What is a Biomaterial?

THE CONCEPT ‘BIOMATERIAL’ is fairly frequently encountered these days. Associated with advanced medical solutions and replacement of body parts, it has a slight futuristic nuance to it. But what is a biomaterial? And where and why is this material used?

A biomaterial is a material designed to interact with the body

Contrary to what the word may implicate, a biomaterial is not necessarily biological or based on bio-related matter. The material itself can be anything from a metal to a plastic to varieties of composites, but it can also be bio-inspired and derived from nature. The definition of a biomaterial is a material that is designed with the purpose to interact with the body, i.e. it is designed to reside in a biological environment.

Where and why are biomaterials used?

Typically, the purpose of a biomaterial is to replace a missing piece of a body part, by replicating the structure that is no longer there, or to enhance function. Think of implants, such as hip joints, and heart valves, skin transplants, vascular grafts, and stents. Biomaterials are also used in less intrusive contexts, such as in contact lenses and wound care

From foreign material being tolerated by the body to purposeful material design

Although the biomaterial concept has a futuristic nuance to it, the desire and urge to mend a broken body is ancient. Attempts to replace or fix damaged or diseased body parts has existed for thousands of years. There are recordings of dental implants already from the Mayan era, where the tooth implants were made of nacre from seashells. Throughout history, there are plenty of recordings of foreign material being more or less successfully introduced into the body. We have carbon particle-based tattoos, sutures made of catgut and heads of biting ants, glass eyes, and stainless-steel hips to name but a few. The scientific area of biomaterials science as we know it today, however, is relatively new. It started around the 60s. At this time, we went from using the materials we had at our disposal to engineer materials with the intent of increasing the material integration success rate, and the area of biomaterial science was born.

The concept of biocompatibility and functional materials  

In the old days, of course, the concept and understanding of biocompatibility did not exist. Most likely, it was mere luck if the implanted material was tolerated by the body, and the patient did not suffer from any severe side-effects. Today we have a good understanding of biocompatibility and tailor materials for desired interaction with the body. So, the material usage evolution has taken us from seashells in the Mayan period, to off-the-shelf materials such as polymers, metals, and ceramics after World War II, to engineered materials designed for biocompatibility in modern times. Here we find silicones, hydrogels, and hydroxyapatite, which today are commonly used. Now it is time for the next era. This aims to engineer materials which are not only tolerated by the body, but which also have functional properties, i.e. properties that can be tuned for example to control the physiological environment and induce a response, such as tissue repair.

What is Biocompatibility?

IN BIOMATERIALS LITERATURE, one often meets the term biocompatibility. The definition is, however, somewhat vague and ambiguities about what biocompatibility is are common. So, how is biocompatibility defined and what does this property entail?

Biocompatibility refers to the contextual host-response

Already early in biomaterials research, attempts were made to define a material’s biocompatibility. Today, the most commonly used definition is "the ability of a material to perform with an appropriate host response in a specific application"[1].

Taking a closer look at this definition, “appropriate host response” means that the material, as a minimum requirement, does not induce any unwanted responses, such as toxic reactions, in the tissue where the material is placed. ‘Appropriate’ could, however, also refer to a desire to have some positive responses, such as promoting the healing in process and reducing the time until the material or device is functional.

The definition above also refers to “a specific application”, which means that biocompatibility is contextual. For example, a biomaterial may be biocompatible in bone but not in blood and vice versa, or it may be biocompatible for short-time use in a specific tissue, but not in a long-term application in the same tissue.

How biocompatible is the material?

It is notable that the quality of “being biocompatible” can be a grey area, where it is not necessarily a matter of either-or. One material can be more biocompatible than another in a specific application, but both can be classified as biocompatible. For example, if two specific materials are working well as bone-anchored materials, but one of them also heals-in faster to a functional state than the other, we can say that the latter one is more biocompatible.

Biocompatibility of devices

The discussion above refers to individual (bio)materials. It is appropriate to extend the biocompatibility concept to also include devices, like implants, pacemakers, and drug release devices that consist of more than one material, and talk about the biocompatibility of these. For example, in a device consisting of two materials, both materials must be biocompatible in the tissue(s) where they are placed. Also, there must not be any negative crosstalk between the materials or the tissue responses that they induce. We can then talk about a biocompatible device.

Biocompatibility in tissue engineering and other contexts

Usually, when we talk about biocompatibility, we implicitly have in mind materials or devices intended for medical use in humans. But the definitions and discussion above are also extendable to other areas, like veterinary medicine or to templates and scaffolds for tissue engineering.

Sharper or modified definitions may be expected as we move forward

As we learn more and more about biological responses of living tissues to (bio)materials, some ambiguities will resolve, and definitions will become sharper. There is a great need and desire to be able to perform biocompatibility testing in vitro, and then from such data predict biocompatibility in vivo, in the real application. This is, however, not yet the case.