The Definition of Biocompatibility

Etymologically, the term biocompatibility sounds simple to interpret since it implies compatibility, or harmony, with living systems. This concept, however, is a little too simple to be useful and the meaning of compatibility has to be explored further. 

This concept of biocompatibility, which equates the quaUty to inertness and biological indifference, has resulted in the selection of a portfolio of acceptable or standard biomaterials which have widespread usage. These range from the passivatable alloys such as stainless steel and titanium alloys, the noble metals gold and platinum, to some oxide ceramics such as alumina and zirconia, various forms of carbon and a range of putatively stable polymeric materials including silicone elastomers (poly-siloxanes), polyolefins, fluorocarbon polymers and some polyacrylates. Of course, if this was all there was to biocompatibility, there would be few problems other than optimizing inertness and there would be little to write about. 

The third reason why biocompatibility cannot be equated with inertness is that there are several, and indeed an increasing number, of applications which involve intentionally degradable materials. The two most widely quoted situations here are absorbable sutures and implantable drug delivery systems but many more circumstances where degradable scaffolds and matrices could form an essential component of a device are envisaged. If biocompatibility is predicated on inertness, then degradable materials cannot, by definition, be biocompatible. This clearly does not make sense and suggests that the concept of biocompatibility needs to be altered. 

The fourth reason is even more compelling, especially when considering biomaterials used in devices for tissue reconstruction. If a device is made from materials which are inert and which do not interact with the body in any way, then it is unlikely that it can be truly incorporated into the body. For effective long term performance in the dynamic tissue environment, it is far more preferable for there to be functional incorporation, which implies that the device should be stimulating the tissues to be reactive to it positively rather them negatively. Thus biocompatibility should not be concerned with avoiding reactions but selecting those which are the most beneficial to device performance. 

On the basis of these ideas, biocompatibility was redefined a few years ago (2), as the ability to perform with an appropriate host response in a specific situation. Clearly this definition encompasses the situation where inertness is still required for the most appropriate response in some situations is indeed no response. A traditional bone fracture plate is most effective when it is attached mechanically to the bone and does not corrode no response of the tissue to the material is normally required. Even here, however, we have to concede that a material that could actively encourage more rapid bone healing might be beneficial so that a specific osteoinductive response would be considered appropriate. 

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In the context of the definition of biocompatibility, therefore, it is important that the interaction between the material and the tissues is one which leads to an acceptable balance between inflammation and repair. A few points may serve to explain this further and qualify appropriateness. First, the nature of the host response and those features which constitute acceptability will vary very considerably from one host to another and from one location (or set of circumstances) to another within a particular host. It is often forgotten that host variables are as important as material variables in the determination of biocompatibility. This is particularly important when the wide variety of tissue characteristics is considered. Obviously bone is very different from nerve tissue or a vascular endothelium and there will be very considerable difference in the details of their... 

Williams D. Revisiting the definition of biocompatibility. Med Device Technol. 2003 14 10-3. 

In all intra- and extracorporeal applications, the textiles are in direct contact with living tissue and bodily fluids. This requires that the material must not interfere with the organism in any way nor must the used materials be damaged by the biological environment of the body (Planck, 1993). A material that meets both requirements is regarded as biocompatible. The definition of biocompatibility is complex and can only be regarded as the sum of certain properties reflecting the above-mentioned demands. The term itself is described in DIN ISO 10993. A simplified definition that is suitable for most textiles focuses on either chemical, structural, or mechanical properties (compliance). 

When inspecting the definition of biocompatibility, provided by Williams (2008), it is noticeable that the presence of a fibrous capsule does not present the most appropriate beneficial cell or tissue response. For some applications this is exceptionally clear as the fibrous capsule intervenes with transport of signalling molecules or nutrients (eg, biosensors, pacemaker electrodes and cochlear implant electrodes). Moreover, it has been shown to be a risk factor when the fibroblasts in the capsule transform into myofibroblasts and start contracting the capsule, as observed for example in women having silicone breast implants (Steiert et al., 2013). 

Although there are several definitions of biocompatibility, the concept named as biocompatibility is usually used to describe the ability of a material to perform a desired function without producing a negative effect on biological systems in specific... 

The main general consideration that can be stated so far is 1) the smoothness of the separations 2) the absence of cell prelabeling and 3) the development of biocompatible instrumentation, which allows subpopulation lineage to be produced, which are not only usable for fundamental studies such as differentiation pathways or apoptosis studies, but also for transplantation or genetic engineering. One must have in mind that the cell is definitively the place, home, and native localization of genes and proteins. The possibility of rapid, nondestructive separation, purification, and characterization of cells (cellulomics) opens fabulous dimensions for proteomics and genomics. 

The definition of a biomaterial that has been arrived at by consensus is A biomaterial is one which possesses the ability to perform with an appropriate host response in a specific application (Williams, 1999). As subsequently stressed by Hench (2014), this definition emphasises that the term biocompatibility does not just mean absence of cytotoxicity but provides for the requirement that a material performs appropriately. This also means that different applications of a particular material enforces different conditions. As a consequence, a material, be it a metal, a ceramics or a polymer may or may not be biocompatible in different applications. 

The most commonly used term to describe appropriate biological requirements of a biomaterial or biomaterials used in a medical device is biocomparibifity. A simplistic definition of biocompatibility is that a material does not aeate any adverse tissue reactions. A more helpful definition of biocompatibility is the ability of a material to perform with an appropriate host response in a specific application. This definition is helpful in that it links material properties or characteristics with... 

The key distinction between material and biomaterial revolves arormd the word biocompatible. A biomatetial must be biocompatible while closely related materials used in construrtion or automotive applications will not necessarily be biocompatible. An often-dted definition of biocompatible is... 

The above definition of biocompatibility helps to explain the subject area but cannot describe exactly what it is. For this purpose we have to consider the various components that are involved in biocompatibility processes. Biocompatibility refers to the totality of the interfacial reactions between biomaterials and tissues and to their consequences. These reactions and consequences can be divided into four categories. These involve different mechanisms and indeed quite separate sectors of science but are, nevertheless, inter-related. 

However, body response to implanted material remains heavily dependent on the patient. That is why it should be understood that definition of biocompatible materials and associated smart characteristics fall in exploratory fields that show promising... 

Going strictly by the definition of haemocompatibility, apart from endothelial cells and platelets fliat are directly involved in the process of thrombosis and antithrombosis, although they are present in the blood, the other blood cells can be used to assess only biocompatibility instead of haemocompatibility because they are not directly involved in thrombus formation and/or prevention of thrombus formation. Thus, the interaction of DLC with other blood cells such as neutrophils, lymphocytes, monocytes and erythrocytes (RBCs) can only give an indication of biocompatibility and not really haemocompatibility . 

Once implanted into the body medical polymers must show specific properties without interaction with surrounding tissues or with the body as a whole. So far there is no recognized definition of biocompatibility since there is no material parameter or biological tests which could be used as a quantitative characteristic of this property of the polymer [1]. 

The properties of a pH electrode are characterized by parameters like linear response slope, response time, sensitivity, selectivity, reproducibility/accuracy, stability and biocompatibility. Most of these properties are related to each other, and an optimization process of sensor properties often leads to a compromised result. For the development of pH sensors for in-vivo measurements or implantable applications, both reproducibility and biocompatibility are crucial. Recommendations about using ion-selective electrodes for blood electrolyte analysis have been made by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) [37], IUPAC working party on pH has published IUPAC s recommendations on the definition, standards, and procedures... 

The term biocompatibility needs some explanation and has to be defined for each application separately. The historical definition was based on unimpaired cell growth in the presence of the test article [12]. More recent definitions of the biocompatibility of implantable devices ask for the ability of the device to perform its intended function, without eliciting any undesirable local or systemic effects in the host. 

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