Porous biomimetic matrices

With tissue engineering damaged tissue and organs shall be replaced by biological compatible and functional implants from primary cells. For the development of autologous transplants primary cells are isolated and proliferated in cell culture until sufficient cells are available for the colonisation of a matrix or for direct cell therapy. Porous biomaterials, which can be manufactured by several processes, might be suitable matrices for such cells to get three-dimensional cell cultures.

Figure 1: Asymmetric cellulose acetate membranes produced by a phase inversion process.
Figure 2: Porous polylactic acid matrices; the pores were created using template processes (salts) and freeze-drying.

Porous membranes are often manufactured by the so-called phase inversion process. With this process especially asymmetric structures from various polymers can be obtained (Fig. 1). For freeze drying a polymer solution is frozen which leads to a phase separation. Then the solvent is evaporated and thereby the generated structure is fixed. Depending on the used solvent ordered structure can be obtained by crystallisation effects (Fig. 3). The so-called template method uses insoluble particles (e.g. salts) which are introduced in polymer solutions. After drying the solution the salt crystals can be solved by suitable solvents leading to defined pores in the polymer matrix (Fig. 2).

Figure 3: Electrospun nanofibres (left: polylactic acid, right: polyvinylpyrrolidone).

A completely different method is electrospinning. Here a jet of a liquid is accelerated in an electrical field and thereby the liquid jet is stretched. If the liquid is a polymer solution, suitable solvents are evaporate during spinning and polymer fibres with diameters in the sub-micron and nanometre range can be obtained (Fig. 3). The dimension of the fibres is controlled by the spinning conditions (mainly strength of electrical field, distance of electrodes and concentration of the polymer solution). Various polymers, like PVP, PEO or PLA can be processed by electrospinning. If inorganic precursors are added to the spinning solutions, composite fibres or after sintering even pure inorganic fibres can be obtained.