Multifunctionality of carboxyl groups carried by cellulose nanocrystals for mechanical and conductive properties of acrylic-based hydrogels—mechanism study

Poly (N-isopropylacrylamide) (PNIPAm) hydrogels have been proposed for application as promising biomaterials and stimuli-responsive materials in biomedical fields, but their poor mechanical properties limited their development. Here we report that the mechanical properties of PNIPAm-clay nanocomposite hydrogels were enhanced significantly through in situ polymerization by addition of very low content of graphene oxide (GO). GO sheets were used as both the crosslinking agent and reinforcing agent. Compared to nanocomposite hydrogels without GO, a more than 200% increase in tensile strength and a nearly 400% improvement of Young’s modulus were achieved with the addition of 0.1 wt. % of GO, which suggests an excellent load transfer between the GO and hydrogel matrix. KEYWORD: Graphene oxide; hydrogel; N-isopropylacrylamide International Conference on Industrial Technology and Management Science (ITMS 2015) © 2015. The authors Published by Atlantis Press 1102 with uniform thickness (∼ 1.1 nm) according to cross-section analysis and lateral dimensions ranging from several hundred nanometers to several micrometers. While individual graphene nanosheet is atomically flat with a well-known thickness of ~0.34 nm, GO is much thicker due to the attachment of oxygen-containing functional groups such as epoxy, hydroxyl and carboxyl groups, which disrupt the sp2-bonding nature of carbon atoms and introduce structure defects to form folds on the GO sheets. The results are consistent with those reported in the literature (Haraguchi et al 2006), which proving the graphene oxide is capable of forming uniform dispersion of single thickness sheet. Figure 1. The AFM images of graphene oxide 2.2 The mechanical properties of NC-GO hydrogels 2.2.1 The images of NC and NC-GO hydrogels It can be observed from Fig. 2a and Fig. 2b, compared to NC hydrogel, NC-GO hydrogel has better strength and toughness. GO has high specific surface energy, good hydrophilicity, good mechanical properties, and can be well dispersed in water. Composites consisting of GO and polymer can realize the complementary of the two components which result in mechanical enhancement. Since the excellent mechanical properties of graphene oxide, it is considered desirable reinforcing fillers in polymer composites (Wang et al 2010). In the process of forming the hydrogel, the epoxy, hydroxyl and carboxyl groups on the edge of GO with the polymer chains form dense three-dimensional network structure which is crucial for the improvement of the mechanical properties. Figure 2. The photographs of ClayPNIPAm (NC) hydrogel (a) and NC-GO hydrogel (b) 2.2.2 Mechanical behavior Fig. 3 shows the stress-strain curves of NC hydrogel with different content of GO. One can see that the addition of GO significantly improved the tensile properties of NC hydrogels. The tensile strength and Young’s modulus increased sharply by more than 200% from 50 KPa to 111 KPa (Figure 3a) and by nearly 400% from 5.6 KPa to 20.4 KPa, respectively. The addition of GO made the mechanical property of the hydrogel enhanced dramatically due to the increasing of crosslinking points. On the other hand, the hydrogen bond between epoxy, hydroxyl and carboxyl groups on GO made the hydrogel network more dense which led to the improvement of tensile strength. It was strongly obvious that even a small amount of GO could significantly improve the mechanical properties (Fig. 3b). The content of GO increased from 0.01wt% to 0.1 wt. %, the tensile strength increased with the increasing of GO. Surprisingly, when the content of GO increased from 0.1 wt. % to 0.15 wt. %, the tensile strength decreased on the contrary. This may be caused by excessive crosslinking points of excessive GO, thus affecting the mechanical properties of the entire system. Figure 3. The stress-strain curves of NC2 hydrogel and NC2GO1 hydrogel (a) and NC2-GO hydrogels with different contents of GO (b) (a) (b)

Structural and mechanical characterization of crosslinked and sterilised nanocellulose-based hydrogels for cartilage tissue engineering

Effects of molecular mass of polymer and composition on the compressive properties of hydrogels composed of Laponite and sodium polyacrylate

Thermo-responsive hydrogels containing poly(N-isopropylacrylamide) (PNIPAAm), reinforced both with covalent and non-covalent interactions with cellulose nanocrystals (CNC), were synthesized via free-radical polymerization in the absence of any additional cross-linkers. The properties of PNIPAAm-CNC hybrid hydrogels were dependent on the amounts of incorporated CNC. The thermal stability of the hydrogels decreased with increasing CNC content. The rheological measurement indicated that the elastic and viscous moduli of hydrogels increased with the higher amounts of CNC addition, representing stronger mechanical properties of the hydrogels. Moreover, the hydrogel injection also supported the hypothesis that CNC reinforced the hydrogels; the increased CNC content exhibited higher structural integrity upon injection. The PNIPAAm-CNC hybrid hydrogels exhibited clear thermo-responsive behavior; the volume phase transition temperature (VPTT) was in the range of 36 to 39 °C, which is close to normal human body temperature. For wound dressing purposes, metronidazole, an antibiotic and antiprotozoal often used for skin infections, was used as a target drug to study drug-loading and the release properties of the hydrogels. The hydrogels showed a good drug-loading capacity at room temperature and a burst drug release, which was followed by slow and sustained release at 37 °C. These results suggested that newly developed drugs containing injectable hydrogels are promising materials for wound dressing.

In this study, polyurethane (PU) hydrogels were synthesized via mercapto curing reaction to elucidate the effect of molecular interactions between isocyanate and soft segments on the properties of hydrogels. Further, the mesh size, mechanical properties, hydrophilicity, and biological properties of the PU hydrogels were determined. In the isocyanate series, the structural regularity and rigidity of 4,4′‐dicyclohexylmethane diisocyanate (HMDI) favored the formation of hydrogel materials with small mesh size, high modulus, and low water absorption. In contrast, l‐lysine diisocyanate (LDI) favored the materials with large mesh size, low modulus, and good hydrophilicity. In the soft‐segment series, the strong hydrogen bonds of polycarbonate diol (PCDL) favored the formation of materials with small mesh size, dense cross‐link points, and high modulus, whereas weak hydrogen bonds of polytetrahydrofuran ether glycol (PTMG) favored the hydrogel materials with small mesh size, few crosslink points, and low modulus. PU hydrogels exhibit excellent cytocompatibility, anti‐cell adhesion, and anti‐inflammatory properties. Therefore, this study offers valuable insights into understanding the chain structure and macroscopic properties, thus contributing to preparing PU hydrogels with varying performances, as desired.

The aim of this work was the manufacture and characterisation of novel chemically cross-linked mucoadhesive PVA-GANT hydrogels prepared by using autoclaving. Particularly, the study was focused on the physicochemical and pharmaceutical properties of these hydrogels with regards to potential applications for drug delivery and wound dressing. PVA-GANT hydrogels with different molar ratios and total concentrations of polymers in solution were prepared using a standard sterilisation autoclave. The physico-chemical properties were characterised by various techniques including IR spectroscopy, Texture Analysis and SEM and thermo-analytical techniques (DSC and TGA). Pharmaceutical characteristics were obtained in drug loading/release tests and microbiological assays. The results have shown that the properties of hydrogels (swelling degree, mechanical properties, internal structure, drug loading/release and antimicrobial properties) are very dependent on the polymer composition.

Hydrogels are considered good biomaterials for soft tissue regeneration. In this sense, collagen is the most used raw material to develop hydrogels, due to its high biocompatibility. However, its low mechanical resistance, thermal stability and pH instability have generated the need to look for alternatives to its use. In this sense, the combination of collagen with another raw material (i.e., polysaccharides) can improve the final properties of hydrogels. For this reason, the main objective of this work was the development of hydrogels based on collagen and chitosan. The mechanical, thermal and microstructural properties of the hydrogels formed with different ratios of collagen/chitosan (100/0, 75/25, 50/50, 25/75 and 0/100) were evaluated after being processed by two variants of a protocol consisting in two stages: a pH change towards pH 7 and a temperature drop towards 4 °C. The main results showed that depending on the protocol, the physicochemical and microstructural properties of the hybrid hydrogels were similar to the unitary system depending on the stage carried out in first place, obtaining FTIR peaks with similar intensity or a more porous structure when chitosan was first gelled, instead of collagen. As a conclusion, the synergy between collagen and chitosan improved the properties of the hydrogels, showing good thermomechanical properties and cell viability to be used as potential biomaterials for Tissue Engineering.

Manufacturing of hydrogel biomaterials with controlled mechanical properties for tissue engineering applications.

Properties of as-prepared and freeze-dried hydrogels made from poly(vinyl alcohol) and cellulose nanocrystals using freeze-thaw technique

Biosynthetic hydrogels have proved to be a credible solution in developing cost effective scaffolds with superlative mechanical properties for biomedical applications. Elastic hydrogels have emerged with advanced application possibilities for cartilage tissue regeneration and cell implantation. However, a hydrogel scaffold that mimics the properties of biological tissues in terms of elasticity, provision of favorable environment for cell growth and biocompatibility are rarely reported. In this research, we developed photocrosslinked hyaluronic acid/pluronic F127 (HA/PF) porous hydrogels with exceptional mechanical and water sorption properties. In order to retain the micellar phase of PF in the hydrogels, we restrained their concentrations to 8 wt% in the hydrogel matrices. Further optimization such as duration of photocrosslinking resulted in hydrogel scaffolds with remarkable mechanical properties. Topical and cross-sectional scanning electron microscopy images depicted the dense interconnected porous networks within the hydrogel matrices. Rheology studies confirmed the importance of PF concentrations in obtaining the hydrogels with enhanced toughness and mechanical properties. The results from the microscopic rheology studies were further testified by applying macroscopic mechanical distortions over the hydrogels. Hydrogels with higher PF concentrations restrained the degradations and displayed enhanced mechanical properties. By retaining the micellar structures in the HA/PF hydrogel scaffolds, the polypropylene blocks in PF were able to reversibly fold and unfold to favor the energy dissipation during the mechanical deformation and aid in improving the mechanical properties of the hydrogels. The overall properties of the HA-PF hydrogels show optimum feasibility for hard tissue engineering application.

Polyionic liquid hydrogels attract increasing attention due to their unique properties and potential applications. However, research on amino acid-based polyionic liquid hydrogels is still in its infancy stage. Moreover, the effect of amino acid types on the properties of hydrogels is rarely studied to date. In this work, amino acid-based polyionic liquid hydrogels (D/L-PCAA hydrogels) are synthesized by copolymerizing vinyl choline-amino acid ionic liquids and acrylic acids using Al3+ as a crosslinking agent and bacterial cellulose (BC) as a reinforcing agent. The effects of amino acid types on mechanical and antimicrobial properties are systematically investigated. D-arginine-based hydrogel (D-PCArg) shows the highest tensile strength (220.7 KPa), D-phenylalanine-based hydrogel (D-PCPhe) exhibits the highest elongation at break (1346%), and L-aspartic acid-based hydrogel (L-PCAsp) has the highest elastic modulus (206.9 KPa) and toughness (1.74MJ m-3). D/L-PCAsp hydrogels demonstrate stronger antibacterial capacity against Escherichia coli and Staphylococcus aureus, and D/L-PCPhe hydrogels possess higher antifungal activity against Cryptococcus neoformans. Moreover, the resultant hydrogels exhibit prominent hemocompatibility and low toxicity, as well as excellent self-healing capabilities (86%) and conductivity (2.8 S m-1). These results indicate that D/L-PCAA hydrogel provides a promise for applications in wound dressings.

Endoscopic submucosal dissection (ESD) is a widely used procedure for the treatment of early and precancerous gastrointestinal lesions and has become the standard treatment. In this procedure, the commonly used materials have a short retention time and a limited lifting capacity, which will prolong the duration of the ESD procedure. Furthermore, these liquids tend to diffuse after ESD surgery, failing to adequately protect the wound. Therefore, we designed and developed injectable hydrogels based on hyaluronic acid. A series of oxidized hyaluronic acid (OHA) and hydrazide hyaluronic acid (AHA) were synthesized, and 16 kinds of injectable hydrogels were fabricated to investigate the effects of molecular structures on the properties of the hydrogels. Among these, the O1A3 hydrogel exhibited a suitable injection performance, gelation time, and mechanical properties, along with good blood and cell compatibility in vitro. Subsequently, in a porcine model of the ESD procedure, the results demonstrated that the O1A3 hydrogel exhibited a good retention time and lifting performance while also significantly reducing the operation time from 1-2 h to ∼10 min. Furthermore, the adhesive property of the O1A3 hydrogel on small bleeding spots and wounds could be observed, which was beneficial in protecting the wound from the complex environment of the gastrointestinal tract. The present work of injectable hyaluronic acid-based hydrogels could be promising to improve the efficiency of ESD surgery.

Nanocellulose-reinforced ionic conductive hydrogels were prepared using cellulose nanofiber (CNF) and polyvinyl alcohol (PVA) as raw materials, and the hydrogels were prepared in a dimethyl sulfoxide (DMSO)/water binary solvent by a one-pot method. The prepared hydrogels were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The mechanical properties, electrical conductivity, and sensing properties of the hydrogels were studied by means of a universal material testing machine and LCR digital bridge. The results show that the ionic conductive hydrogel exhibits high stretchability (elongation at break, 206%) and firmness (up to 335 KPa). The tensile fracture test shows that the hydrogel has good properties in terms of tensile strength, toughness, and elasticity. The hydrogel as a conductor medium is assembled into a self-powered strain sensor and the open-circuit voltage can reach 0.830 V. It shows good sensitivity in the bend sensing testing, indicating that the hydrogel has good sensing performance. The water retention and anti-freezing performance experiments show that the addition of dimethyl sulfoxide solvents can effectively improve the anti-freezing and water retention properties of hydrogels.

Role of active nanoliposomes in the surface and bulk mechanical properties of hybrid hydrogels

ABSTRACTKonjac glucomannan (KGM) hydrogel has good potential application in food and medical science, although to achieve this, the physical and mechanical properties need further improvement. In this study, graphene oxide (GO) was used to improve the functionality of KGM hydrogel. KGM/GO hydrogels were prepared by freezing the alkaline KGM/GO sols. Rotational rheometer was used to study the rheological properties of different alkaline KGM/GO sols. Fourier transform infrared, Raman, differential scanning calorimetry, thermogravimetric analyses, and scanning electron microscopy were used to evaluate the structure and properties of the hydrogels. In addition, different pH solutions and an in vitro assay were used to study the swelling property and the release behavior of KGM/GO hydrogels, respectively. The result revealed strong hydrogen‐bond interaction between KGM and GO. The incorporation of GO highly improved the gel properties of KGM/GO sol, higher thermal stability, and more compact structure of KGM/GO hydrogels. KGM/GO hydrogels showed better swelling properties in deionized‐distilled water and pH 7.2 PBS. The release of 5‐aminosalicylic acid (5‐ASA) from KGM/GO (KG4) hydrogel was different in various pH media, but the initial burst release effect was very severe. Therefore, incorporation of GO have a good potential in enhancing the properties of KGM hydrogel, but KGM/GO hydrogel is not an ideal carrier for 5‐ASA release. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45327.