Polymers Flashcards

1
Q

POLYMERS

A

• 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

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2
Q

Thermoplastic

A

• Polymers with linear molecules are likely to be thermoplastic
(substances that soften upon heating and can be remolded and recycled)

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3
Q

Thermosets

A

• 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)

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4
Q

Biomedical Uses for Polymers

A
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
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5
Q

Inert Polymers

A

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 ®)

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6
Q

Polymers 1

A
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7
Q

Polymers 2

A
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8
Q

Polymers 3

A
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9
Q

Polymers 3

A
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10
Q

Polyethylene & Polypropylene

A
  • 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
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11
Q

Perfluorinated Polymer

A

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

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12
Q

Acrylic Polymers

A
  • PMMA

* Used for contact lenses, bone cement, dentures, maxillofacial prostheses

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13
Q

• Polyurethanes

A

• Vascular tubes, artificial heart assist devices

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14
Q

Polyamides

A

• Applications in intracardiac catheters, components in

dialysis device, sutures

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15
Q

Silicone Rubber

A

• The most widely used is polydimethylsiloxane
• Catheters, lines
• Silicone prostheses: finger, toe,
mammary, maxillofacial surgery

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16
Q

Biodegradable Polymers Used for Medical Applications

A
Natural polymers
▪ Fibrin
▪ Collagen
▪ Chitosan
▪ Gelatin
▪ Hyaluronan ...
Synthetic polymers
▪ PLA, PGA, PLGA, PCL, Polyorthoesters …
▪ Poly(dioxanone)
▪ Poly(anhydrides)
▪ Poly(hydroxybutyrates)
▪ Polyphosphazenes ...
17
Q

Bioresorbable Polymers

A
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)

18
Q

Polymer Synthesis

A

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

19
Q

Polymer Synthesis

A
• 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)

20
Q

Polymer Chemical Structure

A

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.

21
Q

Polymer Physical Structure

A

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.

22
Q

Degradation Mechanisms

A
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

23
Q

Bioresorbable

A
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
24
Q

Advantages of bioresorbable polymers

A
  • 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.
25
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.
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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 ↑↓
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Degradation rates for a range of bioresorbable polymers
28
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)
29
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
30
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
31
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 ```
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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
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High Performance Bioresorbable Fibres
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High Performance Bioresorbable Fibres
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“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.
35
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