Artificial skin, dressing materials and novel drug delivery systems Part 2

Wound contamination caused by cotton gauze
Problems of microbial contamination have been observed with cotton gauze. Moisture is an essential component of any wound, whether a burn or a boil. In a study conducted by Oie et al at the department of pharmacy, University Hospital, Ube, Japan, seven in-use cotton gauze samples and three cotton balls soaked in sterile distilled water in canisters were investigated 7 days after they were prepared in the hospital, before use. The investigators found all the samples to be heavily contaminated with bacteria including 10(6) to 10(7) colony forming units/ml of Pseudomonas aeruginosa. The cotton balls soaked in sterile distilled water too was found to have a high percentage of contamination with P. aeruginosa, Serratia marcescens and Candida albicans.

Chitin beads with a carboxymethyl coat
At the department of chemistry, National University of Singapore, Yusof et al have standardised a method of making highly absorbent chitin beads. Chitin beads when coated with a layer of carboxymethyl cellulose have an absorbing capacity which permits them to absorb up to 95 times their dry weight of water. Such beads are especially useful in wound healing.

The beads are made by first preparing a 0.5% solution of chitin. Chitin dissolved in N, N-dimethylacetamide/5% lithium chloride (DMAc/5%LiCl) forms a 0.5% chitin solution. Chitin beads are made by dropping 0.5% chitin solution into a nonsolvent coagulant, ethanol.

Ethanol-gelled chitin beads have an average diameter of 535 microm. The chitin beads when activated in 50% (w/v) NaOH solution react with 1.9 M monochloroacetic acid/2-propanol solution to develop a carboxymethylated surface layer.

In another study, Stone et al have studied the role of chitin as a wound dressing to promote rapid wound healing. Chitosan is a derivative of chitin, extracted from the exoskeleton of lobsters, crabs and shrimps. As a semi-permeable biological dressing, it forms a sterile wound exudate beneath a dry scab, preventing dehydration and contamination of the wound to optimize conditions for healing.

Chitosan when used as a wound dressing appears to have a role to play in causing rapid wound re-epithelialisation and regeneration of nerves within a vascular dermis. Chitosan films prepared using two different solvents, acetic acid (Chitosan-AA) and lactic acid (Chitosan-LA), for wound dressing have been found to differ significantly in terms of mechanical and bioadhesive strength properties. The lactic acid treated chitosan has been observed to have a lower tensile strength, greater flexibility and bioadhesiveness than Chitosan-AA. The chitosan film has also been found to be permeable to water vapour. In contrast, the use of chitosan-AA films has been shown to cause adverse skin reactions.

Chitin-carboxymethyl bilayer
Loke et al at the National Laboratories, Science Park, Singapore have reported on the remarkable properties of a novel nonadherent wound dressing with sustained anti-microbial action. The wound dressing has an upper layer of carboxymethyl-chitin hydrogel and a lower layer made from chitosan acetate foam, and impregnated with chlorhexidine gluconate.

This unique dressing enhances wound healing as well as exerts anti-microbial action. The hydrogel layer acts as a mechanical and microbial barrier, and absorbs wound exudate. In tissue fluid, the carboxymethylated-chitin hydrogel has been found to absorb up to 4 times its own weight of water and is also highly porous to water vapor. This water absorbing capacity of the dressing material has been found to be especially useful in absorbing wound exudates, as seen in burn wounds.

Polyurethane foams
Polyurethane (PU) foams used in wound management are produced by a reaction between aromatic diisocyanates and polyether polyols. There is serious concern that residues of these starting materials, which may contain functional aromatic amines may leach from the finished polymer during in vivo applications.

Carboxymethylated cellulose fibers
Some studies on carboxymethylated cellulose fibre in fabric form show that the material can immobilise fluid by gel blocking. This can be beneficial in the treatment of chronic wounds by protecting the delicate peri-ulcer area from maceration by exudate.
In other studies as that carried out by Vloemans et al at the Netherlands, the wound healing properties of a newly developed, carboxymethylcellulose based hydrofibre dressing have been investigated. The hydrofiber dressing material has been found to be safe, easy to use and suitable for the treatment of partial thickness burns.

Polylactides
Polylactides are prepared by direct polycondensation of lactic acid, or by ring-opening polymerization of cyclic dilactides (usually called lactides). Copolymerizations of lactides with glycolide or epsilon-caprolactone has led to the development of new wound dressings. Kricheldorf at the University of Hamburg has reviewed the properties of a new type of wound dressing based on transparent or resorbable films of copolylactides.

Octylcyanoacrylate
Octylcyanoacrylate is a medical-grade adhesive with antimicrobial properties. It forms a thin, flexible, occlusive bandage. This external spray bandage is well tolerated and does not cause any signs of histotoxicity or adverse wound healing.

Alginate dressing
Alginates have been used as dressing material for a variety of wounds with moderate to high levels of exudate. These compounds represent a cost-effective treatment option for flat and cavity wounds.
One such material that has been used successfully for wound healing is Algosteril, a natural, pure, non-woven dressing made from calcium alginate fibres. The material rapidly absorbs and retains wound fluid to form an integral gellified structure, thereby helping to form an ideal moist wound healing environment.
Within the network of gellified fibres, macrophage and platelet activity is stimulated along with the trapping and engulfment of bacteria. That leads to achieving adequate control of haemostasis and accelerated wound healing. It has been observed that alginates containing zinc ions have the greatest potentiating effect on prothrombotic coagulation and platelet activation.

Silicon gel coats
Bugmann et al at the Hopital des enfants, Geneva, Switzerland have reported on the remarkable wound healing properties of a new grid like silicone coated nylon dressing which has no additional biological compounds added. Unique features of this dressing material include its easy removability since it adheres only to intact skin and faster healing time due to a direct effect of silicone on epithelial growth.