Surface Modification for Biomaterials Applications Topics: Protein Adsorption Physiochemical Surface Modification Techniques Biological Surface Modification Techniques Surface Patterning Techniques

Factors affecting adsorption: Protein Adsorption Factors affecting adsorption: Surface energy (or tension), g Surface hydrophobicity Surface charge Definitions: Hydrophobic: water fearing Hydrophilic: water loving Definitions: Adsorption: adhesion to solid surface Absorption: penetration of molecules into bulk

Fs/g > Fs/l + Fg/l cos(q) Surface Tension Fgas/liquid q Fsolid/liquid Fsolid/gas For wetting to occur, Fs/g > Fs/l + Fg/l cos(q) Wetting Non-Wetting

Adding molecules that prevent adsorption is called steric hindrance. In this example polyethylene glycol (PEG) attaches to the surface (hydrophobic) preventing protein adhesion Like attracts like

Physicochemical Surface Treatments Covalent and non-covalent coatings describes how materials is attached to the surface Surface modification with no overcoat, and laser methods for surface modification make surface locally attractive for adhesion of desired species

Methods of surface coating: Plasma Discharge Charged particles are attracted to the sample surface, which acts as the cathode. Particles may be positive or negative ions, free radicals, electrons, atoms, molecules or photons. Often used to add OH or NH2 groups to surface as a precursor to further modification

Plasma Discharge Advantages: Disadvantages Coatings are conformal Free of voids/pinhole defects Easily prepared Sterile when removed from reactor Produce low amount of leachable substances Demonstrate good adhesion to substrate Allow unique film chemistries to be produced Easily characterized Disadvantages Chemistry within reactor may be undefined Equipment often expensive Uniform reaction within long, narrow pores may be difficult Care must be taken in sample preparation to prevent contamination

Vapor Deposition: Physical (PVD) Physical Vapor Deposition (PVD) may be from evaporation or sputtering. Sometimes a plasma is used to create high energy species that collide with target (right)

Vapor Deposition: Chemical (CVD) In Chemical Vapor Deposition (CVD) a reactive gas is passed over the substrate to be coated, inside of a heated, environmentally controlled reaction chamber. In this case (right) CH4 gas is introduced to create a diamond-like coating

Radiation Grafting and Photografting Substrate is exposed to a radiation source of high energy, which forms a reactive species at the surface to create covalent bonding of the coating to the underlying material Often employed to bind hydrogels to hydrophobic substrates Biomaterial substrate may be placed in a monomer solution the irradiated by electrons of gamma rays to form a polymerized coating.

Self-Assembled Monolayers (SAMs) SAMs are amphiphilic, having both hydrophilic (polar) and hydrophobic (nonpolar) parts. They are made up of 3 parts: The attachment group A long hydrocarbon chain The functional (polar) head group

A strong exothermic reaction attaches the Silane to the OH In the picture, hydroxyl groups form a strong attachment to the substrate. A strong exothermic reaction attaches the Silane to the OH

Physiochemical coatings Physiochemical coatings are used to coat biomaterials with biologically active molecules. These methods include solution coatings and Langmuir-Blodgett films (right) Coatings are amphiphilic, having a hydrophilic head and hydrophobic tail. This causes the heads to remain in the water and the tails to extend above the surface. The molecules at the head may be tailored to enable crosslinking with other molecules or to the biomaterials surface

Surface Modifying Additives Surface Modifying Additives (SAMs) are atoms or molecules that, when added to the bulk material, will spontaneously rise to the surface, producing a coating with characteristics dictated by the properties of the SMA. SMAs may be used with metals (e.g. Cr in steel) to create a corrosion resistant surface, or in polymers (right). Here the A copolymer anchors into the material, leaving the B copolymer exposed, which provides the desired surface properties.

Physicochemical Surface Modifications with no Overcoat These techniques are designed to modify existing atoms at the surface, and include: Ion beam implantation Plasma treatment Conversion Coatings Bioactive Glasses Conversion coatings create an oxide layer at a metal surface, 5 – 500-nm thick, to prevent corrosion Bioactive glasses come from the range of compositions depicted in the phase diagram. These dissolve and combine with natural biomaterials depending upon the ratios of CaO, Na2O, and SiO2 The IB index is a measurement of the bioactivity of these materials

Ion beam implantation This method can create surfaces with high hardness, wear, corrosion resistance and biocompatibility It can also cause surface damage in the form of sputtering of surface atoms, surface roughness and changes in the crystal structure.

Biological Surface Modification Techniques Biological surface modification attach biologically active molecules to a substrate through a variety of means that they then interact with specific target areas on cells or other tissue components Biomolecule attachment has been successfully achieved on: Soluble polymers Solid Polymers Porous solid polymers Hydrogels

Methods for the covalent attachment of biomolecules to a biomaterial surface. (a-c) attachment via post fabrication methods (d-e) attachment during synthesis. The biomolecule may be attached with or without a spacer arm in any of these methods Heparin, a hydrophobic molecule, may be attached by (a) adding a hydrophobic region to the heparin or (b) adsorption of the heparin (which has a strong negative charge) to a positively charged surface Many of these methods can be used to attach enzymes to solid substrates, and have been used in many areas, including biosensors, controlled release devices and protein analysis

Surface Patterning Techniques Surface or substrate patterning is used to alter the surface properties of biomaterials in a controlled manner, resulting in a geometric design of well-defined regions with very different characteristics. It may be used on both metals and polymers. Microcontact printing (right) creates a “stamp” that is inked with the desired biomaterial and printed on the substrate. This method employs many of the techniques used in making integrated circuits