Traditionally, the term was used in the context of implantable devices for long-term clinical use. Initially, safety was the main requirement of the materials; it was achieved through their chemical and biological inertness. The materials had to be non-toxic, non-carcinogenic, non- allergenic, non-thrombogenic, etc., such a list of negatives becoming the definition of biocompatibility. Materials of this type include titanium or platinum-based metal alloys and polyethylene or silicone-based polymers.

The increasing complexity of clinical applications has led to the understanding that the material still has to enter into specific interactions with the body, rather than simply ignoring the surrounding living tissue. To ensure efficient engraftment the material has to incite the desired response from a tissue. Ceramic nanocoatings of bone substitute implants that can induce bone formation are examples of bioactive material. Finally, for many applications it is important to ensure safe material resorption and replacement with a natural tissue. Classic examples are polyester sutures and orthopedic implants. However, "nonliving" substitute materials can recover only the physical and mechanical properties of the organs, but cannot restore metabolic functions. The last decade has seen a fundamental change in the concept of regenerative medicine: it now focuses not on the substitution of organs with synthetic materials, but on the regeneration of the diseased tissue. The key approach is tissue engineering, whose purpose is to restore the organ through the directed and controlled stimulation of specific cells by molecular and mechanical signals. The important point is to create a bioresorbable and bioactive matrix that can initiate and sustain tissue regeneration. The most promising tissue-engineering constructs are matrices based on biopolymers (collagen, silk, chitosan, etc.) with allogeneic human cells (including stem cells). Nanostructures of many matrix materials (e.g., nanocomposite of hydrophobic and hydrophilic polymers, or specific fibre structures) determine their biological properties.

Thus, depending on the reaction of the tissue to the implant, materials can be subdivided into four categories:

- toxic (that kill the surrounding tissue);

- inert (in the body such materials get surrounded by a fibrous non-contiguous tissue);

- bioactive (an interfacial bond is formed between the material and the tissue, encapsulation is minimal);

- bio-resorbable (as the material dissolves it is gradually replaced by the host tissue; the products of dissolution must be non-toxic).

The above categories, except for toxic, belong to the class of biocompatible materials.