Approximately 5 million pounds of PU per year are consumed making wound dressings [28, 29]. The role of a wound dressing is to provide protection to damaged skin following an operation, a burn, or a laceration, for instance. The dressing can be occlusive such that the wound is completely isolated or can be nonocclusive such that there is a semipermeable membrane or foam capable of absorbing fluids produced by the wound and transmitting gases. Wound dressings can also come with various surface treatments to prevent infection, to prevent adhesion, or to promote more rapid
TABLE 11.1 Formulation and properties of thermoplastic polyurethane commonly employed for catheters with a Shore D hardness of 75 and 55 and Shore A 80
reendothelialization (skin regrowth). PU find use in nearly every facet of wound dressing, but are by no means the only material used for this purpose.
For flexible solid substrate, a thermoplastic material such as the Shore A 80 material defined in Table 11.1 is often used. Softer versions can also be employed by reducing the amount of hard segment in the formulation. In some cases, it is desirable to make a clear substrate through which the surface condition of the wound can be inspected without having to completely remove the dressing. For increasing film transparency, there are many means of proceeding including reducing hard segment, using an aliphatic hard segment, using polyester soft segment, mixing chain extenders, or processing in such a way to minimize hard segment phase growth [30, 31].
For nonocclusive dressings, the semipermeable component is sometimes a foam or nonwoven structure. There has been recent work on making nonwoven dressing by an electrosprayed process [32, 33]. Electrospray can make fibrous thread with very small diameter and allow addition of functional components during the spinning process. Conventional flexible PU foams used for comfort applications are not used for wound dressings because they do not provide adequate hydrophilicity to sufficiently absorb bodily fluids. This is overcome by maximization of the polyethylene glycol content in the foam soft segment . The increased hydrophilicity also provides opportunity to act as a high surface area substrate for wound healing additives.
PU foam structures can be used for a wide variety of wounds but, because of their absorptive capacity, are primarily used in cases where a relatively large amount of fluid must be imbibed. Wounds where foam dressings are chosen are, for example, burn sites, sutured sites, skin grafts, skin graft donor sites, and covering tube sites that transit through to the body exterior (i.e., tracheostomy). The foam is usually covered by a solid or air permeable film structure on the outside and a liquid permeable membrane facing the wound to prevent abraded urethane from entering into the wound. A negative pressure is sometimes employed by evacuating the foam structure through a solid-surfaced/airtight bandage covering. The partial vacuum draws the exudates into the bandage to more efficiently prevent festering .
Examples of PU foam hard segment are TDI-water, MDI-water, and various aliphatic structures. The soft segment is usually designed to provide the wetting interface either for the fluids or for specialized functional coatings. Common soft segment compositions are based on propylene oxide (PO) and ethylene oxide (EO), with EO reported to be as high as 75%. Froth foams have also been prepared for wound dressings using all polyethylene glycol soft segment, 1,4-butanediol (BDO), glycerine, and water as a chain extender/blowing agent and hexane diisocyanate for the polyisocyanate. The foaming is assisted by whipping large volumes of air into the reacting mass at elevated temperature and then allowing the blow reaction to expand the foam to its final potential. Such pads can be relatively soft, made at relatively higher densities, and are of course very hydrophilic .
Foam dressing size is also optimized for various functions. Foam pad thickness is usually in the range of 1.0-lO.Omm to optimize for capacity and facile attachment to the wound and surrounding skin. In contrast to a foam prepared for comfort where foam densities may be less than 2 lbs/ft3, a wound dressing density can be more than 6 lbs/ft3 to facilitate contact and wettability. The upper density is usually controlled such that the open-cell content is very high and the foam is not perceived to be hard.
Recent work has made electrosprayed non woven absorbent layers with commercial equipment specially designed to heat a molten polymer to a high enough temperature to reduce viscosity. Low viscosity is required so the TPU can exit a small orifice spinneret. Alternatively, the PU elastomer can be dissolved in a solvent at a concentration such that the viscosity is optimal for extrusion and removal of the solvent. Nonwoven absorbent TPUs have been reported electrospun from polymer melts . With specialized equipment, this can be achieved using a standard TPU formulation. A TPU with Shore A 85 and composition by weight 60% PTMEG 1000, 34% 4,4'-MDI, and 6% BDO has been demonstrated to extrude fibers less than 10|jm in diameter. In contrast, a Shore A 97 TPU based on a polyester soft segment was dissolved at 10% (weight) in a 50/50 mixture of dimethylformamide and methyl ethyl ketone to electrospray fibers less than in diameter . A similar result was obtained by a 50/50 solvent mixture of dimethylformamide and tetrahydrofuran.