Recent progress in the replication of hierarchical biological tissues

Bioinspired materials research is a continuously growing field in interdisciplinary materials science and engineering with large potential for novel functional materials. One possibility of combining the hierarchical structure of natural materials with the broad range of available constituent materials is biotemplating, i.e., the use of biological materials as scaffolds or casting molds for the synthesis of ceramics, semiconductors, metals, polymers or composites thereof. However, the replication of structural details of complex biological tissues over several levels of hierarchy down to the nanometer level is still a big challenge. The biological templates need to be (made) accessible for the precursor materials down to the nanometer scale, and the desired inorganic replica often require a final thermal treatment with the danger of collapse of the nanostructure. Here, some of the current knowledge about the replication of hierarchical biological materials into ceramics or carbons with special emphasis on wood as a template is reviewed. It is concluded that real hierarchical replication down to the nanometer scale requires a careful preparation of the biological template, followed by elaborate template infiltration via gas or liquid phases and their transformation into a solid, and finally, the gentle removal of the template. Not surprisingly, retaining the replicated material in an amorphous or nanocrystalline state is a key requirement for a successful nanometer-scale replication of biological materials.