New research directions

As a result of the experience of the past two decades, the problems involved in developing an adhesive system for both soft and hard tissues have been addressed. Nevertheless, it is difficult to reconcile short- and long-term biocompatibility needs with chemical adhesion mechanisms that use reactive monomer systems.

Where relatively temporary adhesion is required, as in wound healing, systems based on natural models that allow biodégradation of the adhesive and interface and subsequent normal tissue remodeling appear to merit further development. For longer term durability in both soft and hard tissues, hydro-philic monomers and polymers of low toxicity which can both diffuse into the tissue surface and form ionic bonds across the interface seem to be the most promising approaches. Evidence has been obtained of the need for hydrophobic—hydrophilic balance in adhesive monomer systems (Nakabayashi et al., 1991). The use of hydrophilic monomers such as hydroxyethyl methacrylate in commercial materials has facilitated surface penetration.

On calcified surfaces, the use of hydrophilic electrolytes such as the polycarboxylates has demonstrated that proven ionic bonding in vitro can also be achieved in vivo. An advantage of such systems is that surface molecular reorientations can improve bonding with time. Encouraging preliminary results have been obtained with such glass ionomer cements in orthopedics and there is considerable scope for the future development of such polyelectrolyte cements.

A practical limitation in many systems is ease of manipulation and application. For example, the effectiveness of the fibrin sealant is critically dependent on proper mixing of the ingredients and uniform application. Further technology transfer could improve this often-neglected area of adhesive development. For example, the visible-light polymerization technology developed in dentistry that allows extended working time and curing "on demand" in a few seconds could usefully be applied to medical applications. Laser activation of fibrin sealants has received an initial trial.

The development of more efficient adhesives and sealants that, in addition to enhancing the durability of current applications, would permit new applications such as osteogenic bone space fillers, percutaneous and permucosal seals, and functional attachment of prostheses is a challenging problem for the future.

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