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Anisotropic and super-strong conductive hydrogels were prepared by mechanical stretching combined with the Hofmeister effect. Anisotropic hydrogels exhibit anisotropic mechanical and electrochemical properties.

l-methionine sulfoxide (Met(O)–OH) as a CPA for cell cryopreservation.

Hydrogel-based flexible electronics, especially stretchable human motion sensors, have been intensely investigated in the past few decades. However, challenges remain in the preparation of anti-freezing hydrogels with balanced properties of high sensitivity, large sensing range and suitable mechanical properties in a broad temperature range. In this study, we fabricate a fully-physically crosslinked poly(hydroxyethyl acrylamide)/silk fibroin-LiCl (PHEAA/SF-LiCl) hydrogel with excellent mechanical properties at both room temperature and subzero temperature. Strain sensitivity and durability at both room temperature (25 °C) and low temperature (−30 °C) have been characterized. Strain sensors which can monitor human movements in different situations, including wireless sensing and low temperature sensing have been fabricated, indicating their huge potential in flexible electronics, remote health control and electronic skins.

Recently, flexible and wearable electronics have gained considerable research interest due to their potential applications in wearable devices, energy storage materials, electronic skins, sensors, etc. Compared to elastomers, hydrogels demonstrate more potential for flexible electronics because of their biomimetic structures, suitable mechanical properties and excellent biocompatibility. Among all the designs, nanocomposite hydrogel-based strain and pressure sensors which can transmit external stimulus to electrical signals have been intensely investigated due to their high mechanical strength, considerable conductivity and outstanding sensitivity. Numerous reports have been dedicated to the designs, preparations and applications of nanocomposite hydrogels. This review provides an up-to-date and comprehensive summary of research progresses of nanocomposite hydrogel-based strain and pressure sensors including designing strategies, preparing methods and applications of the five nanofiller based hydrogel sensors including carbon nanotube based, graphene oxide based, MXene based, polymer nanofiller based and other nanofiller based sensors. Representative cases are carefully selected and discussed regarding the fabrication, merits and demerits, respectively. Finally, perspectives and challenges are presented for the designs of future nanocomposite hydrogel-based strain and pressure sensors.

Flexible and stretchable electronics have received tremendous attention for next-generation human-friendly electronic applications. However, fabrication of transparent, fully recyclable and stretchable electronic sensors with low-temperature stability using biocompatible natural polymer-based hydrogels still remains a great challenge. In this study, a green and fully recyclable stretchable electronic sensor with high transparency and ultra-low operating temperature is constructed using ionic conductive gelatin organohydrogels. These gelatin organohydrogels are prepared by a simple strategy of immersing gelatin pre-hydrogels in citrate (Na3Cit) water/glycerol solutions. The existence of Na3Cit in the organohydrogel not only induces the formation of multiple non-covalent cross-linking points, endowing the organohydrogel with high mechanical performances, but also makes the organohydrogel have excellent ionic conductivity. The organohydrogel is also highly transparent and exhibits outstanding antifreezing properties. The mechanical robustness, conductivity and transparency of the organohydrogel can be well maintained even at −60 °C. As a result, a stretchable and transparent electronic sensor based on this organohydrogel is fabricated, which is strain-sensitive with a large linear sensing window and excellent stability. More importantly, the organohydrogel-based electronic sensor can be fully recycled due to the reversible non-covalently crosslinked structure, and the recycled organohydrogel regains its mechanical and sensing properties. The obtained sensors could precisely detect various human activities even below −30 °C, indicating the potential applications of the organohydrogel-based electronic sensor in flexible and stretchable electronics in a broad range of temperature.

The development of three-dimensional conductive scaffolds is vital to support the adhesion, proliferation and myocardial differentiation of stem cells in cardiac tissue engineering. Herein, we describe a facile approach for preparing a poly(3,4-ethylenedioxythiophene)/alginate (PEDOT/Alg) porous scaffold with a wide range of desirable properties. In the PEDOT/Alg scaffold, chemically crosslinked alginate networks are formed using adipic acid hydrazide as the crosslinker, and PEDOT is synthesized in situ in the alginate matrix simultaneously. PEDOT exists in the alginate matrix as particles and its morphology can be modulated by adjusting the ratio of PEDOT/alginate. The results also show that the swelling properties, degradation behaviors, mechanical strength and conductivity of the PEDOT/Alg scaffold can be controlled via adjusting the PEDOT/alginate ratio. The introduction of PEDOT can overcome the brittle nature of the pure alginate scaffold. Moreover, the PEDOT/Alg scaffold exhibits excellent conductivity (as high as 6 × 10−2 S cm−1). The introduction of PEDOT improves the protein absorption capacity of the alginate scaffold. To explore its potential application in cardiac tissue engineering, brown adipose-derived stem cells (BADSCs) are seeded in the prepared PEDOT/Alg porous scaffold. The results suggest that the PEDOT/Alg porous scaffold can support the attachment and proliferation of BADSCs. Moreover, it is beneficial for the cardiomyogenic differentiation of BADSCs, especially under electrical stimulation. Overall, we conclude that the PEDOT/Alg porous scaffold may represent an ideal platform to modulate the biological behaviors of BADSCs.

Graphene aerogels (GAs) were synthesized via a one-step hydrothermal method. Generally, the pore shape and diameter of GAs are difficult to control or the preparation process is complicated, requiring a multi-step operation. Herein, a soft-template one-step hydrothermal synthesis process was proposed to produce GAs with controllable pore sizes. Cyclohexane and n-butanol were added to a graphene oxide suspension to form a uniform aqueous dispersion under emulsification by sodium lauryl sulfate. The reduction process may have occurred around the organic droplets during the hydrothermal reaction, and a large number of organic droplets became countless physical barriers inside the hydrogel. In the later freeze-drying and high-temperature calcination procedures, the droplets evaporated to form a rich pore structure. Compared to the conventional templating method, the organic template was volatilized during the drying process such that no additional process for removing the template was required. In addition, GAs prepared by the template method possessed lower density (2.66 mg cm−3) and better compression performance and, as an adsorbent material, absorbed organic matter and petroleum from wastewater more efficiently than GAs obtained by the traditional one-step hydrothermal method; Q for n-hexane reached 116, and Q for xylene reached 147; also, the GAs prepared by the soft template method can absorb all crude oil in water samples within 30 s.

Herein, we develop a fluorescence sensing platform based on boron and nitrogen co-doped graphene quantum dots (B,N-GQDs), which are synthesized by a one-step bottom-up hydrothermal method. The prepared B,N-GQDs exhibit high-fluorescence quantum yield (75%) and long fluorescence lifetime (12.86 ns). In the B,N-GQDs–Hg2+ system, the fluorescence of B,N-GQDs is quenched with increasing Hg2+ concentration due to the affinity interaction between the surface functional groups of B,N-GQDs and Hg2+. Subsequently, the quenched fluorescence of the B,N-GQDs–Hg2+ system is gradually recovered with the addition of F− ions due to their strong affinity with Hg2+. The linear ranges for the detection of Hg2+ and F− ions are 0.2–2.6 μM and 0.25–7.0 mM, respectively, and the detection limits are 0.16 μM and 0.18 mM for Hg2+ and F− ions, respectively, which are below the regulatory level of 0.25 μM and 0.526 mM in the Integrated Wastewater Discharge Standard. Therefore, B,N-GQDs provide a facile and effective approach for the detection of both Hg2+ and F− ions with real-time monitoring and high-speed measuring.

In this study, nano-hydroxyapatite crystals (nHAs) were prepared in the presence of zwitterionic poly(3-carboxy-N,N-dimethyl-N-(3′-acrylamidopropyl) propanaminium inner salt) (PCBAA) at different concentrations and pH microenvironments, denoted as Z-nHAs. The phase, morphology, size of apatite, and interactions between nHAs and PCBAA were systemically characterized via XRD, TEM, FTIR, and TGA. The results showed that Z-nHAs could be formed at all pH ranges in the presence of PCBAA in our experiment. It was different from the case of pure nHAs that could not be formed under an acidic microenvironment without PCBAA. In addition, a higher pH value was beneficial for the formation of nHAs with a smaller size. Z-nHAs exhibited a fiber-like structure at the initial 2 h and became rod-like in shape after 5 h, whereas pure nHAs prepared in water had an irregular flake-like structure. Moreover, 14–20% PCBAA could be incorporated into Z-nHAs via interactions between the quaternary ammonium group (–R3N+) and carboxyl group (–COO−) in the PCBAA chain and Ca2+/PO43− of nHA. A possible mechanism for the formation of Z-nHAs in the presence of PCBAA was proposed based on the experimental results.

The lack of sufficient mechanical properties restricts the application of polysaccharide-based hydrogels in the field of biomedicine, especially load-bearing tissue repair. Nowadays, double network (DN) hydrogels have aroused great interest through special cooperation between two contrasting networks. Inspired by this idea, here, we devised a new strategy to prepare a pectin–Fe3+/polyacrylamide hybrid DN hydrogel using a simple two-step method. The introduction of Fe3+ ions into a pectin network to produce strong reversible ionic complexation, results in excellent toughness. Under optimal conditions, our hybrid DN hydrogels possessed tensile strength as high as 0.9 MPa, corresponding to a high strain of 1300%. Besides, our hybrid DN hydrogels also exhibited superb stiffness (elastic modulus ∼ 1.46 MPa), toughness (fracture energy ∼ 3785 J m−2), and water absorption capacity (85%). Loading–unloading tests showed that the internal fracture process of the hydrogels was continuous. Owing to the reversible structure of Fe3+–pectin complexation, the hybrid DN hydrogels also showed good fatigue resistance, notch-insensitivity and recoverability. This type of polysaccharide-based hydrogel has potential to broaden the application in the load-bearing tissue repair field.

To improve the circulation stability of polyamidoamine (PAMAM) based drug delivery systems in complex biological microenvironments, a series of generation-3.0 PAMAM-graft-poly[3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate] (PAMAM3.0-g-PDMAPS) copolymers are synthesized via atom transfer radical polymerization. The zwitterionic PDMAPS segments serve as a shell to stabilize the unimolecular micelles, whereas the PAMAM3.0 dendrimers constitute a hydrophobic core. The sizes of the PAMAM3.0-g-PDMAPS unimolecular micelles range from 6.5 to 8.5 nm. Furthermore, PAMAM3.0-g-PDMAPS can keep the micelle-like structure when it is diluted by large volumes of fluids. More importantly, the PDMAPS shell layer can suppress non-specific protein adsorption on the surface of the micelles. The excellent stability to dilution and anti-biofouling are beneficial for prolonged circulation time in a complex biological microenvironment. In addition, anticancer doxorubicin (DOX) can be encapsulated both in the PAMAM3.0 core via hydrophobic interactions and the PDMAPS shell layer via hydrogen bonds. Drug release studies confirm the pH-responsive nature of PMAMA3.0-g-PDMAPS micelles by achieving 65.23% DOX release at pH 5.1 as compared to 16.38% at pH 7.4. Based on these results, the cytotoxicity and anticancer effects against human hepatocellular carcinoma cells (HepG2) of the PAMAM3.0-g-PDMAPS system loaded with DOX are investigated. The results suggest that the PDMAPS shell layer can significantly decrease the cytotoxicity via decreasing/shielding of the positive charges on the PAMAM dendrimers. After internalization by HepG2 cells, DOX is released from the micelles to the nucleus and further inhibits the proliferation of HepG2. Therefore, these PAMAM3.0-g-PDMAPS unimolecular micelles are a potential platform for anticancer drug delivery.

Non-specific protein adsorption adversely affects the application of thermoresponsive polymers in the biomedical field. To overcome this disadvantage, thermoresponsive N-vinylcaprolactam (VCL) and anti-biofouling zwitterionic sulfobetaine methacrylate (SBMA) monomers with various VCL/SBMA ratios were used for the synthesis of poly(VCL-co-SBMA) (P(VCL-co-SBMA)) copolymers via free radical solution polymerization. The P(VCL-co-SBMA) copolymers exhibited both a lower critical solution temperature (LCST) and an upper critical solution temperature (UCST) in aqueous solutions. Furthermore, both the UCST and LCST of the copolymer shift to higher temperatures with the increase of PSBMA segments, and they shift to lower temperatures with the increase of salt concentrations in the solution. Based on these results, P(VCL-co-SBMA) hydrogels were prepared using N,N′-methylenebisacrylamide (MBAA) as the crosslinker. Compared with the PVCL hydrogel, the P(VCL-co-SBMA) hydrogels exhibit better mechanical properties. Notably, the P(VCL-co-SBMA) hydrogel retained the temperature sensitivity of PVCL, and it could be modulated by varying the PVCL/PSBMA segment ratios. In addition, all the hydrogels exhibit good cytocompatibility. More importantly, the protein adsorption and cell adhesion of the hydrogel can be controlled by temperature. The non-specific protein adsorption was effectively suppressed at physiological temperatures. The switchable anti-biofouling nature of P(VCL-co-SBMA) hydrogel together with their temperature sensitivity can be potentially used in drug, cell or enzyme delivery.

Poly(vinyl alcohol) (PVA) hydrogels have gained comprehensive attention in the biomedical area. However, their resistance to cell adhesion is a drawback for applications such as tissue engineering. Besides, the controllability of the porous structure of PVA-based hydrogels during lyophilization needs to be further improved. Herein, we prepared PVA–carrageenan (CAR) composite hydrogels as tissue engineering scaffolds using the freeze–thaw technique. The hydrogels were found to possess deformation resistance, preserving their shape during the lyophilization process without shrinkage. Besides, ATDC5 cells showed good adherence and proliferation activity on the composite hydrogels. In addition, the PVA–CAR composite hydrogels possess good hemocompatibility and did not cause any adverse effects in the inflammatory response from RAW 264.7 macrophage cells. Overall, the results obtained indicate that the PVA–CAR composite hydrogels show potential applications in the field of tissue engineering based on their good structural stability, excellent biocompatibility and mild fabrication process.

To improve the stability of micelles and decrease the burst release behaviours of hydrophobic drugs, poly(lactic acid)/poly(ethylene glycol) (PLA/PEG) block copolymer based shell or core cross-linked micelles are successfully fabricated. First, PLA–PEG diblock and PLA–PEG–PLA triblock copolymers terminated with acryloyl end groups are synthesized and characterized by 1H NMR and Fourier Transform Infrared (FTIR). These PLA/PEG block copolymers can spontaneously form micelles, exposing hydrophilic PEG segments outside while hiding hydrophobic PLA segments inside the micelles. The methacryloyl groups, exposed on the outer of shell in the PLA–PEG methacrylate copolymer micelles, are copolymerized with N-vinylpyrrolidone and lead to the formation of shell cross-linked (SCL) micelles. On the contrary, the core cross-linked (CCL) micelles are fabricated through the photo-crosslinking reaction of acryloyl end groups inside the core of PLA–PEG–PLA diacrylate copolymer micelles using poly(ethylene glycol) diacrylate as cross-linker. TEM and DLS are used to investigate the morphology and size of SCL and CCL micelles. Results suggest that the size of these micelles depends on the length of PLA segments in the PLA/PEG diblock micelles and the cross-linking degree. Besides, the shell cross-linking increases the size of the micelles, while the core cross-linking decreases the size of the micelles. Notably, both SCL and CCL micelles retain higher stability than that of uncross-linked micelles. Based on these results, hydrophobic tetrandrine (TED), as the model drug, is used to evaluate the controlled release behaviours of SCL or CCL micelles. Results show that both SCL and CCL micelles can decrease the burst release phenomenon in the initial period. The release performance can be controlled via changing the length of PLA segments in the copolymers. It is indicated that these SCL or CCL micelles are useful for a hydrophobic drug-carrier system.

Herein, we developed a novel biodegradable gene carrier for rapid endothelialization of endothelial cells (ECs) in vitro. Three triblock amphiphilic copolymers, methoxy-poly(ethylene glycol)-block-poly(3(S)-methyl-2,5-morpholinedione-co-glycolide)-graft-polyethyleneimine (mPEG-b-P(MMD-co-GA)-g-PEI) with different 3(S)-methyl-2,5-morpholinedione and glycolide contents were synthesized. Microparticles (MPs) were obtained via self-assembly of these copolymers. The hydrophobic core composed of P(MMD-co-GA) segments provide crosslinking points for numbers of PEG and short PEI chains to form a highly hydrophilic and positively charged corona/shell of MPs. Using these MPs, potential genes (ZNF580) for rapid endothelialization were efficiently transported into EA.hy926 cells. Because of the hydrophilic PEG chains and low molecular weight PEI in the triblock copolymers, the cytotoxicity of these MPs and their complexes with pEGFP–ZNF580 was decreased significantly. The transfection efficacy of MPs/pEGFP–ZNF580 complexes was as high as Lipofectamine™ 2000 reagent to EA.hy926 cells in vitro. The proliferation and migration of EA.hy926 cells were improved greatly by the expression of pEGFP–ZNF580 after 60 hours. Our results indicated that the mPEG-b-P(MMD-co-GA)-g-PEI based MPs could be a suitable non-viral gene carrier for ZNF580 gene to enhance rapid endothelialization.