Rapid Heat Transfer Dynamics and Cold Gamma Sterilisation Methods for Soft Tissue Allografts

Abstract

The sterilisation of soft tissue allografts presents a particular challenge. Methods must be sufficiently robust to effect bacterial and viral kill while maintaining native structure as well as the biochemical and biomechanical properties of the grafts. The use of cold temperature gamma irradiation has been described as a method for sterilising tissue while minimising deleterious effects. In our experience, cold temperatures alone did little to prevent matrix damage of a freeze-dried dermal product when exposed to sterilising gamma doses. Our studies confirmed that a negligible change of product temperature was detected during gamma processing (photo-heating). Assuming tissue damage was not instantaneous, some accumulation of energy from all sources could contribute to a local damage ‘event’ over time. We focused on optimising the design and composition of the gamma container to increase target density and improve heat transfer. We theorised that increasing the rate of energy transfer away from the tissue would prevent harmful chemical reactions by preventing the required energy thresholds from being attained. Numerous tests have produced good results even with 15kGy of accumulated dose absorbed by the tissue. Localisation of the reaction kinetics by cold temperatures may limit the number of opportunities for adverse reactions at any given time during gamma exposure, by nature of the probability of a given number of photons intercepting molecules of interest. This rate may fall within the capability of special materials designed at LifeCell to disperse energy to a -80° C cold reservoir. Gamma processing of human sheet dermis, tendon and other tissue derived products has been accomplished with these special methods. Preliminary tests indicate good results employing accumulated doses up to 40kGy upon osteoinductive materials. Current research is investigating the application of 12kGy upon cryoprotected skin prior to processing, followed by a second 12kGy exposure of the same tissue in protective packaging in the same environment.