kei Wan

Anisotropic polyvinyl alcohol-Bacterial cellulose nanocomposite for biomedical applications.

Compliance mismatch between the synthetic graft and the surrounding native tissue has been reported as a major factor in ultimate failure of the currently used cardiovascular graft replacements. Thus, developing biomaterials that display close mechanical properties as the tissue it is replacing is an important objective in biomedical devices design. Polyvinyl alcohol (PVA) is a biocompatible hydrogel with characteristics desired for biomedical applications. It can be crosslinked by a low temperature thermal cycling process. By using a novel thermal processing method under an applied strain and with the addition of a small amount of bacterial cellulose (BC) nanofibers, an anisotropic PVA-BC nanocomposite was created. The stress-strain tensile properties of porcine aorta were closely matched in both the circumferential and the axial directions by one type of anisotropic PVA-BC nanocomposite (10% PVA with 0.3% BC at 75% initial strain and cycle 2) within physiological range, with improved resistance to further stretch beyond physiological strains. The PVA-BC nanocomposite gives a broad range of mechanical properties, including anisotropy, by controlling material and processing parameters. PVA-BC nanocomposites with controlled degree of anisotropy that closely match the mechanical properties of the soft tissue it might replace, ranging from cardiovascular to other connective tissues, can be created.

Hydrogel/electrospun fiber composites influence neural stem/progenitor cell fate

Cell replacement therapy with multi-potent neural stem/progenitor cells (NSPCs) into the injured spinal cord is limited by poor survival and host tissue integration. An injectable and biocompatible polymeric cell delivery system serves as a promising strategy to facilitate cell delivery, promote cell survival and direct cell behaviour. We developed and characterized the use of a physical hydrogel blend of hyaluronan (HA) and methylcellulose (MC) for NSPC delivery, and incorporated electrospun fibers of either collagen or poly(e-caprolactone-co-D,L-lactide) (P(CL:DLLA)) to promote cell–matrix interactions and influence cell behaviour. The shear-thinning and thermally reversible HAMC had a zero-shear viscosity of 1.2 Pa s at 25 °C, formed a weak gel at 37 °C with a yield stress of 0.5 Pa, and swelled to 115% of its original volume after one day. HAMC was both cytocompatible and allowed NSPC differentiation in vitro, similar to what one would observe in media. Interestingly, cells cultured in HAMC remained homogeneously dispersed over the 7 d culture period, unlike those cultured in media controls where significant cell aggregation was observed. Inclusion of electrospun fibers in the HAMC hydrogel further influenced cell behaviour. Composite systems of collagen fibers in HAMC resulted in reduced survival/proliferation and differentiation relative to HAMC itself whereas composites of P(CL:DLLA) fibers in HAMC maintained cell survival/proliferation and enhanced neuronal and oligodendrocytic differentiation similar to HAMC. In this study, the importance of the cell delivery vehicle to NSPC survival and cell fate was demonstrated in vitro and is being tested in on-going studies in vivo.

Microbially Mediated Mineral Carbonation: Roles of Phototrophy and Heterotrophy

Ultramafic mine tailings from the Diavik Diamond Mine, Canada and the Mount Keith Nickel Mine, Western Australia are valuable feedstocks for sequestering CO₂ via mineral carbonation. In microcosm experiments, tailings were leached using various dilute acids to produce subsaline solutions at circumneutral pH that were inoculated with a phototrophic consortium that is able to induce carbonate precipitation. Geochemical modeling of the experimental solutions indicates that up to 2.5% and 16.7% of the annual emissions for Diavik and Mount Keith mines, respectively, could be sequestered as carbonate minerals and phototrophic biomass. CO₂ sequestration rates are mainly limited by cation availability and the uptake of CO₂. Abundant carbonate mineral precipitation occurred when heterotrophic oxidation of acetate acted as an alternative pathway for CO₂ delivery. These experiments highlight the importance of heterotrophy in producing sufficient DIC concentrations while phototrophy causes alkalinization of waters and produces biomass (fatty acids = 7.6 wt.%), a potential feedstock for biofuel production. Tailings storage facilities could be redesigned to promote CO₂ sequestration by directing leachate waters from tailings piles into specially designed ponds where carbonate precipitation would be mediated by both chemical and biological processes, thereby storing carbon in stable carbonate minerals and potentially valuable biomass.

Compression properties of polyvinyl alcohol--bacterial cellulose nanocomposite.

Despite the established use of total joint replacement for the treatment of advanced degeneration of articular cartilage, component loosening due to wear and osteolysis limits the lifespan of these joint prostheses. In the present study, nanocomposites consisting of poly(vinyl alcohol) (PVA) and bacterial cellulose (BC) nanofibers were investigated as possible improved cartilage replacement materials. Nanocomposites were synthesized by adding small amounts (<1%) of BC to PVA, and subjecting the mixture to thermal cycling. The mechanical properties of the resulting material were evaluated using unconfined compression testing. By the addition of BC nanofibers to the PVA matrix, a nanocomposite with a wide range of compressive mechanical properties control was obtained, with elastic modulus values similar to those reported for native articular cartilage. The nanocomposite also showed improved strain-rate dependence and adequate viscoelastic properties. The PVA-BC nanocomposite is therefore a promising biomaterial to be considered as a possible replacement material for localized articular cartilage injuries and other orthopedic applications such as intervertebral discs.

Design and simulation of a poly(vinyl alcohol)—bacterial cellulose nanocomposite mechanical aortic heart valve prosthesis

Abstract In this study, a polymeric aortic heart valve made of poly(vinyl alcohol) (PVA)—bacterial cellulose (BC) nanocomposite is simulated and designed using a hyperelastic non-linear anisotropic material model. A novel nanocomposite biomaterial combination of 15 wt % PVA and 0.5 wt % BC is developed in this study. The mechanical properties of the synthesized PVA—BC are similar to those of the porcine heart valve in both the principal directions. To design the geometry of the leaflets an advance surfacing technique is employed. A Galerkin-based non-linear finite element method is applied to analyse the mechanical behaviour of the leaflet in the closing and opening phases under physiological conditions. The model used in this study can be implemented in mechanical models for any soft tissues such as articular cartilage, tendon, and ligament.

Advanced modeling strategy for the analysis of heart valve leaflet tissue mechanics using high-order finite element method

Modeling soft tissue using the finite element method is one of the most challenging areas in the field of biomechanical engineering. To date, many models have been developed to describe heart valve leaflet tissue mechanics, which are accurate to some extent. Nevertheless, there is no comprehensive method to modeling soft tissue mechanics, This is because (1) the degree of anisotropy in the heart valve leaflet changes layer by layer due to a variety of collagen fiber densities and orientations that cannot be taken into account in the model and also (2) a constitutive material model fully describing the mechanical properties of the leaflet structure is not available in the literature. In this framework, we develop a new high-order element using p-type finite element formulation to create anisotropic material properties similar to those of the heart valve leaflet tissue in only one single element. This element also takes the nonlinearity of the leaflet tissue into consideration using a bilinear material model. This new element is composed a two-dimensional finite element in the principal directions of leaflet tissue and a p-type finite element in the direction of thickness. The proposed element is easy to implement, much more efficient than standard elements available in commercial finite element packages. This study is one step towards the modeling of soft tissue mechanics using a meshless finite element approach to be applied in real-time haptic feedback of soft-tissue models in virtual reality simulation.

Use of degradable and nondegradable nanomaterials for controlled release.

Drug-delivery devices are fundamentally important in improving the pharmacological profiles of therapeutic molecules. Nanocontrolled-release systems are attracting a lot of attention currently owing to their large surface area and their ability to target delivery to specific sites in the human body. In addition, they can penetrate the cell membrane for gene, nucleic acid and bioactive peptide/protein delivery. Representative applications of nanodrug-delivery systems include controlled-release wound dressings, controlled-release scaffolds for tissue regeneration and implantable biodegradable nanomaterial-based medical devices integrated with drug-delivery functions. We review the present status and future perspectives of various types of nanocontrolled-release systems. Although many of the well-established degradable and nondegradable controlled-release vehicles are being investigated for their processing into nanocarriers, several new emerging nanomaterials are being studied for their controlled-release properties. The release of multiple bioactive agents, each with its own kinetic profile, is becoming possible. In addition, integration of the nanocontrolled-release systems with other desirable functions to create new, cross-discipline applications can also be realized.

Poly(Vinyl Alcohol) Cryogels for Biomedical Applications

Poly(vinyl alcohol) (PVA) is a hydrophilic and biocompatible polymer that can be crosslinked to form a hydrogel. When physically crosslinked using a freeze–thaw cycling process, the product hydrogel or cryogel (PVA-C) possesses unique mechanical properties that can be tuned to closely match those of soft tissues, thus making it an attractive candidate for biomedical and especially medical device applications. We review the freeze–thaw cycling process and processing parameters that impact on the properties of PVA-C and its nanocomposite products. Both the mechanical properties and diffusion properties relevant to biomedical application are discussed. Applications to orthopedic and cardiovascular devices are summarized and discussed. The concept of biomaterial–tissue hybrids that can impart the necessary hemocompatibility to PVA-C for cardiovascular device is introduced and demonstrated.

Preparation and characterization of a poly(2-hydroxyethyl methacrylate) biomedical hydrogel

Abstract Heparin a commonly used anticoagulant was immobilized onto a poly(2-hydroxyethyl methacrylate) hydrogel using glutaraldehyde as a coupling agent in a heterogeneous and homogeneous phase. The amount of immobilized heparin was determined using a colorimetric assay. The thrombogenicity of the gels was assessed using modified activated partial thromboplastin time (aPTT) and thrombin time (TT) tests. Statistically significant differences in both TT and aPTT were observed between the heparinized gels and the non-modified gels (i.e. controls). Antithrombogenic activity was still observed on the gels even after three weeks rinsing where all the non-immobilized heparin was deemed to have leached out of the gel. Thus the significant residual heparin like activity was attributed to the immobilization of heparin in its bioactive form.

Simultaneous measurement of Young’s and shear moduli of multiwalled carbon nanotubes using atomic force microscopy

Carbon nanotubes (CNTs) are widely hailed as the strongest material known to mankind. However, experimental measurements—and even theoretical estimates—of their mechanical properties span a wide range. We present an atomic force microscopy study of multiwalled CNTs, which, unlike previous such studies, measures the tube compliance as a function of position along suspended tubes. This permits a simultaneous determination of the effective Young’s and shear moduli of CNTs: 350±110 and 1.4±0.3GPa, respectively.