Questions: Biomaterials

Question 1

Titanium and its alloys (Ti-6Al-4V) are the most widely used metallic biomaterials for orthopedic and dental implants. What chemical property of titanium is primarily responsible for its biocompatibility?

Model answer: Titanium spontaneously forms a thin (3-7 nm), stable, self-healing TiO2 passive oxide layer on its surface in air or aqueous environments. This oxide layer is chemically inert, resistant to corrosion by body fluids, and does not release toxic metal ions at biologically significant rates. The oxide surface also promotes direct bone bonding (osseointegration) by adsorbing calcium and phosphate ions from the biological fluid, forming a bone-like apatite layer at the interface. It is the surface chemistry of TiO2, not the bulk properties of titanium metal, that makes it biocompatible.

This parallels silicon's dominance in electronics (due to SiO2 quality) — titanium's dominance in biomedical implants is due to TiO2 quality. The oxide must be stable, non-toxic, and biologically favorable. Other metals with stable oxides (e.g., zirconium) also show good biocompatibility, while metals with less stable oxides (nickel, cobalt) can release toxic ions and cause adverse reactions.

Question 2

Poly(lactic-co-glycolic acid) (PLGA) is used for resorbable sutures and drug delivery because it degrades in the body. This degradation occurs by:

• A. Enzymatic breakdown by proteases in the tissue
• B. Hydrolysis of the ester bonds in the polymer backbone, producing lactic acid and glycolic acid that are metabolized through normal biochemical pathways
• C. Oxidative degradation by reactive oxygen species from immune cells
• D. Dissolution of the polymer in body fluids without chemical change

Answer: (B)

PLGA degrades by bulk hydrolysis: water diffuses into the polymer matrix and cleaves ester bonds randomly throughout the material. The degradation products (lactic acid and glycolic acid) are natural metabolic intermediates that enter the citric acid cycle and are eventually eliminated as CO2 and water. The degradation rate can be tuned by adjusting the lactic/glycolic acid ratio (higher glycolic content = faster degradation), molecular weight (lower MW = faster), and crystallinity (amorphous regions degrade first). This tunability makes PLGA the most widely used biodegradable polymer in medicine.

Question 3

A biomaterial implant's biological response is primarily determined by its bulk mechanical properties.

• T. True
• F. False

Answer: False

The biological response is primarily determined by surface chemistry. Within seconds of implantation, proteins from blood and tissue fluids adsorb onto the material surface (the Vroman effect). The identity, conformation, and orientation of these adsorbed proteins determine which cells are recruited, how they adhere, and whether the immune system tolerates or attacks the implant. Surface chemistry (composition, charge, hydrophilicity, topography) controls protein adsorption and thus controls the biological cascade. This is why surface modification (plasma treatment, PEG coatings, bioactive coatings like hydroxyapatite) is central to biomaterials engineering — you can have excellent bulk properties that fail biologically due to poor surface chemistry.