Dendritic ice morphology in unidirectionally solidified collagen suspensions

Collagen is a fibrillar protein which is commonly used as a biodegradable b iomaterial. A typical application of such a biomaterial is a freeze-dried c ollagen sponge which is primarily used as a permanent skin replacement for the treatment of deep dermal burns. Especially to diminish scar formation c aused by a severe loss of skin, it is necessary to induce a cell migration into the sponges and the regeneration of endogenous tissue within the spong es. Thereby, the three-dimensional structure and the pore sizes of the coll agen sponges strongly influence the wound healing. In order to study this i nfluence, the development of a process to manufacture collagen sponges with an adjustable and homogeneous three-dimensional structure is necessary. Th e porous structure of freeze-dried sponges corresponds to the ice crystal m orphology after freezing. In order to form an adjustable and homogeneous ic e morphology, the unidirectional solidification of collagen suspensions was investigated. All experiments were performed in a cryomicroscope according to the Bridgman technique. To induce a constitutional supercooling leading to the breakdown of the planar ice front and the formation of a stable den dritic ice crystal morphology (without side branches), the addition of solu tes to the basic collagen suspension is required. We used acetic acid and e thanol as soluble additives, because they are suitable for medical applicat ion. The effects of the temperature gradient, the ice front velocity, and t he solute concentration on the primary spacing and the dendrite sizes were examined. In order to find a simplified experimental system, acetic acid so lutions without collagen were solidified under the same freezing conditions . Although the primary spacings and the dendrite sizes almost varied in the same range for solutions and suspensions, they were influenced in a differ ent manner by the freezing parameters and the solute concentrations. Varyin g the aforementioned parameters the primary spacing could be adjusted in th e range from 40 to 60 mu m. Furthermore, the ice crystal sizes, which corre spond to the pore size after freeze-drying, could be varied between 30 and 50 mu m. (C) 2000 Elsevier Science B.V, All rights reserved.