Physical Cross-linking Points Research Articles

Strong and tough conductive hydrogels via an amphiphilic macromolecular crosslinker for flexible strain sensor

The strength and toughness of hydrogel are usually achieved through the synergy of chemical cross-linking and physical cross-linking from multi-component contributions. However, there are few reports on hydrogels that rely on a single component to obtain two cross-linked structures. Herein, we successfully synthesized an amphiphilic macromolecular crosslinker (polyoxyethylene 20 oleyl ether acrylate (O20AC)), and introduced it into a system composed of acrylamide (AAm), sodium chloride (NaCl) and water to prepare the P(AAm-O20AC)/NaCl hydrogel with high strength, outstanding toughness, and excellent good conductivity. Notably, the O20AC possess both crosslinker properties, which not only provide hydrogels with a stable chemical cross-linking network, but also its long hydrophobic chain act as physical crosslinking points to give the hydrogel a new pathway of energy dissipation.Depending on the O20AC crosslinker, the P(AAm-O20AC)/NaCl hydrogels exhibit good strength (tensile stress 714.46 kPa), toughness (elongation at break 3005 %), and fatigue resistance (overlap of hysteresis curves in repeated loading–unloading cycles). Besides, the existence of sodium ions and chloride ions also provides the hydrogel with highly sensitive deformation-dependent conductivity (GF is 5.8 in the 400 %-500 % strain range). Collectively, the strategy of preparing chemical and physical double-crosslinked hydrogels by amphiphilic O20AC will provide a bran-new insights for the development of hydrogel sensing materials.

Sustainable Polymers with High Performance and Infinite Scalability

AbstractOur society has been pursuing high‐performance biodegradable polymers made from facile methods and readily available monomers. Here, we demonstrate a library of enzyme‐degradable polymers with desirable properties from the first reported step polyaddition of diamines, COS, and diacrylates. The polymers contain in‐chain ester and thiourethane groups, which can serve as lipase‐degradation and hydrogen‐bonding physical crosslinking points, respectively, resulting in possible biodegradability as well as upgraded mechanical and thermal properties. Also, the properties of the polymers are scalable due to the versatile method and the wide variety of monomers. We obtain 46 polymers with tunable performance covering high‐Tm crystalline plastics, thermoplastic elastomers, and amorphous plastics by regulating polymer structure. Additionally, the polymerization method is highly efficient, atom‐economical, quantitatively yield, metal‐ and even catalyst‐free. Overall, the polymers are promising green materials given their degradability, simple and modular synthesis, remarkable and tunable properties, and readily available monomers.

Sustainable Polymers with High Performance and Infinite Scalability.

Our society has been pursuing high-performance biodegradable polymers made from facile methods and readily available monomers. Here, we demonstrate a library of enzyme-degradable polymers with desirable properties from the first reported step polyaddition of diamines, COS, and diacrylates. The polymers contain in-chain ester and thiourethane groups, which can serve as lipase-degradation and hydrogen-bonding physical crosslinking points, respectively, resulting in possible biodegradability as well as upgraded mechanical and thermal properties. Also, the properties of the polymers are scalable due to the versatile method and the wide variety of monomers. We obtain 46 polymers with tunable performance covering high-Tm crystalline plastics, thermoplastic elastomers, and amorphous plastics by regulating polymer structure. Additionally, the polymerization method is highly efficient, atom-economical, quantitatively yield, metal- and even catalyst-free. Overall, the polymers are promising green materials given their degradability, simple and modular synthesis, remarkable and tunable properties, and readily available monomers.

Sustainable ultra-strong polyesteramide elastomers with rapid degradation and high resilience

Degradable polymers typically require prolonged exposure to harsh conditions for degradation, but this extended duration and severe treatment often result in incomplete breakdown, leading to the formation of harmful microplastics that pose significant environmental pollution risks. In this work, degradable alternating polyamide hard segments and soft crystallizable segments were combined by amide groups to design thermoplastic bio-based polyesteramide elastomers (PEAEs) with customizable properties and rapid degradability. The hard segment of the polymer features an alternating arrangement of amide and ester bonds. This arrangement facilitates the formation of small molecule monomers when the ester bond is broken, preventing microplastic contamination that may arise from incomplete degradation of the hard segment. Additionally, the inclusion of side groups in the soft segment disrupts the crystallization, enhancing its susceptibility to solvent penetration and thereby accelerating the degradation rate. After step-cyclic tensile tests, additional physical cross-linking points were formed in PEAEs to enhance their mechanical strength and elasticity. Furthermore, PEAEs demonstrated outstanding processing capabilities, including compounding, extrusion, pelletization, and spinning. This innovative approach introduces fresh design concepts for the industrial application of bio-based degradable polymers, playing a pivotal role in advancing resource conservation and environmental protection.

Construction of porous and hydrophilic polypropylene-based membrane with microphase separation in amorphous region

Polypropylene (PP) membrane with low porosity and poor hydrophilicity limits the performance of lithium-ion batteries. In mechanism, the function of amorphous structure of precursor film on microporous structure of PP membrane is yet unknown. In this work, the porous and hydrophilic PP/reduced hydroxyl-modified polypropylene (PPrOH) membrane was fabricated by blending esterified-PPOH firstly and then hydrolyzing to introduce the hydrogen bond in amorphous region and avoid phase separation. The results showed that the hydrogen bond was successfully introduced into the amorphous region of PP/PPrOH film and led to an increase of physical crosslinking points. Therefore, a large region of microphase separation emerged in the PP/PPrOH membrane after cold stretching, which resulted in a 45.6 % increase of porosity at maximum compared to the PP membrane. Besides, the hydrophilicity of PP/PPrOH membrane was improved with the blend content exceeding 10 wt%. Eventually, the lithium-ion conductivity of PP/PPrOH-10 membrane enhanced by 37.5 % over the base PP membrane. This work provides a novel perspective for preparing hydrophilic PP membrane with high porosity.

High mechanical properties nanocomposite hydrogel achieved based on montmorillonite and tailored microgel suspensions reinforcing polyacryamide networks

Hydrogels have attracted much attention because of their softness and water retention ability, but their poor mechanical properties have severely limited their application in various aspects. Herein, we reported a strategy to fabricate polyacrylamide/montmorillonite nanocomposite hydrogel by introducing montmorillonite via a facile and universal two-step method composed of microgel suspension system and in situ free radical polymerization. Compared with microgel, microgel suspension is simpler to prepare and better dispersed in hydrogels, which it could act as physical crosslinking points, interpenetrate with polyacrylamide networks embedded, and serve as sacrifical bonds to strengthen the hydrogels. Simultaneously, montmorillonite synergistically facilitated the formation of a robust and uniform polymer structure through the interaction of hydrogen bonds and polyacrylamide networks, resulting in significant improvements in mechanical properties. As a result, the satisfactory mechanical properties of the nanocomposite hydrogels are achieved at a relative high water content (80 wt%), including a superior compressive strength (95% deformation) of 37.96 MPa, tensile strength of 311.1 ± 8.2 kPa, elongation at break of 316.5 ± 21.1%, Young's modulus of 165.8 ± 7.5 kPa, and excellent toughness of 644.1 ± 21.8 kJ/m3, respectively. In addition, it was found that the hydrogel had excellent cycle compression stability through 10 cycles of loading and unloading at 80% strain. These new high-performance nanocomposite hydrogels are expected to be used in the fields of structural or load-bearing materials.

Dynamic Shape Change of Liquid Crystal Polymer Based on An Order‐Order Phase Transition

AbstractLiquid crystal actuators conventionally undergo shape changes across an order‐disorder phase transition between liquid crystal (LC) and isotropic phases. In this study, we introduce an innovative Liquid Crystal Polymer (LCP) actuator harnessing an order‐order LC phase transition mechanism. The LCP film is easily stretchable within the LC phase, facilitated by the π–π stacking of phenyl groups serving as robust physical crosslinking points, and thereby transforms to a stable monodomain structure. The resultant monodomain LCP actuator shows a distinctive reversible dynamic shape change, exhibiting extension followed by contraction along the LC director on cooling. The extension is propelled by the reversible smectic C to smectic A phase transition, and the contraction is attributed to the re‐entry to the smectic C phase from smectic A phase. Thermal annealing temperature determines this peculiar dynamic shape change, which occurs during both heating and cooling processes. This pivotal attribute finds manifestation in gripper and flower‐shaped actuators, adeptly executing grabbing and releasing as well as blooming and closure motions within a single thermal stimulation. In essence, our study introduces an innovative approach to the realm of LCP actuators, ushering in a new avenue for the design and fabrication of versatile and dynamically responsive LCP actuators.

Dynamic Shape Change of Liquid Crystal Polymer Based on An Order-Order Phase Transition.

Liquid crystal actuators conventionally undergo shape changes across an order-disorder phase transition between liquid crystal (LC) and isotropic phases. In this study, we introduce an innovative Liquid Crystal Polymer (LCP) actuator harnessing an order-order LC phase transition mechanism. The LCP film is easily stretchable within the LC phase, facilitated by the π-π stacking of phenyl groups serving as robust physical crosslinking points, and thereby transforms to a stable monodomain structure. The resultant monodomain LCP actuator shows a distinctive reversible dynamic shape change, exhibiting extension followed by contraction along the LC director on cooling. The extension is propelled by the reversible smectic C to smectic A phase transition, and the contraction is attributed to the re-entry to the smectic C phase from smectic A phase. Thermal annealing temperature determines this peculiar dynamic shape change, which occurs during both heating and cooling processes. This pivotal attribute finds manifestation in gripper and flower-shaped actuators, adeptly executing grabbing and releasing as well as blooming and closure motions within a single thermal stimulation. In essence, our study introduces an innovative approach to the realm of LCP actuators, ushering in a new avenue for the design and fabrication of versatile and dynamically responsive LCP actuators.

Mussel-inspired adhesive and anti-swelling hydrogels for underwater strain sensing.

The application of hydrogels in an underwater environment is limited due to their swelling behavior and the existence of a hydration layer. In this study, a hydrogel based on poly(sulfobetaine methacrylate) (PSBMA), tannic acid (TA) and montmorillonite (MMT) was prepared with excellent anti-swelling properties and underwater self-adhesion properties. The PSBMA hydrogel has excellent anti-swelling properties due to the strong electrostatic interaction between charged groups of PSBMA chains. Inspired by marine mussels, tannic acid modified montmorillonite (TA@MMT) was introduced. Natural polyphenol tannic acid, as a catechol donor, provides a large number of catechol groups for hydrogels. Montmorillonite acts as the physical cross-linking point of PSBMA chains through electrostatic interaction to improve the cohesion of the hydrogel. By combining the adhesion mechanism of zwitterions and catechol, the hydrogel maintains adhesion in air and underwater environments. In addition, a strain sensor was prepared based on the PSBMA/TA@MMT hydrogel, which can closely fit the human skin and stably monitor different movements in air and in underwater environments. Through a Bluetooth communication system, long-distance information transmission can be achieved. Therefore, the PSBMA/TA@MMT hydrogel broadens the application prospect of wearable devices in the underwater environment.

Organic-inorganic polyureas with benzyl hindered urea bonds and POSS cages in the main chains: Synthesis, shape memory and reprocessing properties

The organic–inorganic polyureas with benzyl hindered urea bonds (HUBs) and POSS cages as the main-chain structural units were synthesized via the polyaddition of 4,4′-diphenylmethane diisocyanate with α,ω-dibenzyl diamines including α,ω-dibenzylamino-terminated poly(ethylene oxide), N,N’-dibenzylhexamethylene-1,6-diamine and 3,13-dibenzylaminoanilino double decker silsesquioxane. The organic–inorganic PUreas were microphase-separated, in which the POSS cages were self-assembled into the microdomains via POSS-POSS interactions. The POSS microdomains acted as the physical crosslinking points and thus the organic–inorganic PUreas behaved as the crosslinked elastomers; the physical crosslinking remained thermally stable even at elevated temperature as high as 200 °C. In spite of the high thermal stability, the networks can be reprocessed with the plasticity in solids via the dynamic exchange of benzyl HUBs. Due also to the physical crosslinking, the organic–inorganic PUreas possessed the excellent shape memory properties; the original shapes were re-configurable by taking advantage of the benzyl HUBs in the main chains.

Fabrication of recyclable and biodegradable PBAT vitrimer via construction of highly dynamic cross-linked network

The emergence of biodegradable materials has not satisfactorily addressed the issue of plastic waste management, owing to their high cost and poor mechanical performance and durability that impair their potential to replace traditional plastics. In this study, a dynamic cross-linked network with robust remoldability was fabricated in PBAT. Transesterification catalysts were employed to catalyze the reaction of β-hydroxyl groups produced from the branching reaction of polyester end groups and epoxy branching agents with the abundant ester groups of PBAT to produce biodegradable PBAT vitrimers. Swelling experiments, DMA tests and stress relaxation experiments confirmed the presence of the dynamic cross-linking structure, while rheological analysis demonstrated that the highly catalyst-loaded PBAT vitrimer dynamic cross-linked network could achieve chain relaxation by exchanging bonds quickly at physical entanglement and chemical cross-linking points. Multiple recycling experiments further demonstrated that the robust dynamic cross-linked network of PBAT vitrimers possessed both strong stress relaxation ability and remoldability, thus conferring excellent recyclability to PBAT. Upon recycling 4 times, the elongation at break of the PBAT vitrimer remained at 93.46 % (with a tensile strength retention of 85.99 %), whereas the raw PBAT could only maintain an elongation at break of 28.01 % (with a tensile strength retention of 56.09 %). TGA and hydrolysis experiments revealed that PBAT vitrimers exhibited better degradation performance relative to raw PBAT. PBAT vitrimers, featuring excellent mechanical performance and durability, can be recycled several times in easily recyclable settings, thus reducing production costs, and finally biodegrade after disposal, thereby serving as a viable replacement for traditional plastics. This straightforward and practicable approach provides a solution for plastic waste management.

Hydrophobic-oleophobic polyurethane with mechanical reinforcement: Synthesized by thiol-ene click chemistry

Polyurethane (PU) is a universal polymer material with excellent performance, widely used as foam materials, adhesives, coatings, rubber, etc. Nevertheless, the considerable presence of ester and ether bonds in the polyurethane backbone, combined with the hydrophilic –OH group located at the end of molecular chain, renders it intrinsically hydrophilic. This inherent hydrophilicity presents substantial constraints on its potential utilization in flexible electronic displays, wearable electronic devices, and various other fields. It is a great challenge for hydrophobic polyurethane to simultaneously achieve mechanical reinforcement. Herein, the fluorinated chain extenders were introduced into PU, which not only resulted in low surface energy but also improved the surface roughness through the microphase separation due to the difference in the solubility parameters between the fluorine-containing side chain and the main chain. The long-chain perfluorinated extender was achieved via thiol-ene click chemistry, thus regulating the fluorine content by adjusting the amount of chain extender. The low surface energy and high roughness, developed by microphase separation, make the DFX-PU films exhibit excellent hydrophobicity (120°) and oleophobicity (121°). Additionally, the fluorine-containing side chains aggregate to form physical cross-linking points, reinforcing the mechanical properties of the film. When the fluorine-containing chain extender accounted for 10% (DF0.5-PU), the PU film showed the best mechanical properties. Compared with the control group, tensile strength is enhanced by 165%, the toughness is increased by 287%, and the elongation-at-break was improved by 156%.

Nano-injectable pH/NIR-responsive hydrogel for chemo-photothermal synergistic drug delivery.

Conventional cancer treatments are highly toxic and ineffective; therefore, it is essential to develop less toxic and minimally invasive treatment methods. A pH/Near Infra-red (NIR) dual-responsive, nano-injectable smart hydrogel was fabricated by incorporating CuS nanoparticles into the hydrogel networks formed by a random copolymer of N-isopropylacrylamide (NIPAM) and double-bond functionalized uracil. Microstructural characterizations of synthesized polymer and hydrogels were carried out using transmission electron microscope (TEM), scanning electron microscope (SEM), nuclear magnetic resonance (NMR) and fourier transform infrared spectroscopy (FT-IR). Multiple hydrogen bonding interactions between uracils function as physical cross-linking points to construct the network structure of the polymeric nanogel without the addition of additional cross-linking agents, ensuring the material's safety. The amino group on the structure of uracil gives the uracil-modified polymeric hydrogel excellent pH responsiveness. Notably, as a temperature-responsive material, poly (N-isopropylacrylamide) (PNIPAM) nanogel solution can achieve in situ gel formation (within 100 s at 37°C) above its lower critical solution temperature (LCST), granting injectability to polymeric solutions. Moreover, using a hierarchical construction strategy, the variable loading of DOX and CuS was achieved. First, a heterogeneous system was created by encapsulating doxorubicin (DOX) inside the nanogel via hydrophobic and π-π stacking interactions, followed by the introduction of CuS nanoparticles as photosensitizers outside of the nanogels. Due to the presence of CuS nanoparticles, the gel is able to convert NIR light into local heat to enhance the destruction of tumor cells while simultaneously achieving rapid in situ gel formation. The in situ-forming hydrogel showed promising tissue biocompatibility. The in vitro antitumor test demonstrated the capacity of the nanocomposite hydrogel for chemo-photothermal synergistic therapy. Therefore, this prepared platform has the potential to become a safe and effective, smart-responsive drug carrier for chemotherapy and PTT synergy, a minimally invasive material for tumor treatment.

Robust and stretchable Ti3C2Tx MXene/PEI conductive composite dual-network hydrogels for ultrasensitive strain sensing

The development of high-performance hydrogels with superior strength, stretchability, and conductivity is critical to the practical application of flexible sensors. In this study, we employed a strategy that involves introducing physical cross-linking points, creating a double network, and adding nanofillers. By utilizing a two-step method that involves thermal cross-linking and post-immersion cross-linking, and using two common metal ions to crosslink different polymer networks, we successfully developed a conductive composite dual-network hydrogel. In addition, the incorporation of modified MXene has significantly improved the strength and toughness of the hydrogel. The resulting hydrogel exhibits impressive mechanical properties, including a tensile strength of 2.64 MPa and elongation at break of 689%, as well as high toughness of 10.25 MJ·m−3 and conductivity of 1.89 S/m. When applied as a flexible sensor in electronic skin, the sensor demonstrates a wide operating range (>300%), high sensitivity (GF = 4.64), and excellent linear detection ability (R2 = 0.99).

Liquid crystal elastomer film actuators with anti-strain robustness

Thermotropic monodomain liquid crystal elastomers (mLCEs) exhibit reversible contractile actuation properties along the collimated direction of mesogens when stimulated by external temperature, similar to those of biological skeletal muscle, and have great potential applications in flexible actuators and artificial muscles. However, the existence of flexible chain segments causes mLCEs to elongate under a small load, making calibration under different loads necessary when they are used as an actuator. In this study, a physical crosslinking point between flexible segments is provided through the introduction of designed long-chain molecules (LHS) containing amide bonds on the main chain of LCEs. The 5% LHS-mLCEs with enhanced anti-strain robustness achieved essentially constant original length over a range of loads (0–0.25 MPa) and maintained a reversible contraction ratio of 39.4%, comparable to biological skeletal muscle. Furthermore, a nonreconfigurable topological structure of a metal electrothermal layer with lossless compression properties was designed and prepared on the surface of mLCEs. The film actuators exhibit higher frequency oscillatory actuation (0.1 Hz 9.2%; 0.2 Hz 6.3%; 0.5 Hz 2%) relative to previously reported actuators, benefitting from the optimized heat dissipation structure of LCEs/Ag/air. This study provides valuable references and ideas for applying thermotropic mLCEs in practice.

A green and efficient synthetic strategy for the preparation of PBS ionomers with high molecular weight, high ionic group content and good combined properties

The conventional direct esterification and polycondensation is difficult to obtain kilogram scale of poly(butylene succinate) (PBS) ionomers with both high molecular weight and high ionic group content. Therefore, the investigation of relationship between the content of ion group and performance (such as mechanical properties) is impossible. In this work, the preparation of PBS ionomers with high molecular weight and high ion group content was achieved by transesterification method using sodium dimethyl isophthalate-5-sulfonate (SIPM) as the ionic monomer, and the polycondensation time decreased significantly with the increase of SIPM content. The mechanism of reducing the polycondensation time of PBS prepared by transesterification is proposed. The tensile strength, flexural strength and impact strength of PBS ionomers increased significantly with increasing SIPM content. At high ion group content, the formed multiplets-clusters among ion groups, which act as reversible physical crosslinking points and nano-fillers, substantially improved the mechanical properties. The tensile strength, flexural strength and impact strength improved and reached to 50.1 MPa, 40.9 MPa and 13.1 kJ/m2 at high ion group content from 34.2 MPa, 24.1 MPa and 5.8 kJ/m2 for PBS, respectively. It showed a totally opposite trend from the previous research. In addition, the hydrophilicity and processing properties of PBS ionomers improved significantly with the increase of the ion group content, and the contact angle reduced to 41.1° after the introduction of ions.

Construction and Evaluation of Alginate Dialdehyde Grafted RGD Derivatives/Polyvinyl Alcohol/Cellulose Nanocrystals IPN Composite Hydrogels.

To enhance the mechanical strength and cell adhesion of alginate hydrogel, making it satisfy the requirements of an ideal tissue engineering scaffold, the grafting of Arg-Gly-Asp (RGD) polypeptide sequence onto the alginate molecular chain was conducted by oxidation of sodium periodate and subsequent reduction amination of 2-methylpyridine borane complex (2-PBC) to synthesize alginate dialdehyde grafted RGD derivatives (ADA-RGD) with good cellular affinity. The interpenetrating network (IPN) composite hydrogels of alginate/polyvinyl alcohol/cellulose nanocrystals (ALG/PVA/CNCs) were fabricated through a physical mixture of ion cross-linking of sodium alginate (SA) with hydroxyapatite/D-glucono-δ-lactone (HAP/GDL), and physical cross-linking of polyvinyl alcohol (PVA) by a freezing/thawing method, using cellulose nanocrystals (CNCs) as the reinforcement agent. The effects of the addition of CNCs and different contents of PVA on the morphology, thermal stability, mechanical properties, swelling, biodegradability, and cell compatibility of the IPN composite hydrogels were investigated, and the effect of RGD grafting on the biological properties of the IPN composite hydrogels was also studied. The resultant IPN ALG/PVA/CNCs composite hydrogels exhibited good pore structure and regular 3D morphology, whose pore size and porosity could be regulated by adjusting PVA content and the addition of CNCs. By increasing the PVA content, the number of physical cross-linking points in PVA increased, resulting in greater stress support for the IPN composite hydrogels of ALG/PVA/CNCs and consequently improving their mechanical characteristics. The creation of the IPN ALG/PVA/CNCs composite hydrogels' physical cross-linking network through intramolecular or intermolecular hydrogen bonding led to improved thermal resistance and reduced swelling and biodegradation rate. Conversely, the ADA-RGD/PVA/CNCs IPN composite hydrogels exhibited a quicker degradation rate, attributed to the elimination of ADA-RGD by alkali. The results of the in vitro cytocompatibility showed that ALG/0.5PVA/0.3%CNCs and ADA-RGD/PVA/0.3%CNCs composite hydrogels showed better proliferative activity in comparison with other composite hydrogels, while ALG/PVA/0.3%CNCs and ADA-RGD/PVA/0.3%CNCs composite hydrogels displayed obvious proliferation effects, indicating that PVA, CNCs, and ADA-RGD with good biocompatibility were conducive to cell proliferation and differentiation for the IPN composite hydrogels.

Natural UCST hydrogel with long-term stability, recyclability, thermoplasticity, frost resistance and adhesive properties

Thermosensitive hydrogels have received great research interest, but the elaborate experimental design, inferior mechanical and anti-freeze properties have limited the practical application of thermosensitive hydrogels. For address these challenges, here, polyol (glycerol, xylitol, and sorbitol) and deionized water were selected as mixed solvents to fabricate multifunctional natural gelatin UCST hydrogels by simple physical blending. XRD and FT-IR indicate that the polyol assists the gelatin molecular chain to form short-range β-sheets without affecting the triple helix structure. The short-range β-sheets acts as the physical cross-linking point, which leads to a denser three-dimensional network of the hydrogel. The results indicate that the phase transition temperature and mechanical properties of UCST hydrogels can be modulated by simply changing the amount of polyol in the solvent mixture. In addition, the polyol forms extensive hydrogen bonds with water, which gives the hydrogel excellent freeze resistance, anti-drying and adhesive properties. In this paper, natural UCST gelatin hydrogels with long-term stability, recyclability, thermoplasticity, anti-freeze and adhesive properties were prepared by a simple scheme, and the facile process and cheap cost lays the foundation for the application of UCST hydrogels in drug delivery, skincare products and wearable devices.

A dual-crosslinked zwitterionic hydrogel with load-bearing capacity and ultra-low friction coefficient

Due to similar properties to that of cartilage, hydrogels are considered as promising candidates for cartilage repair. However, it is still a challenge to synthesize hydrogel combining high strength and low friction like natural cartilage. In this work, we synthesized a dual-crosslinked zwitterionic P(AAm-co-AAc-co-SBMA)/Fe3+ hydrogel, in which a zwitterionic monomer [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)(SBMA) was used to improve the hydrophilicity of the polymer chain. The results showed that the hydrogel exhibited significantly improved tensile strength (9.9 MPa) and compressive strength (207.6 MPa), which was due to the physical crosslinking points introduced by SBMA. Moreover, the zwitterionic hydrogel achieved an ultra-low friction coefficient (CoF) of 0.04 in water. The results provide a new approach for cartilage repair materials with considerable strength and low friction.

Initiator-Free Photocuring 3D-Printable PVA-Based Hydrogel with Tunable Mechanical Properties and Cell Compatibility.

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.