Advances in medical technology come with a large amount of materials that are currently under consideration for possible implantation into the human body. With the object to be compatible with the body these potential materials need do not cause any further harm. Biodegradable materials are dispelling the current stereotype in biomaterial science to research and produce only corrosion resistant materials. Especially, materials which consist of nutrients existing in the human body are high-potential expectants for this approach. The main point is that biodegradable implants and coatings can support tissue regeneration and healing in the process of material degradation and can gradually dissolve with replacement by natural tissue.
Biodegradable metals take priority over current biodegradable materials for example bioactive glasses, polymers or ceramics as applied to loads in body that require a higher tensile strength and a Young’s modulus that is closer to bone.
The magnesium has drawn great consideration in capacity of biodegradable material for bone joint replacement due to its similar mechanical properties with human bone, biodegradation and biocompatibility. By the way, the high degradation rate of magnesium delimitates its application as bioimplant materials. In the past decade, various approaches have been investigated to improve the degradation behaviour of magnesium. Development of biocompatible coatings and alloying are the most common methods that have been investigated. It has to be said, biocompatible calcium phosphate (CaP) has limelighted great attention in recent years as a coating material on magnesium and its alloys.
The development of biocompatible CaP coating on the surface of magnesium alloys is one of the way to control their degradation rate. Thick CaP coatings that were prepared by wet-chemical methods are well known. Nevertheless, wet-chemical methods of CaP coating deposition on the surface of magnesium alloys often is afflicted with weak adhesive strength. Currently, researchers are prone to establish a thin protective surface layers on the surface of alloys, which allows to keep the initial substrate topography and enhance the corrosion resistance. However, even by now, there is insufficient information on the corrosion resistance of these coatings. The RF magnetron sputtering is a preeminent method for deposition of pure HA. On top of everything else, a HA coating received by RF magnetron sputtering is well-adhered to the substrate.
At present in medicine, the question is about creation of artificial tissue reproducing the functions and structure of living tissue for a limited period. Therefore, the development of bulk (3-D) scaffolds based on biodegradable polymers is an important area of bioengineering.
Biodegradable polymers are specific materials that are capable of degradation caused by microbiological and chemical processes. The development of biomaterials from biodegradable polymers will allow us to form the bone implants for the treatment of a broad spectrum of fractures that not only will support and serve as a damaged bone, but also to stimulate its growth and recovery. Biodegradable implants have all the advantages of conventional implants (from metal). Additional advantage is gradually breakdown in a patient organism to result in non-toxic products. After that, the decomposition products are extracted from the body during metabolism. It is therefore possible to remove the implants from a patient's body without repeated surgery. All these factors determined the great interest in search of technology, focused on a construction of materials and implants based on biodegradable polymeric materials.
The purpose of this research is the formation of a new hybrid composite material based on biodegradable polymers for regenerative medicine. New material should have gradient porosity to make imitation of bone structure. It should have mechanical properties similar with properties of damaged part of bone and repeat chemical composition of hard tissue, which contains 90% of calcium phosphate. In addition, implant surface should provide a good cell adherence to stimulate ingrowth of tissue into the matrix material.
Work is carried out with the collaboration of Technology Centre of Institute of Physics and Technology National Research Tomsk Polytechnic University, Fraunhofer Institute for Interfacial Engineering and Biotechnology, the University of Duisburg-Essen and Karlsruhe Institute of Technology KIT in Germany.
At present, synthesis and formation of 3-D bulk scaffolds are performed based on Technology Centre by using method of electrospinning. Materials are produced from next biodegradable polymers: polycaprolactone, poly-3- hydroxybutyrate and copolymer poly (3-hydroxybutyrate-co- 3-hydroxyvalerate). The method of electrospinning allows to synthesize 3-D bulk materials with fibrous structure and to control pore size and mechanical properties.
In Fraunhofer Institute for Interfacial Engineering and Biotechnology the polymeric films and 3-D scaffolds were modified by using radio-frequency (RF) reactive plasma. Studies have shown that surface of untreated polymer was hydrophobic, poor wettability, had low surface energy, that leads to bad cell adherence on the material surface. After plasma treatment the polymer's wettability improved significantly and surface energy increased. The advantage of plasma treatment is that it enables to form functional groups on the surface, which then allows an attachment of macromolecular compounds to the polymer. The results of performed researches
are presented in follow articles [Materials Letters 163 (2016) 277–280, Materials Science and Engineering: C, 62 (2016) 450–457].
In one direction our research group study the fundamental principles of RF-magnetron sputter deposition of calcium phosphate coating as an approach to enhance biocompatibility of metallic and polymer materials applied in regenerative medicine.
Due to the current trend of the population ageing the development of biomaterials for tissues regeneration plays a major role in the field of medical material science. Nowadays, the doctor should have a number of implant systems at his disposal allowing to choose the device for individual patient’s needs to solve any even the most difficult clinical problems. To achieve the favorable implant interaction with the living body the specified interface properties of devices should be created.
As it is considered the interaction between the implant and organism occurs by the aggression path. Human body interprets implant as a foreign substance and tries to get rid of it. The interaction between the cells and tissues with the biomaterials is strongly defined by the tissue-implant interface. The surface properties determine both the biological response to the implants and the material response to the physiological environment. Hence, surface engineering of biomaterials is aimed at modifying the material properties through changes in surface properties meanwhile maintaining the bulk properties of the implant. The application of a biocompatible coating to the implant is a well-documented strategy. Biocompatible coatings must be free from toxic substances, do not cause negative immune reactions, don’t degrade (or degrade in a predictable manner) in contact with living tissues, and show a high adhesion to the implant. Calcium-phosphates are proved to be a natural metabolite of bone. In the field of medical material science, there are numerous studies devoted to increasing of the biocompatibility and osteoinduction of metal implants using calcium phosphate coatings (CaP). Hydroxyapatite (HA, Ca10(PO4)6(OH)2) – is a typical example belonging to the family of CaP materials. As this material defines the bone matrix, it possesses a high biocompatibility.
Radio frequency (RF) magnetron sputtering method is used to obtain the functionally-graded HA thin films. Magnetron sputter deposition is a very attractive method due to the high adherence of the coating to the substrate material, the thickness uniformity of the deposited layer, and the ability to control the coating structure (amorphous or crystalline) and the Ca/P ratio by varying process conditions. Method allows to deposit coatings with predetermined features and to improve the performance of medical implants including devices with complex geometries. In order to develop coatings with additional functional properties there are some studies carried out in the direction of using the modified HA. In particular, we deal with HA material substituted with silver to obtain antibacterial effect and silicate ions to increase coating bioactivity.
Currently, there is no reliable information on the growth mechanisms of CaP based coatings during RF-magnetron sputter deposition. Generally, researchers focus only on the controlling of process parameters to optimize the coating properties in connection with biological studies without investigation of the fundamental aspects of the film growth. This fundamental understanding is the key approach for further technological improvements. This makes it necessary a detailed description of the different processes occurring at the substrate level and an in-depth characterization of the thin film properties. Only the combination of both thin film and plasma characterisation allows to elucidate the growth mechanisms.
Hence, the main objective is the investigation of the fundamental aspects of the deposition of HA coatings by means of RF-magnetron sputtering and the development of strategies to obtain coatings with tailored properties