Optimisation of Collagen Films Produced via Pulsed Current Electrophoretic Deposition
Electrophoretic deposition (EPD) has been used for the deposition of insoluble collagen type I in aqueous suspensions. In this process, two electrodes are immersed in an aqueous collagen suspension and a voltage is applied. The charged particles in the suspension move towards the oppositely-charged electrode resulting in the deposition of a coating, which can be removed to create a free-standing collagen film. The aim of this project was to maximise processing efficiency and yield, and improve material properties of the collagen films produced using EPD. The behaviour of collagen I from two different suppliers was compared when processed using pulsed current electrophoretic deposition (PC-EPD). It was found that, at voltages required for adequate yield (at least 5 V), formation of bubbles or corrosion of the electrode occurred for suspensions in aqueous media. However, dialysis of collagen suspensions against deionised water for 30 hours was shown to remove residual charge-carrying ions, notably Na+, K+, Cl- and Ca2+, arising from the collagen extraction process. This allowed deposition to be carried out at higher voltages (up to 10 V), leading to higher yield and the production of consistent films, avoiding the processing problems. PC-EPD was mathematically modelled, in order to broaden understanding of the process and aid in future investigations. A pulsed current factor was developed and introduced to previously-described direct current EPD models, derived from the Hamaker equation. To validate this, collagen suspensions were characterised, and measured variables were implemented within the model. The model was compared with experimental data and was found to be accurate at applied potentials of 10 V and above. Deviations between the model and experimental values for voltages of 5 V or less were explained by factors within the pulse cycle that were not accounted for, such as molecule acceleration time. Collagen deposits, once dried, adhered strongly to the electrode and made removal from the substrate challenging. To solve this problem, a sacrificial cellulose acetate layer was developed to assist detachment of collagen deposits from the substrate, giving free- standing films. It was also shown that more complex collagen films could be created by depositing onto shaped electrodes, and that these could also be removed successfully using a cellulose sacrificial layer. Analysis of grooved films showed that features from the substrate were reproduced successfully onto the collagen deposits, with a resolution of 10 μm for topographical surface features, however, the larger grooved features measured approximately 100 μm (±50 μm) greater than the substrate due to the film thickness. The density of collagen biomaterials can influence their stability and degradation properties. The densities of cast films and EPD films were compared, and deposited films were found to be at least 15% more dense. Furthermore, it was found that the density of EPD collagen films could be controlled by changing the deposition voltage, with a densities ranging from 400 kg m−3 at 5 V to 700 kg m−3 at 10 V. Films with a higher density were shown to have increased enzymatic degradation resistance, potentially reducing the need for chemical cross-linkers to stabilise these collagen devices, giving potential benefits for bioactivity. Surgical application of membranes often requires the use of suturing and, therefore, the biomaterials are required to be tear-resistant. It was hypothesised that alignment of collagen fibres within the membranes would increase their ultimate tensile strength (UTS). Through optimisation of the EPD rig and processing parameters, alignment of collagen was induced during deposition by generating interfacial shear between the substrate and suspension. This was achieved using two different EPD setups, and the collagen alignment created was assessed using birefringence measurements. It was found that successful alignment was achieved (with corresponding birefringence values of 0.0025) using (1) a rotating cylindrical electrode with a tangential velocity of 0.0006 m s−1 at 10 V PC-EPD, and (2) volumetric flow rate of suspension pumped through the EPD cell of 1.67 mL s−1 at 40 V direct current EPD. This alignment was found to increase the UTS of cross-linked collagen films in the direction of alignment to 6.26 MPa, compared to 3.5 MPa in non-aligned films. Multilayer, cross-ply films were then produced by changing the film orientation between deposition steps. The development of the multi-orientated cross-ply collagen films could result in membranes with increased tear resistance. This thesis has developed the capabilities of EPD processing for collagen films, providing tunability of material properties and forms, and allowing for the design of collagen membranes for a range of medical applications.
Collagen, Films, Electrophoretic deposition, Pulsed Current
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