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Medical Device Design > Polymers > Flashcards

Polymers Flashcards

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USMLE® Step 1 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

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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
Q

State of the Art in Bioresorbable polymers

A

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.

26
Q

Factors affecting degradation rate

A 
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 ↑↓

27
Q

Degradation rates for a range of bioresorbable polymers

A
28
Q

Degradation of PLA/PGA Polymers

A

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
Q

Opportunities for improving bioresorbable

A

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
Q

Biocompatibility and Long-Term Response

A

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
Q

High Strength Bioresorbables

A 
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
32
Q

High strength-solid phase deformation

A

▪ 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

33
Q

High Performance Bioresorbable Fibres

A
33
Q

High Performance Bioresorbable Fibres

A
34
Q

“Shape Memory” Effects

A
  • 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
Q

Future Developments

A

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

Medical Device Design (3 decks)

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