Advanced Biomaterials Chapter 4 - Bioinert Polymers

5 types of bioinert polymers

1. Polyolefin (PE/PP)

2. Polyethylene Terephthalate (PET)

3. Acrylate Polymer (PMMA)

4. Fluorocarbon Polymer (PTFE)

5. Silicone

Polyolefin

Group of polymers produced from chemically unsaturated C_nH_2n as a monomer. Includes PE and PP

Polyethylene (PE)

Most produced polymer worldwide. Available commercially in 3 major grades: low-molecular weight PE, high-molecular weight PE, and ultra-high-molecular weight PE.

Polypropylene (PP)

Second most produced polymer worldwide. Belongs to sub-group of polyolefins called poly-a-olefin which are polymerized from a-defins.

PP Properties and Applications

Flexible and can be used in applications requiring some elasticity such as packaging bags, bottle caps, and yogurt containers. Recyclable in many areas. Used in finger joint and cardiac support devices.

UHMWPE

Ultra-high-molecular weight polyethylene. A successful biomaterial for use in various joint implants, first used in 1962 but became highly crosslinked in 1998.

UHMWPE Properties and Applications

Typically used in the line part of a total hip replacement. Has exceptional strength and toughness which is suitable for heavy-duty applications like bone scaffolds and joint prosthesis.

a-olefin (alpha-olefin)

an alkene where the C=C double bond starts at the a-carbon atom AKA the C=C is between the 1st and 2nd carbon in the molecule

UHMWPE Properties

Melting temp (C°): 125-138

Density (g/cm³): 0.932-0.945

Degree of Crystallinity (%): 40-75

Young’s Modulus (GPa): 0.8-1.6

Ultimate Tensile Strength (MPa): 20-30

Ultimate Elongation (%): 350-525

PP Properties

Melting temp (C°): 160-170

Density (g/cm³): 0.898-0.920

Degree of Crystallinity (%): 30-50

Young’s Modulus (GPa): 1.0-1.5

Ultimate Tensile Strength (MPa): 30-35

Ultimate Elongation (%): 400-900

Polyethylene Terephthalate (PET)

Third most produced polymer worldwide, 18%. Belongs to polyester family and formed from condensation polymerization. Can be completely amorphous or semicrystalline. Highly recyclable and used in bottles and packaging.

Recyclability Factors

Chemistry-related: chemical structure and composition, additives and contaminants, and melting temperature.

Other: Market demand and recycling facilities

Acrylate Polymer (PMMA)

Family of polymers made by the polymerization of an acrylic acid or acrylate.

PMMA Properties and Applications

Is transparent, hard, excellent chemical resistance (AKA inert), and is biocompatible. Used as bone cement for prosthesis fixation.

Acrylic Acid

CH₂CHCOOH, the simplest unsaturated carboxylic acid

Acrylate

CH₂CHOO(R) where R: -CH₃ ; -CH₂-CH₃ ; -CH=CH₂

Bone Cement Design

Must be fluid before implantation and be able to solidify (polymerize and crosslink) once implanted in the body. This comes with the issue of shrinkage during polymerization once solidified. Primarily made of PMMA powder and monomer methyl methacrylate liquid to reduce shrinkage.

PMMA Bone Cement

PMMA powder and monomer methyl methacrylate liquid is primarily used in bone cement. The viscosity changes over time from liquid into a dough-like state for application before finally hardening into solid material.

Mechanical Drawbacks of PMMA Cement

Brittle with low endurance limit, prone to fatigue failure, mechanically weak if it entraps impurities like air or blood, and it spawns small particles from its surface containing hard crystals (barium sulfate) which damage the fine joint surfaces of the artificial joint.

Biological Drawbacks of PMMA Cement

Small cement particles may cause osteolysis (“bone dissolving disease”), very large surface that may support bacteria colonization and development of postoperative infections, and may cause allergy during the operation.

Fluorocarbon Polymers (PTFE)

Most common example is polytetrafluoroethylene which is well known by the brand name Teflon.

PTFE Properties and Applications

chemically inert/corrosion resistant, neither water nor oil wettable, biocompatible, flexible, and resistant to thrombosis (AKA blood clotting). Applications include non-stick pans and artificial vessels.

PTFE Properties

Melting temp (C°): 335

Density (g/cm³): 2.1-2.2

Young’s Modulus (GPa): 0.55

Ultimate Tensile Strength (MPa): 27

Ultimate Elongation (%): 300

Silicone

More precisely called polymerized siloxanes or polysiloxanes. Mixed organic-inorganic polymers where R: -CH₃ ; -CH₂CH₃ ; -C₆H₅ .

Crosslinking Mechanisms of Silicone

Condensation curing of silicone via an acid cross linker OR addition curing of silicone via an oligomer containing a vinyl group.

Silicone Properties and Applications

very stable, inert, flexible due to large bond angles and bond lengths, good O₂ permeability due to loosely packed polymer chain network, and highly hydrophobic. Applications include breast implants, catheters, and contact lens.