Polymers Flashcards
POLYMERS
• POLY – Many
• MERS – Units
Polymers are made up of large numbers of similar
repeat units linked to each other by covalent bonding
• Repeat units – monomers
• Based on hydrocarbons
• Polymerisation is the process that links all
monomers into large macromolecules
Thermoplastic
• Polymers with linear molecules are likely to be thermoplastic
(substances that soften upon heating and can be remolded and recycled)
Thermosets
• The other group of polymers is known as thermosets (substances that
do not soften under heat and pressure and must be re-machined or incinerated to
remove from the environment)
Biomedical Uses for Polymers
Polymer cranium plates, sutures, articulating surfaces, syringes, tubes, filters, contact lenses, heart valves, polymers meshes Why? • Easier to produce • Biocompatibility • Often cheaper • Designed to mimic • Replacement to old practices • Designed to prevent additional surgery/trauma to patient
Inert Polymers
3 Basic Materials - PMMA, acrylic, silicone
The structures of polymers determine their utilization in various medical
domains (i.e. surgery, dermatology, ophthalmology
Popular thermoplastic polymers for biomaterials include:
• polyolefin, Teflon® (fluorinated hydrocarbons), poly (methyl methacrylate) (PMMA), poly (hydroxyethyl methyacrylate) (PHEMA, Hydron ®), polyvinyl chloride (PVC), polycarbonate, nylon, polyester (Dacron ®)
Popular thermosetting polymers for biomaterials include:
• butyl rubber, chlorosulfonated polyethylene (Hypalon ®), epichlorohydrin rubber (Hydrin ®), polyurethane (Biomer ® etc.,) natural rubber, & silicon rubber (Silastic ®)
Polymers 1
Polymers 2
Polymers 3
Polymers 3
Polyethylene & Polypropylene
- Ultra High Molecular Weight Poly (Ethylene) is used to fabricate acetabular cups in artificial hips, bearing surface of some knee prostheses, blood contacting tube
- Polypropylene (Prolene ®) sutures are widely used clinically
Perfluorinated Polymer
PTFE
• For a heart valve, it serves as a sewing ring / receptor for sutures
• Other application- shunts to carry cerebral spinal fluid from hydrocephalic patient
• Middle ear drain tubes, sutures
Acrylic Polymers
- PMMA
* Used for contact lenses, bone cement, dentures, maxillofacial prostheses
• Polyurethanes
• Vascular tubes, artificial heart assist devices
Polyamides
• Applications in intracardiac catheters, components in
dialysis device, sutures
Silicone Rubber
• The most widely used is polydimethylsiloxane
• Catheters, lines
• Silicone prostheses: finger, toe,
mammary, maxillofacial surgery
Biodegradable Polymers Used for Medical Applications
Natural polymers ▪ Fibrin ▪ Collagen ▪ Chitosan ▪ Gelatin ▪ Hyaluronan ...
Synthetic polymers ▪ PLA, PGA, PLGA, PCL, Polyorthoesters … ▪ Poly(dioxanone) ▪ Poly(anhydrides) ▪ Poly(hydroxybutyrates) ▪ Polyphosphazenes ...
Bioresorbable Polymers
Poly(caprolactone) (PCL): •Biodegradable polymer •Semi-crystalline •Modulus ≈ 0.5 GPa, Strength ≈ 16 MPa •Low melting point (60 °C)
Polylactic acid (PLA): •Biodegradable polymer •Two forms: semi-crystalline P-L-LA •amorphous P-DL-LA •Modulus ≈ 1.8 GPa, Strength ≈ 50 MPa
• PGA (poly glycolic acid) - relatively very fast resorbing
polymer
• PLGA (polylactic-co-glycolic acid) is one of the most widely investigated biodegradable polymers for drug delivery
• Lactide/glycolide copolymers - have been subjected to extensive animal and human trials without any significant harmful side effects
• PLLA is also an excellent biomaterial and safe for in vivo studies and use (lactic acid contains an asymmetric αcarbon atom with three different isomers as D-, L- and DL-lactic acid)
Polymer Synthesis
The study of polymer science begins with understanding the methods in
which these materials are synthesised
• Chemical reactions in which high molecular mass molecules are formed
from monomers is known as polymerisation
• There are two basic types of polymerisation, chain-reaction (or addition)
and step-reaction (or condensation) polymerization
Polymer Synthesis
• Chain reaction - this type of polymerisation (also known as addition reaction) is a three-step process involving two chemical entities (monomer and catalyst)
Step-reaction - or condensation polymerisation
E.g. polymerisation reaction involves: terephthalic acid and ethylene glycol (both of which are bifunctional)
Polymer Chemical Structure
Both addition and condensation polymers can be linear, branched,
or cross-linked
• Linear polymers are made up of one long continuous chain
• Branched polymers have a chain structure that consists of one main chain with smaller molecular chains branching from it.
• Cross-linking in polymers occurs when primary valence bonds are formed between separate polymer chain molecules.
Polymer Physical Structure
Segments of polymer molecules can exist in two distinct physical structures:
• crystalline or amorphous
Most polymers are a combination of tangled and disordered regions surrounding the crystalline areas.
Degradation Mechanisms
Surface erosion (poly(ortho)esters and polyanhydrides) ▪ Sample is eroded from the surface ▪ Mass loss is faster than the ingress of water into the bulk
Bulk degradation (PLA,PGA,PLGA,PCL)
▪ Degradation takes place throughout the whole of the sample
▪ Ingress of water is faster than the rate of degradation
Bioresorbable
Materials and/or devices used in repair procedures that: • Breakdown over time to materials that can be eliminated from the body via natural pathways • Ideally leave no evidence of the repair / injury
Advantages of bioresorbable polymers
- Eliminated from the body and replaced by host tissue
- No need for second surgical procedure to remove implant
- Avoid complications of metal implants – stress shielding, corrosion, release of metal ions
- Allow transfer of loads to healing bone
- Revision surgery not complicated by presence of implant
- Compatible with MRI imaging
- Can be used to deliver bioactive agents etc.
State of the Art in Bioresorbable polymers
Current bioresorbable devices
• BioRCI® and BIOSURE ® Interference Screws are used to
rebuild damaged ligaments.
• SURETAC ® Fixation Devices are used to repair the
shoulder
• BIORAPTOR ® and OSTEORAPTOR ® Suture Anchors
are used to repair the shoulder and hip
• TWINFIX ® Suture Anchors are used to repair the
shoulder
TRUFIT ® Bone Scaffold is a graft substitute used to
repair damaged bone.
Factors affecting degradation rate
Material Properties Water uptake (hydrophilicity) ↑ Glycolide/lactide ratio ↑ Crystallinity ↓ Molecular weight ↓ Residual monomer ↑ Additives/fillers ↑↓
Environmental Factors
Temperature ↑
pH ↑↓
Implantation site – fluid flow, cellular activity, enzymes ↑↓
Degradation rates for a range of bioresorbable polymers
Degradation of PLA/PGA Polymers
Current products based on poly(lactic acid), poly(glycolic acid) & co-polymers
• Degrade from inside out (autocatalysis)
• Release acidic degradation products – in an “acid burst”
• PLLA-based materials slow to be resorbed (3-5 years)
Opportunities for improving bioresorbable
Biocompatibility and Degradation Profile
• Acidic breakdown products
• Not consistently replaced by bone
• Non-optimum degradation profile
Strength/Stiffness
• Polymers not as strong/stiff as the metals we are replacing
Added functionality
• Delivery of drugs/actives
• Shape memory
Biocompatibility and Long-Term Response
Foreign body reactions
• Osteolysis
• Extra-articular soft tissue reaction
• Intra-articular synovial reaction
PGA thought to be worse than PLA
• degrades more quickly, higher acidity/toxicity of products
• But: PGA response seen within 8-16 weeks, PLA degrades in 1.5 – 3/5 years so response may go unreported
Reported incidences vary
• depends on implant, anatomical location etc
• even PGA can be very low
• e.g. <2% : Rokkanen et al, Biomaterials, 21 (2000) 2607-2613
High Strength Bioresorbables
Polymer, Crystallinity, Tg(°C), Tm (°C), Tensile Modulus (GPa), Tensile Strength (MPa)
PLLA ,Semi-crystalline, 56, 170, 3, 60 PDLLA, Amorphous, 57, n/a, 2, 45 PGA, Semi-crystalline, 36, 228, 5, 70 Maxon (PGA/TMC), Semi-crystalline, 36, 220, 2.4, 60 PCL, Semi-crystalline, -60, 60, 0.4, 22 PDO, Semi-crystalline, -16, 110, 1.5, 30
High strength-solid phase deformation
▪ Development of process to produce high
performance fibres of PLA, PGA and copolymers
▪ Polymer is extruded into unoriented fibre
▪ High strength fibre is produced by drawing
extruded fibre using custom drawing frame
High Performance Bioresorbable Fibres
High Performance Bioresorbable Fibres
“Shape Memory” Effects
- ActivaScrew™ (Bioretec) demonstrates “autocompression”.
- 1-2% contraction in length and related expansion in diameter.
- Creates compression on the fracture and prevents screw loosening.
- Shape Memory - Change of shape on application of a stimulus.
- Usually heat/temperature but can also be light, radiation etc.
- Specialised processing methods can be used to “programme” the polymer with a new shape.
- Heating above its glass transition temperature (Tg) allows the molecular network to relax and return the polymer to its original shape.
- Fortunately, many lactide/glycolide copolymers are well suited to this as they have a Tg in the right region (just above body temperature).
- Can be exploited in a range of applications such as bone screws, coronary stents etc.
Future Developments
Bocompatibility and degradation rate
- Erodible polymers (no bursting effect)
- Less acidic breakdown products
- Materials having ‘responsive’ degradation - degraded by enzymes/cells during healing
High strength materials
- Advanced fibre composite materials
- Degradable Liquid Crystal Polymers
- Nanocomposites
- Resorbable metals
Added functionality
- Release of drugs/actives
- Shape memory devices
