| Summary: | <p>Artificial tissue expansion is required to generate new skin prior to reconstructive surgery, in
order to compensate for a deficit of healthy tissue. Hydrogel tissue expanders, which expand
anisotropically, show great promise in overcoming clinical limitations in the field, thus
allowing the technique to be used in a wider range of surgeries. These devices consist of
pellets of dry poly(methyl methacrylate-co-vinylpyrrolidone), compressed into discs through a
hot compression moulding process. However, a number of significant problems still exist in
these devices, and this thesis aims to address these issues. </p>
<p>To date, there has been a lack of investigation of the factors governing the behaviour of
anisotropic swelling. For this reason, a range of different compression ratios have been
investigated, with particular focus on the relationship between the material flow during
compression and the swelling behaviour of the resulting device. It was found that samples of
the same initial size expand to the same reference swelling dimensions, regardless of
compression ratio. During hot pressing, the material flow was found to be governed by slip stick behaviour at the interface between the hot press and the device, affecting the properties
and swelling behaviour of the devices. </p>
<p>Based on these findings, devices were developed which could expand from a disc into a non prismatic shape (dome or wedge). Such devices could reduce complication rates and allow
the growth of new tissue with anisotropic resting tension. The devices were tested in a small
in vivo trial, where it was shown that there were no adverse effects on the tissue produced,
and that the shape of the expander (dome) was retained.</p>
<p>As devices are being produced for medical use, understanding the effect of sterilization by γ irradiation is essential, but to date this has been overlooked in the literature. It was found that
γ-irradiation caused an increase in cross-linking in the P(MMA-co-NVP). Whilst this produced
little change in swelling behaviour for isotropic devices, in the case of anisotropic devices it
caused a change in the shape of expansion, reducing the area of new skin which could be
generated by the device. It was found that by reducing the concentration of impurities
(residual molecules from the polymer synthesis) the impact of γ-irradiation could be greatly
reduced. </p>
<p>Finally, controlling the rate of expansion is essential in order to avoid clinical complications.
In order to control the rate of expansion, particularly during the initial period of swelling, semi permeable PDMS coatings were applied to the compressed devices. Coatings of thickness
greater than 0.375mm were found to effectively control the rate of swelling, for both
cylindrical and non-prismatic shapes. As the coating thickness increased, the maximum
swelling size decreased. However, it has been shown that change in height (the parameter
which governs the area of skin produced) is affected less than the change in mass or
diameter.</p>
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