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What’s “bio” about biomaterials?

With people now living longer and tending to take part in more sports in their leisure time, there is a requirement for materials to replace the natural substances of the locomotor system and the internal organs when these begin to wear out or no longer function properly. Implants in the form of replacement bones, joints, organs, tissue and teeth are the main applications for biomaterials. They also play an important part in the relatively new field of tissue engineering. meditec writer Andrea Gerber examines the trends in biomaterials.
Suitable materials for biomaterials are metals, (primarily noble metals such as titanium or stainless steel), ceramics, plastics, such as polyester, and natural materials (collagen and chitin derivates). Sterile natural material, such as heart valves from pigs or tissue grown in cell cultures, is also used. Strictly speaking, bone cement, plaster, surgical sutures, contact lenses, dental fillings, artificial heart valves, pacemakers and surface coatings also come under the heading of biomaterials.
Biomaterials are the interfaces between living and non-living materials, between medicine and materials science. Doctors and researchers in disciplines such as orthopaedics, trauma surgery, oral and maxillofacial surgery, implantology, bio-based materials, materials sciences, interfaces and surfaces and pharmacology work together to optimise applications, composition, design and tolerance. Biomaterials must be either “biostable” or absorbable, depending on how they are used. Biostable materials, such as those for joint implants, must not degrade in a biological environment. Metals must not corrode, polymers must not become brittle; they are, as a rule, designed to be permanent. Material used for sutures, on the other hand, must be absorbable. Absorbable polymer or magnesium stents are also being tested. 

Biomaterials which remain in the body for longer periods must comply with strict requirements. In summary, they include technical functionality, biostability, biocompatibility and sterilisability. The unwelcome phenomenon of biofilm is an example of how closely biomaterials can bind to living cells. Its elimination or prevention always forms part of any discussion about biomaterials. The risk of bacteria forming a stubborn film on joint endoprotheses and also catheters, replacement blood vessels and dental implants should not be underestimated. Bacteria in this type of aggregate are largely resistant to antibiotics: the lowest concentration that will still be effective on biofilms is around 1000 times higher than for free-floating bacteria. Hygiene has absolute priority in the avoidance of biofilms: 85% of infections occur during or after operations; only a small proportion of pathogens originate inside the body. However, using antibacterial coatings, such as antibiotics or nanosilver, or adding antibiotics to the bone cement used with implants can also reduce the risk of infection. A project conducted jointly by aap biomaterials and researchers at the University Hospital Regensburg has shown that the nanosilver additive AgPURE from the company ras materials in Regensburg is strongly antibacterial when mixed with bone cement. 

“God created solids, but the Devil created surfaces,” said the physicist Wolfgang Pauli, remarking on their very specific properties. The “skin” of a material is directly exposed to environmental influences, such as temperature or humidity. Depending on their structure, the molecules of a material react with the gases in the air, or they are resistant. Biofunctional surfaces are a very special case. The outermost layer of a material, instrument or  device interacts actively with its biological environment. “It filters out certain molecules, receives signals or stimulates a reaction. Biofunctionality is therefore more than just tolerance – the surface communicates!” enthuses a researcher from the Fraunhofer Institute for Interfacial Engineering and Biotechnology in Stuttgart. For example, enzymes, antibodies, cancer drugs or antibiotics develop their full effect on the surface of polymers. However, biosensory cell chips, DNA chips, antithrombogenic surfaces and some diagnostic applications are also possible applications for this biofunctionality. 

More and more knee replacements
Biomaterials are used primarily in the manufacture of implants. Heart support systems are one example, but the most common implants are hip and knee joints, which are usually ceramic. The shaft is often made of titanium, a light, ductile, corrosion and heat-resistant metal that is well tolerated, meaning biocompatible. Implants are now available as modular revision prostheses, allowing partial replacements. However, “a joint implant is an artificial joint and cannot fully replace a natural joint, so that there can be limitations where some activities and types of sport are concerned,” says Marc D. Michel, Managing Director at Peter Brehm GmbH Chirurgie Mechanik, one of the leading manufacturers of titanium revision endoprostheses, such as hip and knee joints. Michel recently also became deputy chairperson of the Nuremberg-based MedTech Pharma Forum, which is the largest network in the German healthcare sector. 

Researchers have established that demographic change and new ways of spending leisure time are increasing the demand for knee replacements. Transplants using the patient’s own bone and, increasingly, artificial materials based on calcium phosphates, help patients to regain their mobility and therefore also their quality of life.
Tissue engineering is a very special method of manufacturing implants. This process involves not only the multiplication of cells in special cultures for two-dimensional tissue structures, but also cell growth in three-dimensional scaffolds made of carrier materials.
Whole sections of the oesophagus grow in these scaffolds, for example. One of the biggest challenges presented by this method is how to supply the new tissue with blood vessels: a process called vascularisation. 

In dentistry, implants are used to hold artificial teeth. The screw-like inserts are also usually made of titanium and grow into the jawbone to become very firmly attached. Worn joints, brittle bones, useless teeth, damaged organs: the more faultlessly we have to perform in our daily lives and the older we become, the larger the requirement for biomaterials. Implants and bone replacements and the materials used to replace body parts and organs will have to be more flexible and durable in the future. The goal is to mimic the natural properties of the living organism ever more closely. Researchers working with bio-inspired hybrid materials are looking at how nature’s methods of construction can be applied to the design of hybrid materials. There is also a demand for materials which help the human body to heal damaged parts itself through, for example, biomineralisation. 

After a feverish search for the cause of the EHEC infections, it now appears that the source has been found. Nevertheless, one of the main ways that the gut bacteria were transmitted was human-to-human infection. These smear infections result from a lack of hygiene. Surfaces with an antibacterial coating could be a solution, particularly as the effect of antibiotics in many cases is exhausted and resistance has developed widely. Known alternatives are nano-biomaterials, such as nanosilver, which is used to coat catheters, operating instruments, sterile packaging and also prostheses. The advantage of nanoparticles is their tiny size: The surface of the small particles is huge compared to large bodies, allowing silver to be released constantly to inhibit the growth of microorganisms. Innovative anti-infective silver technologies are being developed by Bio-Gate AG in Nuremberg, for example. Research into the effect that the small size of nanoparticles can have on the body’s own cells is being stepped up.
Another promising technology is PVD (physical vapour deposition), whereby thin functional layers are vaporised onto glass, metal or plastic. “Openair plasma technology” from plasmatreat GmbH is suitable for sterilisation and surface cleaning. Extremely thin coatings with antibacterial or fungicidal properties are becoming increasingly important in medical technology and life sciences. They act as barriers or encourage osteogenesis where titanium implants are used, for example. 

In tissue engineering, the trend is towards three-dimensional tissue structures and the scaffolds required to create them. These are made from titanium composites, for example. Absorbable, bioactive bone scaffolds are also being developed. 

Biopolymers show the way
There are more prospects in the direction of biopolymers, which are becoming more important as oil reserves dwindle. Biopolymers can be fabricated from organic compounds, such as starch or saccarose, which are very easy to modify. One trend is the development of hardened polymers and their composites. Even though it is not easy to work with them, and they are not as thermically stable as technical polymers, they are highly biocompatible and break down very easily in a biological environment. New developments include hard biopolymers for scalpel blades or nails, such as those developed by the Fraunhofer Institute for Manufacturing Technology and Advanced Materials in Bremen. Another area of research is looking at bio-interfaces, such as those between cells, biological tissue and other materials. The focus here is primarily on interactions, such as physiological corrosion, the degradability of biomaterials exposed to the metabolic processes of living tissue. In order to curtail the time-consuming observation of changes in biomaterials in vivo, computer models are used for simulations in biomaterials sciences, such as the biological degradability of nanoparticles.
Andrea Gerber 

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German Summary
Eine immer länger werdende Lebenszeit und ein sportlich geprägtes Freizeitverhalten verlangen nach Ersatzmaterialien für die natürlichen Substanzen des Bewegungsapparats und der inneren Organe, wenn diese abgenutzt sind oder nicht mehr richtig funktionieren. So sind Implantate als Knochen-, Gelenk-, Organ-, Gewebe- und Zahnersatz die Hauptanwendungsgebiete von Biomaterialien. Eine wichtige Rolle spielen Biomaterialien aber auch im relativ neuen Feld des Tissue Engineerings. meditec-Autorin Andrea Gerber hat den Trends bei Biomaterialien den Puls gefühlt. Der deutschsprachige Beitrag ist nachzulesen auf www.meditec-international.com/medi0611bio