Zhijun Shi

Electroconductive natural polymer-based hydrogels

Hydrogels prepared from natural polymers have received immense considerations over the past decade due to their safe nature, biocompatibility, hydrophilic properties, and biodegradable nature. More recently, when treated with electroactive materials, these hydrogels were endowed with high electrical conductivity, electrochemical redox properties, and electromechanical properties; consequently, forming a smart hydrogel. The biological properties of these smart hydrogels, classified as electroconductive hydrogels, can be combined with electronics. Thus, they are considered as good candidates for some potential uses, which include bioconductors, biosensors, electro-stimulated drug delivery systems, as well as neuron-, muscle-, and skin-tissue engineering. However, there is lacking comprehensive information on the current state of these electroconductive hydrogels which complicates our understanding of this new type of biomaterials as well as their potential applications. Hence, this review provides a summary on the current development of electroconductive natural polymer-based hydrogels (ENPHs). We have introduced various types of ENPHs, with a brief description of their advantages and shortcomings. In addition, emerging technologies regarding their synthesis developed during the past decade are discussed. Finally, two attractive potential applications of ENPHs, cell culture and biomedical devices, are reviewed, along with their current challenges.

Time-dependent rheological behaviour of bacterial cellulose hydrogel

This work focuses on time-dependent rheological behaviour of bacterial cellulose (BC) hydrogel. Due to its ideal biocompatibility, BC hydrogel could be employed in biomedical applications. Considering the complexity of loading conditions in human body environment, time-dependent behaviour under relevant conditions should be understood. BC specimens are produced by Gluconacetobacter xylinus ATCC 53582 at static-culture conditions. Time-dependent behaviour of specimens at several stress levels is experimentally determined by uniaxial tensile creep tests. We use fraction-exponential operators to model the rheological behaviour. Such a representation allows combination of good accuracy in analytical description of viscoelastic behaviour of real materials and simplicity in solving boundary value problems. The obtained material parameters allow us to identify time-dependent behaviour of BC hydrogel at high stress level with sufficient accuracy.

Fabrication of bacterial cellulose/polyaniline/single-walled carbon nanotubes membrane for potential application as biosensor

Electrically conductive polymeric membranes of BC with polyaniline (PAni) were fabricated through ex situ oxidative polymerization. PAni was densely arrayed along BC fibers and SWCNTs were uniformly distributed in the composites as confirmed by field emission scanning electron microscopy (FE-SEM). Fourier transform-infrared (FT-IR) spectra of the composite membranes exhibited characteristic peaks for specific functional groups of PAni and SWCNTs besides BC. X-ray diffraction (XRD) analysis indicated the presence of specific peaks for BC, PAni, and SWCNTs in the composites. The conjugated backbone of PAni and SWCNTs contributed to improve the degradation temperatures from 232°C for BC to 260°C, 302°C, and 310°C for BC-PAni, BC-PAni/SWCNTs-I (0.05mg/mL), and BC-PAni/SWCNTs-II (0.1mg/mL) composites, respectively. The electrical conductivity of BC was enhanced to 1.04×10-3S/cm, 4.64×10-3S/cm, and 1.41×10-2S/cm upon doping with PAni, and 0.05mg/mL and 0.1mg/mL SWCNTs, respectively in dry state which was further increased to 4.02×10-2S/cm, 3.03×10-2S/cm, 5.93×10-1S/cm, and 7.36×10-1S/cm, respectively in PBS solution. These membranes can potentially be used for applications requiring biocompatibility and electrical conductivity such as biological and chemical sensors.

Effect of microstructure on anomalous strain-rate-dependent behaviour of bacterial cellulose hydrogel.

This study is focused on anomalous strain-rate-dependent behaviour of bacterial cellulose (BC) hydrogel that can be strain-rate insensitive, hardening, softening, or strain-rate insensitive in various ranges of strain rate. BC hydrogel consists of randomly distributed nanofibres and a large content of free water; thanks to its ideal biocompatibility, it is suitable for biomedical applications. Motivated by its potential applications in complex loading conditions of body environment, its time-dependent behaviour was studied by means of in-aqua uniaxial tension tests at constant temperature of 37 °C at various strain rates ranging from 0.000 1s(-1) to 0.3s(-1). Experimental results reflect anomalous strain-rate-dependent behaviour that was not documented before. Micro-morphological observations allowed identification of deformation mechanisms at low and high strain rates in relation to microstructural changes. Unlike strain-rate softening behaviours in other materials, reorientation of nanofibres and kinematics of free-water flow dominate the softening behaviour of BC hydrogel at high strain rates.

A transparent wound dressing based on bacterial cellulose whisker and poly(2-hydroxyethyl methacrylate)

The current study was aimed to develop a transparent wound dressing comprised of bacterial cellulose (BC) and poly (2-hydroxyethyl methacrylate) (PHEMA) hydrogel coated with silver (Ag) nanoparticles. Briefly, different concentrations of BC whiskers (BCWs) were added into the HEMA solution to form PHEMA/BCWs hydrogel with volume ratio of monomer HEMA and BCWs as 7:3 and 1:1. The addition of BCWs into PHEMA matrix improved its equilibrium water content and light transmittance about 20%-40% and 10%, respectively. The Youngs modulus for PHEMA was found to be 0.72MPa, which was improved to 0.57MPa and 0.50MPa for PHEMA/BCWs 7:3 and PHEMA/BCWs 1:1, respectively. Further, immersion of PHEMA/BCWs hydrogel in the AgNO3 and NaBH4 solutions bestowed it with antibacterial property and produced inhibition zones of 0.5±0.15cm and 0.25±0.15cm against Escherichia coli and Staphylococcus aureus, respectively. Similarly, PHEMA/BCWs prepared with 0.001M AgNO3 and 0.001M NaBH4 solutions showed 99% and 90% reduction in colony forming unit (CFU) for E. coli and S. aureus, respectively, after 24h. The PHEMA/BCWs/Ag hydrogel facilitated the growth of NIH3T3 fibroblast, showing their low toxicity. These results demonstrate the suitability of PHEMA/BCWs/Ag hydrogel for its application as potential transparent wound dressing material for skin repair.

Through-thickness stress relaxation in bacterial cellulose hydrogel.

Biological hydrogels, e.g. bacterial cellulose (BC) hydrogel, attracted increasing interest in recent decades since they show a good potential for biomedical engineering as replacements of real tissues thanks mainly to their good biocompatibility and fibrous structure. To select potential candidates for such applications, a comprehensive understanding of their performance under application-relevant conditions is needed. Most hydrogels demonstrate time-dependent behaviour due to the contribution of their liquid phase and reorientation of fibres in a process of their deformation. To quantify such time-dependent behaviour is crucial due to their exposure to complicated loading conditions in body environment. Some hydrogel-based biomaterials with a multi-layered fibrous structure demonstrate a promise as artificial skin and blood vessels. To characterise and model time-dependent behaviour of these multi-layered hydrogels along their through-thickness direction is thereby of vital importance. Hence, a holistic study combining mechanical testing and micro-morphological observations of BC hydrogel with analytical modelling of its relaxation behaviour based on fraction-exponential operators was performed. The results show a good potential to use a fraction-exponential model to describe such behaviour of multi-layered hydrogels, especially at stages of stress decay at low forces and of stress equilibrium at high forces.

Assessing stiffness of nanofibres in bacterial cellulose hydrogels: Numerical-experimental framework

This work presents a numerical-experimental framework for assessment of stiffness of nanofibres in a fibrous hydrogel - bacterial cellulose (BC) hydrogel - based on a combination of in-aqua mechanical testing, microstructural analysis and finite-element (FE) modelling. Fibrous hydrogels attracted growing interest as potential replacements to some tissues. To assess their applicability, a comprehensive understanding of their mechanical response under relevant conditions is desirable; a lack of such knowledge is mainly due to changes at microscale caused by deformation that are hard to evaluate in-situ because of the dimensions of nanofibres and aqueous environment. So, discontinuous FE simulations could provide a feasible solution; thus, properties of nanofibres could be characterised with a good accuracy. An alternative - direct tests with commercial testing systems - is cumbersome at best. Hence, in this work, a numerical-experimental framework with advantages of convenience and relative easiness in implementation is suggested to determine the stiffness of BC nanofibres. The obtained magnitudes of 53.7-64.9GPa were assessed by calibrating modelling results with the original experimental data.

Fabrication of nanocomposites and hybrid materials using microbial biotemplates

Microbes are important part of life that vary in sizes and shapes, diverse surface chemistry and biology, and porous nature of their cell walls. Besides their importance in industrial processes such as fermentation, these serve as biotemplates and provide a biomimetic approach for fabrication of multifarious complex constructs with predefined features, ordered composites and hybrid nanomaterials, microdevices, and micro/nanorobots through various strategies. The template or building blocks for such approaches can be bacterial, algal, and fungal cells or virus particles. Here, we have summarized recent advancements in biofabrication based on live microbes. Using engineering approaches and suitable methods, live microbes can be manipulated as functional “micro/nanodevices and -robots” to further perform biological functions such as replication, distribution, motility, formation of colonies, and secretion of metabolites at will. Biofabrication based on microbes provides effective methods to control and manipulate microbes as functional live building blocks to create micro/nanodevices and -robots for biomedical and energy applications.

Understanding piezoelectric characteristics of PHEMA-based hydrogel nanocomposites as soft self-powered electronics

AbstractPiezoelectric hydrogel nanocomposites are being developed as interface for connecting biological organs and electronics because of their flexibility, biocompatibility, and electromechanical behaviours, which allow environmental stimulations to be converted into electronic signals. The vision of this work is to develop a series of piezoelectric hydrogel nanocomposites which is capable of generating electric current in aqueous condition. Conductive nanoparticles have been composited in the PHEMA-based hydrogel. Theoretical models and characterisations on the electromechanical properties of such hydrogel have been investigated to assist the understanding of the piezoelectric mechanisms. The hydrogel nanocomposite was demonstrated as a self-powered motion sensor to quantitatively detect human motion and can be considered as candidate material for soft energy harvesting electronics. Overall, the work presented in this paper provides theoretical basis, design guidelines, and technical support for the development of soft self-powered electronics, thus unlocking the potential of piezoelectric hydrogel nanocomposites. Graphical abstractᅟ