Dendritic ice morphology in unidirectionally solidified collagen suspensions

Collagen is a fibrillar protein which is commonly used as a biodegradable biomaterial. A typical application of such a biomaterial is a freeze-dried collagen sponge which is primarily used as a permanent skin replacement for the treatment of deep dermal burns. Especially to diminish scar formation caused 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 sponges. Thereby, the three-dimensional structure and the pore sizes of the collagen sponges strongly influence the wound healing. In order to study this influence, the development of a process to manufacture collagen sponges with an adjustable and homogeneous three-dimensional structure is necessary. The porous structure of freeze-dried sponges corresponds to the ice crystal morphology after freezing. In order to form an adjustable and homogeneous ice 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 dendritic ice crystal morphology (without side branches), the addition of solutes to the basic collagen suspension is required. We used acetic acid and ethanol as soluble additives, because they are suitable for medical application. The effects of the temperature gradient, the ice front velocity, and the solute concentration on the primary spacing and the dendrite sizes were examined. In order to find a simplified experimental system, acetic acid solutions 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 different manner by the freezing parameters and the solute concentrations. Varying the aforementioned parameters the primary spacing could be adjusted in the range from 40 to 60 mu m. Furthermore, the ice crystal sizes, which correspond 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.