrogel along the fibrous directions, and a good self-recovery property emerged after resting for a period of time. The hydrogel with hierarchical fibrous structures constructed by alginate and PVA can be employed in biomedical applications in the future.

Using Stereochemistry to Control Mechanical Properties in Thiol-Yne Click-Hydrogels

The stereochemistry of polymers has a profound impact on their mechanical properties. While this has been observed in thermoplastics, studies on how stereochemistry affects the bulk properties of swollen networks, such as hydrogels, are limited. Typically, changing the stiffness of a hydrogel is achieved at the cost of changing another parameter, that in turn affects the physical properties of the material and ultimately influences the cellular response. Herein, we report that by manipulating the stereochemistry of a double bond, formed in situ during gelation, materials with diverse mechanical properties but comparable physical properties can be obtained. Click-hydrogels that possess a high % trans content are stiffer than their high % cis analogues by almost a factor of 3. Human mesenchymal stem cells acted as a substrate stiffness cell reporter demonstrating the potential of these platforms to study mechanotransduction without the influence of other external factors.

Using Stereochemistry to Control Mechanical Properties in Thiol-Yne Click-Hydrogels

The stereochemistry of polymers has a profound impact on their mechanical properties. While this has been observed in thermoplastics, studies on how stereochemistry affects the bulk properties of swollen networks, such as hydrogels, are limited. Typically, changing the stiffness of a hydrogel is achieved at the cost of changing another parameter, that in turn affects the physical properties of the material and ultimately influences the cellular response. Herein, we report that by manipulating the stereochemistry of a double bond, formed in situ during gelation, materials with diverse mechanical properties but comparable physical properties can be obtained. Click-hydrogels that possess a high % trans content are stiffer than their high % cis analogues by almost a factor of 3. Human mesenchymal stem cells acted as a substrate stiffness cell reporter demonstrating the potential of these platforms to study mechanotransduction without the influence of other external factors.

Hydrogel, a novel therapeutic and delivery strategy, in the treatment of intrauterine adhesions

Intrauterine adhesions (IUAs) are caused by damage to the underlying lining of the endometrium. They' re related to disorder of endometrial repair. In recent years, hydrogels with controllable biological activity have been widely used for treating IUAs. They encapsulate estrogen, cytokines, cells, or exosomes, forming a delivery system to release therapeutic components for the treatment of IUAs. In addition, the hydrogel acting as a barrier can be degraded in the body automatically, reducing the risk of infection caused by secondary surgeries. In this review, we summarize the recent progress of hydrogels and their application in IUAs as both a novel alternative therapeutic and an artificial delivery strategy.

Click Nucleophilic Conjugate Additions to Activated Alkynes: Exploring Thiol-yne, Amino-yne, and Hydroxyl-yne Reactions from (Bio)Organic to Polymer Chemistry

The 1,4-conjugate addition reaction between activated alkynes or acetylenic Michael acceptors and nucleophiles (i.e., the nucleophilic Michael reaction) is a historically useful organic transformation. Despite its general utility, the efficiency and outcomes can vary widely and are often closely dependent upon specific reaction conditions. Nevertheless, with improvements in reaction design, including catalyst development and an expansion of the substrate scope to feature more electrophilic alkynes, many examples now present with features that are congruent with Click chemistry. Although several nucleophilic species can participate in these conjugate additions, ubiquitous nucleophiles such as thiols, amines, and alcohols are commonly employed and, consequently, among the most well developed. For many years, these conjugate additions were largely relegated to organic chemistry, but in the last few decades their use has expanded into other spheres such as bioorganic chemistry and polymer chemistry. Within these fields, they have been particularly useful for bioconjugation reactions and step-growth polymerizations, respectively, due to their excellent efficiency, orthogonality, and ambient reactivity. The reaction is expected to feature in increasingly divergent application settings as it continues to emerge as a Click reaction.

Double-network hydrogels with adjustable surface morphology and multifunctional integration for flexible strain sensors

The next generation of high-performance flexible electronics has put forward new demands on the development of ionic conductive hydrogels. In recent years, many efforts have been made toward developing double-network (DN) hydrogels due to their excellent mechanical properties and unique network structures. However, profound challenges remain in achieving controllable surface morphology and multifunctional integration within DN hydrogels. In this work, we report the fabrication of a multifunctional DN hydrogel by multiple cross-linking between an innovative K+-containing poly(ionic liquid) (PIL) and κ-carrageenan. The resulting hydrogel possesses fascinating physicochemical properties, ranging from remarkable mechanical properties and machinability to adjustable surface morphology and superior adhesion ability. The extremely versatile DN hydrogels exhibited outstanding potential for the future of wearable strain sensors in real-time monitoring of human health, and the optimized design strategy opens new possibilities for the fabrication of multiscale structured and multifunctional integrated ionic conductive hydrogels.

Preparation of conductive self-healing hydrogels via an interpenetrating polymer network method

Conductive self-healing hydrogels and related soft sensor devices are gaining extensive attention from academia to industry because of their impacts on the lifetime and ergonomic design of artificial skins and soft robotics, as well as health monitoring systems. However, so far the development of such a material has been limited considering performance and availability. In this work, we developed composite hydrogels of acrylamide, polyacrylamide, dialdehyde-functionalized poly(ethylene glycol) and conductive carbon black through an interpenetrating polymer network strategy. After optimizing the composition ratio, the resultant hydrogel exhibited self-healing reversibility mechanically and electrically when cut and self-healed. We used ¹H NMR and FT-IR spectroscopy to determine the self-healing mechanism of the system, thus demonstrating that the cooperative effect of the dynamic covalent and noncovalent interactions contributes to the self-healing capability of the gel. Rheology, scanning electron microscopy and light-emitting diode circuits were carried out to examine its macroscopic and microscopic properties, making it possible to apply in soft and conformable electronics.

A new type of conductive interpenetrating polymer network hydrogel exhibited self-healing reversibility mechanically and electrically when cut and self-healed, making it possible to apply in soft and conformable electronics.

Hydrogel Properties and Their Impact on Regenerative Medicine and Tissue Engineering

Hydrogels (HGs), as three-dimensional structures, are widely used in modern medicine, including regenerative medicine. The use of HGs in wound treatment and tissue engineering is a rapidly developing sector of medicine. The unique properties of HGs allow researchers to easily modify them to maximize their potential. Herein, we describe the physicochemical properties of HGs, which determine their subsequent applications in regenerative medicine and tissue engineering. Examples of chemical modifications of HGs and their applications are described based on the latest scientific reports.

Biocompatible polypeptide-based interpenetrating network (IPN) hydrogels with enhanced mechanical properties

Hydrogels are widely used for biomedical applications such as drug delivery, tissue engineering, or wound healing owing to their mimetic properties in relation to biological tissues. The generation of peptide-based hydrogels is a topic of interest due to their potential to increase biocompatibility. However, their usages can be limited when compared to other synthetic hydrogels because of their inferior mechanical properties. Herein, we present the synthesis of novel synthetic polypeptide-based interpenetrating network (IPN) hydrogels with enhanced mechanical properties. The polypeptide single network is obtained from alkyne functional polypeptides crosslinked with di, tri and tetra azide functional PEG by copper-catalysed alkyne-azide cycloaddition (CuAAC). Interpenetrating networks were subsequently obtained by loading of the polypeptide single network with PEG-dithiol and orthogonally UV-crosslinking with varying molar ratios of pentaerythritol tetraacrylate. The characteristics, including the mechanical strength (i.e. compressive strength (UCS), fracture strain (ε_break), and Young's modulus (E)) and cell compatibility (i.e. metabolic activity and Live/Dead of human Mesenchymal Stem Cells), of each synthetic polypeptide-based IPN hydrogel were studied and evaluated in order to demonstrate their potential as mechanically robust hydrogels for use as artificial tissues. Moreover, ¹H NMR diffusometry was carried out to examine the water mobility (D_H2O) within the polypeptide-based hydrogels and IPNs. It was found that both the mechanical and morphological properties could be tailored concurrently with the hydrophilicity, rate of water diffusion and 'swellability'. Finally it was shown that the polypeptide-based IPN hydrogels exhibited good biocompatibility, highlighting their potential as soft tissue scaffolds.

Fabrication of self-healing hydrogel from quaternized N-[3(dimethylamino)propyl]methacrylamide copolymer for antimicrobial and drug release applications

Self-healing hydrogels have attracted great attention in recent years because of their wide application in bioscience and biotechnology. In this study, P(DMAPMA-stat-DAA) were synthesized by Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization and quaternized to import antimicrobial properties. Then quaternized P(DMAPMA-stat-DAA) was used to prepare hydrogel containing acylhydrazone groups with Polyethylene oxide (PEO) diacylhydrazide as a cross-linking agent. The acylhydrazone groups imparted a variety of properties, including group responsiveness and self-healing properties to the hydrogel. At the same time, the quaternary ammonium endowed the hydrogel with the antimicrobial property. The mechanical property, self-healing properties, and antimicrobial property of hydrogels were investigated intensively. Results showed hydrogels formed in neutral conditions, and the luminescent property was introduced with PEO₂₃ dinaphthhydrazide (DNH) cross-linking. The hydrogels showed a controlled pH-sensitive DOX·HC l and Ovalbumin (OVA) release profile. In addition, the hydrogel showed the antimicrobial property and may have important applications in the biomedical field in the near future.

Robust alginate/hyaluronic acid thiol-yne click-hydrogel scaffolds with superior mechanical performance and stability for load-bearing soft tissue engineering

Hydrogels based on hyaluronic acid (HA) exhibit great potential as tissue engineering (TE) scaffolds as a consequence of their unique biological features. Herein, we examine how the advantages of two natural polymers (i.e. HA and alginate) are combined with the efficiency and rapid nature of the thiol-yne click chemistry reaction to obtain biocompatible matrices with tailored properties. Our injectable click-hydrogels revealed excellent mechanical performance, long-term stability, high cytocompatibility and adequate stiffness for the targeted application. This simple approach yielded HA hydrogels with characteristics that make them suitable for applications as 3D scaffolds to support and promote soft tissue regeneration.

Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Given their durability and long-term stability, self-healable hydrogels have, in the past few years, emerged as promising replacements for the many brittle hydrogels currently being used in preclinical or clinical trials. To this end, the incompatibility between hydrogel toughness and rapid self-healing remains unaddressed, and therefore most of the self-healable hydrogels still face serious challenges within the dynamic and mechanically demanding environment of human organs/tissues. Furthermore, depending on the target tissue, the self-healing hydrogels must comply with a wide range of properties including electrical, biological, and mechanical. Notably, the incorporation of nanomaterials into double-network hydrogels is showing great promise as a feasible way to generate self-healable hydrogels with the above-mentioned attributes. Here, the recent progress in the development of multifunctional and self-healable hydrogels for various tissue engineering applications is discussed in detail. Their potential applications within the rapidly expanding areas of bioelectronic hydrogels, cyborganics, and soft robotics are further highlighted.

A multi-interpenetrating network (IPN) hydrogel with gelatin and silk fibroin

A mechanically strong composite hydrogel was produced based on an interpenetrating network (IPN) between gelatin and silk fibroin. When two layers of the IPN were created, the resulting hydrogel exhibited much improved mechanical properties. This hydrogel is biodegradable and non-cytotoxic and allows for cell adhesion and proliferation on the surface.

Contact Lens Materials: A Materials Science Perspective

More is demanded from ophthalmic treatments using contact lenses, which are currently used by over 125 million people around the world. Improving the material of contact lenses (CLs) is a now rapidly evolving discipline. These materials are developing alongside the advances made in related biomaterials for applications such as drug delivery. Contact lens materials are typically based on polymer- or silicone-hydrogel, with additional manufacturing technologies employed to produce the final lens. These processes are simply not enough to meet the increasing demands from CLs and the ever-increasing number of contact lens (CL) users. This review provides an advanced perspective on contact lens materials, with an emphasis on materials science employed in developing new CLs. The future trends for CL materials are to graft, incapsulate, or modify the classic CL material structure to provide new or improved functionality. In this paper, we discuss some of the fundamental material properties, present an outlook from related emerging biomaterials, and provide viewpoints of precision manufacturing in CL development.

3D-extrusion printing of stable constructs composed of photoresponsive polypeptide hydrogels

Printing of novel linear polypeptide hydrogel bioinks and stabilisation of structures by post-printing UV-triggered crosslinking through catalyst free thiol–yne click chemistry of cysteine and propiolated 4-arm PEG.