Gamma-irradiation of lyophilised wound healing wafers
Introduction
Chronic wounds, such as diabetic and venous ulcers, display a relatively long healing process and present a difficult physical environment for the targeted application of antibacterials, growth hormones, polypeptide growth factors and other therapeutic agents (Loots et al., 2002, Thomas et al., 1996, Puolakkainen et al., 1995, Thomas, 1990). Large variations in the rate at which exudate is produced suggest that there is no single topical delivery system suited to all wound types. In the absence of applied therapeutic agents, containment of a moist environment in the immediate wound area, left to heal by secondary intention, is generally recognised to be beneficial whether the suppuration is high or low. Relatively dry wounds require the application of hydrated or pre-swollen substrates whereas wet wound beds require materials that can maintain a balance between the absorption and retention of wound fluid, and the release of water vapour to the atmosphere.
Moisture-retentive dressings such as hydrogels and hydrocolloids can be used to maintain the ideal conditions for the complex and highly regulated healing cycle but the inevitable colonisation of the wound site by potentially pathogenic bacteria can compromise this process. As both endogenous and exogenous bacteria can cause wound infection the risk is minimised if proper care is taken. For endogenous bacteria this normally involves surgical debridement of devitalised tissue and control of bacterial load and inflammation (Bowler, 2002). Sterilisation of applied treatments may be expected to minimise the risk of contamination by exogenous foreign bodies and the dressings themselves can retain harmful bacteria within their swollen structures, minimising dispersion when removed (Walker et al., 2003).
The production of lyophilised wafers as matrices for the direct delivery of therapeutic agents to chronic wounds has been described (Matthews et al., 2003, Matthews et al., 2005). A wide range of viscous behaviour was demonstrated for wafers composed of sodium alginate (SA) and xanthan gum (XG) modified with varying amounts of high molecular weight methylcellulose (MC). XG wafers were of particular interest due to the existence of a yield stress that retarded the viscous flow of rehydrated wafers, permitting a longer residence time on the target surface. Use of a non-animal model for medium to heavily suppurating wounds (Matthews et al., 2005) demonstrated the potential of these systems as stable vehicles for the storage and delivery of both soluble and insoluble wound healing drugs.
In recent years, the exposure to gamma-irradiation has been increasingly used to sterilise or reduce bacterial charge in drug-delivery devices (Maggi et al., 2003). Autoclaving procedures can compromise the stability of thermally labile antibacterial agents (Traub and Leonard, 1995) and the temperatures involved may be expected to degrade proteinaceous growth factors (Bare et al., 1994). Alternatively, ethylene oxide can be used but concerns over its safety to operators and patients, and the environmental impact of its diluents (CFCs) has resulted in a trend towards the use of gamma-irradiation, especially for the sterilisation of disposable medical devices (Woolston and Davis, 1994). Gamma-rays, generated by a Cobalt-60 source in a specially designed irradiation cell, should be powerful enough to completely destroy biological systems but not to damage the material being sterilised. In practice, the sterilisation of pharmaceutical preparations such as, powders, liquids, creams, ointments, tablets and capsules is undertaken, however, the effects of ionising radiation on the novel systems described in our previous paper (Matthews et al., 2005) is unknown.
Presumably many excipients, which include water-soluble polymers, are not unduly damaged at the expense of their function. It is known that cellulose and its commonly encountered cellulose esters will tolerate sterilisation doses and that sterilisation by radiation is frequently the preferred method for dressings (Woolston and Davis, 1994) however, there does not appear to be any directly relevant information on the effect of irradiation on lyophilised cellulose ethers, alginates or xanthan, preferred polymers for the fabrication of wafers.
Generally for polymers, free radicals and ionic species produced by the radiation either result in crosslinking or chain scission, both serving to alter the molecular weight with associated changes in physical properties. Although both mechanisms will take place, one will usually predominate. In the case of sodium alginate it is well established that the overall effect of gamma-irradiation is to degrade the polymer (Hartman et al., 1975, Kume and Takehisa, 1983, King, 1994). The three studies cited used irradiation levels of 25, 50 and 10 kiloGrays (kGy), respectively, the lowest level of 10 kGy being the recommended limit for food products. Another study (Grant and D’Appolonia, 1991) concluded that a lower dose of 300 kRads (3 kGy) actually increased the gel viscosities of highly branched, low molecular weight polysaccharides (pentosans) by the relocation of branch-points or increased branching. Similarly, for starch/xanthan mixtures it has been reported that irradiation increased the apparent viscosity at the higher dosages of 10–30 kGy (Hanna et al., 1997). For modified cellulosic materials, gamma-irradiation of carboxymethylcellulose (CMC) in the solid-state resulted in degradation irrespective of the degree of substitution (DS) (Fei et al., 2000). In solution, however, crosslinking was favoured with high DS grades and increased polymer concentration. The study used doses of irradiation up to an excessive 100 kGy. A more recent study on the stability of prolonged release matrix tablets of hydroxypropyl methylcellulose (HPMC) after gamma-irradiation in the range of 7.5–50 kGy (Maggi et al., 2003) concluded that chemical modifications in the hydrophilic polymer caused a “progressive decrease of the average molecular weight with increasing radiation dose”.
Other than the studies cited in this paper, there does not appear to be any reference to the effects of gamma-irradiation on methylcellulose, MC or indeed, lyophilised blends of MC with both SA and xanthan gum, XG. This work therefore aimed to investigate the effect of sterilising doses of gamma-irradiation on the rheological properties of reconstituted wafers composed of these polymers of interest. Lyophilised wafers are being developed as sterile, drug delivery systems for the treatment of chronic wounds.
Section snippets
Materials
All materials were of the same source and batch as those used and described in our previous paper (Matthews et al., 2005). Sodium alginate was a low viscosity grade of unknown molecular weight from Hopkins and Williams, UK. Methylcellulose (Methocel™ A4M) was supplied by the Dow Chemical Company, USA and pharmaceutical grade (USP/EP) xanthan gum (Xantural™ 180) was obtained from CP Kelco US, Inc., USA. Gelatine powder (approximately 150 bloom from pig skin) and agar, used in the wound models,
Rheological measurement
As previously outlined (Matthews et al., 2005) all gels reconstituted from lyophilised wafers demonstrated shear thinning with pseudoplastic type flow curves (Fig. 2, Fig. 3). All SA/MC curves (Fig. 2(a and b)) intersect the origin whereas the XG/MC curves (Fig. 3(a and b)) show a small yield value (yield stress). Irradiation of SA/MC wafers, subsequently rehydrated by the addition of a volumetric amount (5 mL) of distilled water, produced solutions with drastically reduced apparent viscosities (
Discussion
As highlighted in our previous paper (Matthews et al., 2005) it was clear from the rheological measurements on non-irradiated wafers containing SA/MC and XG/MC that MC was an effective viscosity modifier for both systems studied. The ability to produce lyophilised wafers with such a wide range of gel viscosities upon rehydration was thought to be one solution to the design of delivery systems capable of exercising controlled flow properties on wounds with different rates of suppuration.
Acknowledgement
The authors wish to thank Pfizer Ltd., for the funding with which these studies were possible.
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