Anne Simmons

Biostability and biological performance of a PDMS-based polyurethane for controlled drug release

Polymers have been used to deliver therapeutic agents in a range of medical devices with drug eluting stents being the most widespread current application. Although polymers enable controlled release of a therapeutic agent, the polymeric surface has been reported to provide suboptimal biocompatibility and haemocompatibility and it has been suggested that currently used polymers may be at least partly responsible for the late adverse events observed in intravascular stent systems. In this study, the biostability and biological performance of a siloxane-based polyurethane elastomer (E2A) demonstrating excellent long-term biostability in the unloaded state was investigated following incorporation of a therapeutic agent. After implantation in an ovine model for 6 months, samples were assessed using SEM and ATR-FTIR to determine changes in the surface chemical structure and morphology of the materials and tensile testing was used to examine changes in bulk characteristics. Biological response was assessed using in vitro cytotoxicity testing and histological analysis. Results indicated that incorporation of 25mg/g dexamethasone acetate (DexA) into the siloxane-based polyurethane resulted in no significant difference in the biostability and biocompatibility of the material. Some level of cytotoxic potential was exhibited which was believed to result from residual DexA leaching from samples during the extraction process. These findings suggest that E2A is a potential candidate for a delivery vehicle of therapeutic agents in implantable drug delivery applications.

Extracellular matrix remodelling during cell adhesion monitored by the quartz crystal microbalance.

A cells ability to remodel adsorbed protein layers on surfaces is influenced by the nature of the protein layer itself. Remodelling is often required to accomplish cellular adhesion and extracellular matrix formation which forms the basis for cell spreading, increased adhesion and expression of different phenotypes. The adhesion of NIH3T3 (EGFP) fibroblasts to serum protein (albumin or fibronectin) precoated tantalum (Ta) and oxidised polystyrene (PS(ox)) surfaces was examined using the quartz crystal microbalance with dissipation (QCM-D) monitoring and fluorescence microscopy. The cells were either untreated or treated with cycloheximide to examine the contribution of endogenous protein production during cell adhesion to the QCM-D response over a period of 2h. Following adsorption of albumin onto Ta and PS(ox) there was no difference detected between the response to seeding untreated and cycloheximide treated cells. The QCM-D was able to detect differences in the untreated cellular responses to fibronectin versus serum precoated Ta and PS(ox) substrates, while cycloheximide treatment of the cells produced the same QCM-D response for fibronectin and serum precoatings on each of the materials. This confirmed that the process of matrix remodelling by the cells is dependent on the underlying substrate and the preadsorbed proteins and that the QCM-D response is dominated by changes in the underlying protein layer. Changes in dissipation correspond to the development of the actin cytoskeleton as visualised by actin staining.

Antibacterial polyurethane nanocomposites using chlorhexidine diacetate as an organic modifier.

Polymer nanocomposites (NCs) are hypothesised to have enhanced barrier properties compared with pristine polymer, allowing more sustained drug release from the materials. In these NC systems active agents are typically incorporated into the polymer matrix and the release kinetics are theoretically perturbed by well dispersed nanoparticle inclusions. An alternative approach is to exploit active agent interactions with the nanoinclusion. In the proposed NC system, the driving hypothesis is that active agents can have dual functionality, acting as both drug and dispersant. Polyurethane-montmorillonite (PEU-MMT) NCs were prepared in which the antimicrobial agent chlorhexidine diacetate (CHX) was evaluated as an organic modifier for silicate dispersion. CHX was incorporated at various concentrations through organic modification of MMT or within the bulk polymer. X-ray diffraction and transmission electron microscopy analysis suggested that intercalated and partially exfoliated NCs were achieved, with better dispersion occurring in the presence of free CHX within the bulk. Tensile testing results showed that variations in the level of organic modification and nanoparticle loading modulated the mechanical properties. Material stiffness increased with nanoparticle loading relative to pristine PEU, and the ultimate properties decreased with nanoparticle and free CHX incorporation. Antibacterial activity against Staphylococcus epidermidis was significant in materials with higher exchanged MMT and NCs containing free CHX, for which 2-log reductions in adherent bacteria were found after 24h. CHX was successfully used to modulate the material properties in its dual role as a dispersant and antimicrobial agent, suggesting that alternative biocides of similar structure may behave comparably within PEU-MMT NC systems.

The modulation of platelet and endothelial cell adhesion to vascular graft materials by perlecan.

Controlled neo-endothelialisation is critical to the patency of small diameter vascular grafts. Endothelialisation and platelet adhesion to purified endothelial cell-derived perlecan, the major heparan sulfate (HS) proteoglycan in basement membranes, were investigated using in vivo and in vitro assays. Expanded polytetrafluoroethylene (ePTFE) vascular grafts were coated with perlecan and tested in an ovine carotid interposition model for a period of 6 weeks and assessed using light and scanning microscopy. Enhanced endothelial cell growth and reduced platelet adhesion were observed on the perlecan coated grafts when compared to uncoated controls implanted in the same sheep (n=5). Perlecan was also found to stimulate endothelial cell proliferation in vitro over a period of 6 days in the presence of plasma proteins and fibroblastic growth factor 2 (FGF-2), however in the absence of FGF-2 endothelial cell growth could not be maintained during this period. Perlecan was found to be anti-adhesive for platelets, however after removal of the HS chains attached to perlecan, platelet adhesion and aggregation were supported. These results suggest a role for HS chains of perlecan in improving graft patency by selectively promoting endothelial cell proliferation while modulating platelet adhesion.

Use of gait summary measures with lower limb amputees

Gait summary measures have been developed as a convenient method to communicate overall gait pathology. These measures are primarily used in the context of paediatric cerebral palsy and their use remains largely untested in other disability groups. This study assessed the suitability of gait summary measures for use with lower limb amputees. Modified (m) versions of three published gait summary measures were investigated - the Gillette Gait Index (mGGI), the Gait Deviation Index (mGDI) and the Gait Profile Score (mGPS) in conjunction with the Movement Analysis Profile (MAP). Twenty unilateral lower limb amputees underwent three-dimensional gait analysis. All measures reported significant differences between levels of amputation on the prosthetic limb. The mGGI and mGPS detected significant differences between the levels of amputation on the intact side, but the mGDI did not. All gait summary measures were moderately to strongly correlated with leg-length normalised self-selected walking speed and strong correlations were reported between all measures. The MAP exposed common strategies in amputee gait and showed that sagittal hip and knee kinematics contributed predominantly to overall gait deviation in this population group. The mGGI, mGDI and mGPS identified, quantified and stratified gait pathology, indicating that any of the gait measures investigated in this study can be applied as outcome measures in research and case management in lower limb amputees.

Patient satisfaction following lower-limb amputation: the role of gait deviation

Background: Patient satisfaction is an important measurement in healthcare for administrators, clinicians and patients. Objectives: This study investigated the role of gait deviation in patient satisfaction following lower-limb amputation and prosthesis prescription. Study design: A cross-sectional study was done. Methods: Twenty community-based unilateral lower-limb amputees, 12 transtibial and 8 transfemoral, were recruited from support groups. Participants completed the prosthesis evaluation questionnaire (PEQ) with embedded satisfaction-related questions, the timed-up-and-go test and the six-minute walk test, and also underwent quantitative three-dimensional analysis. Kinematic deviation was summarized using the gait profile score (GPS). Results: Satisfaction levels were generally high (median 80 + /100). Sociodemographic variables did not correlate significantly with any of the satisfaction measures (−0.35 ≤ r ≤ 0.54). Satisfaction correlated strongly with the PEQ scales, particularly ambulation, prosthetic utility, frustration, perceived response and social burden (r ≥ 0.70). By contrast, the relationships between satisfaction and performance-based outcome measures were not significant (−0.45 ≤ r ≤ 0.43), and the GPS did not correlate with any satisfaction measures (−0.23 ≤ r ≤ 0.15). Conclusions: In this study of high functioning amputees, gait deviation was unimportant to the amputee, while self-reported functional ability and attitudes toward the prosthesis were the strongest correlates of satisfaction following lower-limb amputation. Clinical relevance For the high functioning individuals with lower-limb amputation in this study, gait deviation was not a significant correlate of patient satisfaction. Results suggest that improving self-perceived functional ability and attitudes toward the prosthesis, rather than minimizing gait deviation, will improve patient satisfaction.

Development of sustained‐release antibacterial urinary biomaterials through using an antimicrobial as an organic modifier in polyurethane nanocomposites

Urinary catheters are among the most frequently used medical devices in clinical practice. However, their use is associated with high rates of nosocomial infection. This study investigates the use of polyurethane nanocomposites (PUNCs) incorporating an antimicrobial agent, chlorhexidine diacetate (CHX), behaving as nanoparticle dispersant and model drug/active agent, as sustained-release antibacterial biomaterials in urinary devices. A range of PUNCs incorporating organically modified silicate (OMS) nanoparticles with CHX was fabricated using a solution-cast method. PUNCs with free CHX added into the bulk polymer were also made. Materials were assessed for antibacterial activity in an in vitro urinary tract (UT) model and release kinetics of CHX was studied. PUNCs demonstrated sustained antibacterial activity against Staphylococcus epidermidis in the UT model, reaching ~50 days infection-free in materials with 2 wt % free CHX loading. Drug-release profiles demonstrated that, compared with microcomposite and unfilled polyurethane, the initial burst effect was significantly reduced in PUNCs. Prolonged drug release was achieved through incorporation of OMS, hypothesized to be due to a combination of barrier properties created by the nanoinclusions and strong interactions between CHX and MMT within the PUNCs. Use of PUNCs for sustained drug release in long-term urinary applications shows promise in addressing catheter-related nosocomial infections.

The flow field near a venous needle in hemodialysis: A computational study

The vascular access used in hemodialysis can suffer from numerous complications, which may lead to failure of the access, patient morbidity, and significant costs. The flow field in the region of the venous needle may be a source of damaging hemodynamics and hence adverse effects on the fistula. In this study, the venous needle flow has been considered, using three‐dimensional computational methods. Four scenarios where the venous needle flow could potentially influence dialysis treatment outcome were identified and examined: Variation of the needle placement angle (10°, 20°, 30°), variation of the blood flow rate settings (200, 300, 400 mL/min), variation of the needle depth (top, middle, bottom), and the inclusion of a back eye in the needle design. The presence of the needle has significant effect on the flow field, with different scenarios having varying influence. In general, wall shear stresses were elevated above normal physiological values, and increased presence of areas of low velocity and recirculation—indicating increased likelihood of intimal hyperplasia development—were found. Computational results showed that the presence of the venous needle in a hemodialysis fistula leads to abnormal and potentially damaging flow conditions and that optimization of needle parameters could aid in the reduction of vascular access complications. Results indicate shallow needle angles and lower blood flow rates may minimize vessel damage.

Analysis of Drug Distribution from a Simulated Drug-Eluting Stent Strut Using an In Vitro Framework

The mechanisms of delivery of anti-proliferative drug from a drug-eluting stent are defined by transport forces in the coating, the lumen, and the arterial wall. Dynamic asymmetries in the localized flow about stent struts have previously been shown to contribute to significant heterogeneity in the spatial distribution of drug in in silico three-compartmental models of stent based drug delivery. A novel bench-top experiment has been created to confirm this phenomena. The experiment simulates drug release from a single stent strut, and then allows visualization of drug uptake into both lumen and tissue domains using optical techniques. Results confirm the existence of inhomogeneous and asymmetric arterial drug distributions, with this distribution shown to be sensitive to the flow field surrounding the strut.The mechanisms of delivery of anti-proliferative drug from a drug-eluting stent are defined by transport forces in the coating, the lumen, and the arterial wall. Dynamic asymmetries in the localized flow about stent struts have previously been shown to contribute to significant heterogeneity in the spatial distribution of drug in in silico three-compartmental models of stent based drug delivery. A novel bench-top experiment has been created to confirm this phenomena. The experiment simulates drug release from a single stent strut, and then allows visualization of drug uptake into both lumen and tissue domains using optical techniques. Results confirm the existence of inhomogeneous and asymmetric arterial drug distributions, with this distribution shown to be sensitive to the flow field surrounding the strut.

Fibrinogen adsorption and platelet adhesion to silica surfaces with stochastic nanotopography

In this study, the effect of surface nanoscale roughness on fibrinogen adsorption and platelet adhesion was investigated. Nanorough silica surfaces with a low level of surface roughness (10 nm Rrms) were found to support the same level of fibrinogen adsorption as the planar silica surfaces, while nanorough silica surfaces with higher levels of surface roughness (15 nm Rrms) were found to support significantly less fibrinogen adsorption. All surfaces analyzed were found to support the same level of platelet adhesion; however, platelets were rounded in morphology on the nanorough silica surfaces while platelets were spread with a well-developed actin cytoskeleton on the planar silica. Unique quartz crystal microbalance with dissipation monitoring (QCM-D) responses was observed for the interactions between platelets and each of the surfaces. The QCM-D data indicated that platelets were more weakly attached to the nanorough silica surfaces compared with the planar silica. These data support the role of surface nanotopography in directing platelet-surface interactions even when the adsorbed fibrinogen layer is able to support the same level of platelet adhesion.