Peter Chrast
<petr.chrast@tnuni.sk>

Peter Chrast, born in Brno, Czech Republic, is a Ph.D. student of inorganic chemistry and biomaterial science at Funglass, Slovakia, with previous education obtained in the field of analytical chemistry.


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Boron containing bioactive glass and the regeneration of bone tissue
1*Peter Chrast, 1Dusan Galusek and 2Aldo R. Boccaccini
1Centre for functional and surface functionalized glass FunGlass, Študentská 2, Trenčín 91150, Slovakia
2Institute of Biomaterials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Erlangen, Germany

Tissue engineering in 21st century aims at developing of materials, which induce self-regeneration of tissues. This way wounds and defects can be repaired, which do not heal themselves under normal conditions. Bioactive materials are able to heal the wounded tissue and to accelerate the remedial rates without the use of hormone-based drugs. There are generally two main approaches to bioactive materials. First is the creation of a permanent biomaterial-tissue boundary through chemical reactions. The second way is the regeneration of the original tissue via artificial material degradation and subsequent rebuilding of the degradation products into the original tissue via natural cellular regeneration processes.

Both ways are plausible for the medical application of the bioactive materials, but only the second option offers one additional property: retaining the original mechanical properties of the tissue substituted. The balance is often hard to reach for artificial joint or bone implants, since they are manufactured from hard and very durable alloys, which do not offer the same flexibility and torsion resistance as the original bone tissue. The mechanical stress created at the contact points is often the point of failure.

This is caused mainly by the organic nature of the human bone. In general, 30% of the bone consists of the organic, collagenous tissue, which gives them their tensile strength and overall flexibility.  The mineral part of the human bone occupies about 70% of the total bone mass, distributed in outer (cortical, weight bearing) and inner (cancellous, cellular and organic) structures of the bones, and gives them their mechanical durability and resistance. It is created on the surface of collagen fibres by bone-forming osteoblast cells by deposition of calcium phosphate layers, hardened by hydroxide and carbonate ions. The main bone forming mineral is hydroxyapatite (HAp), and other calcium phosphate salts (e.g. tricalcium phosphate).

Bioactive tissue implants aim to induce the natural bone-forming processes simply by introducing the building material for the bone-forming cells to rebuild into the original bone tissue. Dissolution of glass matrix takes place through a sequence of chemical reactions. The glass network bonds break and release the building blocks of the glass network in the ionic form, changing the surrounding pH, which also alters the reaction kinetics. This way, the glass surface produces a lot of material to be taken care of – either by flushing it away into the surrounding environment, or by deposition of crystalline phases of calcium phosphate minerals on top of glass surface. These crystalline phases are useful in the recreation of bone tissue. Bone tissue implants made from bioactive material can substitute for disparities in material physical properties, since the resulting tissue material resembles the original bone. The only drawback is to avoid any mechanical load during the rehabilitation process, because the glass implant (scaffold) is often more brittle than the surrounding bone, being prone to damage. The duration of the healing, in terms of material dissolution and the replacement with the original bone tissue should match. If the implant material dissolves before the bone reconstruction process is finished, material failure would happen. On the other hand, if the process takes too long to finish, local inflammation can occur, greatly complicating the entire procedure.

Bioactive properties of certain glass compositions have been proposed since Larry Hench discovered the signature 45S5 Bioglass©. Modified silicate bioactive glasses (13-93, 45S5) inevitably led to conclusion that not only silicate matrix, but also borate or phosphate glass networks produce hydroxyapatite under corrosion in water or simulated body fluid, with increased degradation rates and additional therapeutic effects, induced by the presence of other chemical elements, such as B, Sr, Zn, Cu, Ag and even heavier elements, such as Ce, Nb, Ga. The major focus now lays in the development of novel borate bioactive glass compositions, and their physicochemical characterization, such as complete chemical analysis and corrosion assays, to completely understand and describe the mechanism, which will lead to rapid and successful regeneration of wounded human bone.

This paper is a part of dissemination activities of project FunGlass.
This project has recieved funding from the European Union´s Horizon 2020 research and innovation programme under grant agreement No 739566.