Biomaterial

A biomaterial is a substance engineered to interact with biological systems for medical purposes. These materials serve either therapeutic roles, such as repairing or replacing tissue, or diagnostic roles to help doctors examine the body.

The study of these substances is known as biomaterials science or biomaterials engineering. This multidisciplinary field combines medicine, biology, chemistry, and materials science. While people often use the term "biomaterial" to describe things like bone, scientists distinguish them from biological materials produced by living systems. Some researchers suggest using the word "bioterial" for materials naturally made by biology, such as bone or fungal biocomposites.

Biomaterials can be derived from nature or synthesized in a laboratory. Scientists create them using metals, polymers, ceramics, or composite materials. Once inside the body, these substances can perform various functions. Some are passive, meaning they perform a simple task like acting as a heart valve. Others are bioactive, meaning they interact directly with living tissue. For example, some hip implants are coated with hydroxyapatite to encourage bonding.

Bioactivity is the ability of a material to induce a physiological response that supports its function. In many cases, this involves the material bonding with surrounding tissue through osteoconduction or osseoproduction.

Modern science also utilizes a process called self-assembly to create advanced materials. Self-assembly is the spontaneous aggregation of particles, such as atoms or molecules, without external forces. These particles organize themselves into stable, well-defined arrays. This process is very similar to the crystal systems found in metallurgy.

Molecular self-assembly is common in biological systems and helps form complex structures. Scientists use these principles in nanotechnology to create highly ordered structures like micelles, emulsions, and thin films. This method relies on the principle of self-organization to build materials from the bottom up.

Biological materials often possess a complex structural hierarchy. This means their organization changes across different spatial scales. In bone, the building blocks are collagen molecules that form a triple helix with a diameter of 1.5 nm. These molecules work with a mineral phase called hydroxyapatite to form fibrils. These fibrils then curl into structures called osteons. The volume of bone is typically distributed as about 60% organic material and 40% mineral phase. Hydroxyapatite crystals in bone are tiny mineral platelets. They have a diameter of 70 to 100 nm and a thickness of only 1 nm.

Other natural structures also show this hierarchical complexity. The abalone shell begins its organization at the nanolevel with an organic layer 20 to 30 nm thick. This leads to layers of aragonite crystals that eventually form a mesostructure about 0.3 mm thick. Crabs have a carapace made of a hard mineral component and a soft organic component called chitin. The mineral rods in a crab shell are about 1 μm in diameter. These rods contain chitin-protein fibrils that are 60 nm in diameter. These fibrils contain tiny 3 nm diameter canals that connect the interior to the exterior.

Biomaterials are used in a vast range of clinical applications. They are used for joint replacements, bone plates, and dental implants. Doctors also use them for intraocular lenses in eye surgery and for heart valves. In the United States, 49% of the 250,000 annual valve replacement procedures use a mechanical valve. One common example is the bileaflet disc valve, or St. Jude valve. This device uses two semicircular discs to manage blood flow and prevent backflow. The valve is often secured with a woven fabric called Dacron to allow body tissue to grow into it.

Safety is the most critical factor in biomaterials engineering. A material must be biocompatible, meaning it is suitable for its specific medical application. Biocompatibility is application-specific; a material that works in a hip might not work in an eye. When a device is implanted, the body initiates a host response to protect itself. This is known as the foreign body response (FBR). The FBR includes an inflammatory response that has an acute phase and a chronic phase. The acute phase happens within hours or days and involves the delivery of proteins and blood to the site. Scientists must carefully study these interactions to prevent device failure.