Various types of artificial materials are being utilized in biomedical devices ranging from permanent implants to diagnostic devices. The surface properties of the materials used, determine their interactions when in contact with bodily fluids. The biocompatibility of these materials is a complex issue starting from the definition of biocompatibility itself. In one application, the material should be completely inert to prevent any interactions whereas in others an integration of the material with the host tissue is needed. Physicochemical properties of the surface, like wettability and surface roughness, are of prime importance for the optimization of protein and cell adhesion, cell spreading, and proliferation of cells.
Wettability correlates with biological interactions
The initial response when a material is placed in the biological surroundings is water molecule adsorption to its surface. This happens within the first few nanoseconds. In the second stage, protein adsorption occurs. It is generally accepted that the small proteins will be the first to adsorb due to their rapid transport to the surface. Over time these proteins are replaced by bigger ones that have a greater affinity towards the surface. The third stage of biological response includes cell attachment to the surface.
Surface energy, which is intimately related to wettability, is known to correlate with biological interactions. The material wettability is a determining factor for protein adsorption and thus also for cell adhesion. It is usually reported that biomaterial surfaces with moderate hydrophilicity improve cell growth and have higher biocompatibility. However, cell adhesion can decrease as the material becomes very hydrophilic. Modification of the material properties, either bulk or surface, makes it possible to find a material in which surface free energy is optimum for the application at hand.
Surface texture and roughness determine the cell behavior
All surfaces are rough to some extent. In some biomedical applications, the surface texture can be utilized to improve the material – host response. Topographical factors such as size, shape, and geometric alignment have a strong influence on the adhesion, migration, arrangement, and differentiation of cells.
The effect of surface topography has been studied on titanium surfaces. Titanium and its alloys are used as dental and bone implants. The biocompatibility of the titanium implant is dependent on the ability of osteoblasts to adhere to the implant surface. The wide use of titanium is not only due to the nature of the material itself but the range of possible surface treatments by which this material can be modified.
Combined wettability and surface roughness measurements
In addition to affecting cell adhesion and differentiation, the surface topography affects also wettability. It is well-known that surface wettability is a combination of surface chemistry and surface roughness.
Wettability is determined through contact angle measurements. When the contact angle is low the surface is said to be wettable. If the liquid used for measurement is water, the liquid used is water, the low contact angle means hydrophilic material and when the contact angle is high, the material is said to be hydrophobic. Possible surface roughness will enhance this as stated by the Wenzel equation.
To be able to study the effect of roughness on wettability, the combined surface roughness and contact angle measurements should be made. To read more about these measurements, please download a white paper through the link below.
Editors note: This blog was originally published 24th of November 2015 and has since been updated for completeness.
Within biomaterials research and development, hydroxyapatite (HA) is well known for its biocompatible properties, particularly in bone bonding, and for its potency of rapid integration into the human body.