4.Polymeric biomaterials Flashcards
Polymers
Organic materials composed by long chains (macromolecules) of repeating
units – monomers – linked together by covalent bonds
Are there additional types of bonds in (bio)polymers?
Secondary bonds:
* Hydrogen bonds
* van der Waals forces
* Dipole–dipole interactions
Polymers - general properties
- Low density (compared to metals)
- Low stiffness, soft and compliant materials
- Stretchable and flexible materials => long flexible polymer chain
- Good thermal and electrical insulators
- Relatively chemical inert
- Highly susceptible to chemicals and temperature
- Low degree of crystallinity (compared to metals and ceramics)
- Often exhibit high molecular mass
- Functional groups amenable to modification/functionalization
- Easy processing
Arrangement of polymer chains – molecular structure
-linear
-branched
-crosslinked
-comb
-star
-ladder
Arrangement of polymer chains – molecular structure
-homopolymer
-random copolymer
-alternating copolymer
-graft copolymer
Types of structure assumed by polymers:
amorphous
semicrystalline
crystalline
What determines the crystallinity of polymers?
-chain architecture
-backbone chemistry
-crosslinking
-processing conditions (TºC and pressure)
-side groups + chain branching
-molecular weight
Tacticity
Affects the ability of the polymer molecules to crystallize
DP-degree of polymerization
Average number of monomer repeat units in each polymer chain
A higher degree of polymerization results in
-higher mechanical strength
-higher melt/solution viscosity
-more difficult processing
Glass Transition Temperature (Tg):
Transition between the glassy region of behavior in which the polymer is relatively stiff and the rubbery region in which it is very compliant;
- Crystalline Melting Temperature (Tm):
Loss of crystallinity (only present when there are crystalline regions in a polymer).
Glassy state
rotation around bonds becomes hindered during cooling
molecules can no longer rearrange
Rubbery state
molecular motion
shape changes
Viscoelasticitiy
solid-like and fluid-like characteristics
How can we synthetize polymers for a specific application?
addition polymerization
or
condensation reactions
How does addition polymerization work?
initiation>propagation>termination
Which polymers can be polymerized by addition polymerization?
PE
PMMA
Which polymers can be polymerized by condensation reactions?
Nylon
PET
Classification by thermal behaviour
thermoplastics
thermosets
Classification by physical state
amorphous
semi-crystalline
crystalline
Classification by origin
Natural
Synthetic
Classification by biological reactivity
Bioniner
Biodegradable
Vinyl Polymers examples
Polyethylene (PE)
Polypropylene (PP)
Polyvinylchloride (PVC)
Polystyrene (PS)
Polytetrafluoroethylene (PTFE) - Teflon®
Polyether ether ketone (PEEK)
Polyethylene (PE) characteristics
- High abrasion and highest impact strength
- Low surface friction coefficient
- High resistance to radiation
- Difficult to process by heating
Polypropylene (PP) characteristics
- Thermoplastic polymer, Tm ~174ºC
- Higher rigidity and mechanical resistance than PE
- Lower impact resistance
- Methyl group (-CH3) contributes to oxidation and degradation
Commercial polymeric meshes for Pelvic Organ Prolapse (POP) repair are
made of synthetic polymers such as PP and PE. What kind of issues can
occur upon implantation?
- Severe foreign body response
- Poor tissue integration
- Prolonged inflammation
- Stiffness mismatch à tissue disruption and
erosion
Polyvinylchloride (PVC)
Amorphous material (7–20%crystallinity)
* Plasticizers (e.g., dioctyl, phthalate) are used to obtain polymers with varying
rigidity
* Flexible polymer
* Unstable at temperatures ~150ºC
Polyvinylchloride (PVC) applications
Flexible
blood bags
tubs
disposable gloves
catheters
intravenous probes
Rigid PVC
lab instruments
medical packaging
containers
Polystyrene (PS)
Amorphous material
* Rigid, hard, transparent, and brittle
* Tg ~ 100ºC
* Resistance to gamma radiation
Polytetrafluoroethylene (PTFE) - Teflon®
- Highly crystalline polymeràthermal stability
- High molecular weight (106–107 Da)
- Chemically inert, hydrophobic, non-adhesive properties
- Expensive
Polytetrafluoroethylene (PTFE) - Teflon® applications
guided bone regeneration
vascular grafts
Acrylic Resins chemistry
Vinyl group + Ester group
Acrylic Resins material examples
Polymethacrylate (PMMA)
Polymethacrylate (PMMA)
- Hydrophobic, rigid and chemically stable
- Amorphous and transparent
- Tg ~100ºC to 125ºC
- Allows for in situ polymerization
Polymethacrylate (PMMA) application
bone cement
Polyesters: example
Polyethylene Terephthalate (PET)
Polyethylene Terephthalate (PET)
- Semicrystalline thermoplastic
- Obtained by polycondensation reaction
- Good flexibility
Polyethylene Terephthalate (PET) applications
ACL ligament reconstruction
grafts
Silicones or polysiloxanes – elastomers
- Linear polymers that remain liquid
- Maintain the flexibility over a wide temperature range
- Excellent biocompatibility and chemical stability
- Hydrophobicity, stability to hydrolysis and oxidation
Poly-ether-ether-ketone (PEEK)
Chemically inert and insoluble in all conventional solvents at room temperature
* 40%crystallinity, Tg ~143ºC, and Tm ~334ºC
* High-performance polymer à replace metal implant components in some
applications
* Difficult to process and high production costs
Poly-ether-ether-ketone (PEEK)
-Plate Fixation System in the vertebras
-Patient-Matched Cranial Implant2
Degradation rate
Anhydrides>Carbonates>Ester>Urethane>Ortho esters> Amides
**Degradation **[Biodegradable Polymers – Key definitions]
A chemical process by which bonds are cleaved
example:
hydrolysis of poly(lactic acid)
Resorbable[Biodegradable Polymers – Key definitions]
The process of eliminating the degradation product
example: bone resorption
Erosion[Hydrolysable Polymers Biodegradable Polymers – Key definitions]
A process that results in the mass loss of a material that may result in a change of size,shape or mass
Example: Release of a drug through layer-by-layer erosion of a nanoparticle
BIodegradation [Biodegradable Polymers – Key definitions]
Cleavage of bonds as a consequence of a biological agent, such as an enzyme, cell or microorganism
Example.: enzymatic degradation of amorphous poly(L-lactide) by proteinase K
Bioerosion [Biodegradable Polymers – Key definitions]
Erosion in a biological setting
Example.: Microborers graze on the surface of coral slowly breaking down limestone
Surface erosion [Biodegradable Polymers – Key definitions]
Erosion that is restricted to the surface of a material and proceeds via an erosion front
Examplo.: Surface erosion of polyanhydrides
Bulk Erosion [Biodegradable Polymers – Key definitions]
Erosion that occurs throughout a sample causing the whole material to degrade
Example.:degradation of poly(glycolic acid)
Chemical degradation routes
- Hydrolysis
- Enzymes
- pH
- Photodegradation
Chemical degradation mechanisms
1) Crosslink degradation
2) Side chain degradation
3) Backbone degradation
Polyesters
Esters
Poly(glycolic acid) PGA
Poly(lactic acid) PLA
Poly(propylene) PP
Poly(vinyl chloride) PVC
Poly(ethylene) (PE)
Poly(lactic acid) (PLA)
Linear, aliphatic polyester
3 forms: L-PLA (PLLA), D-PLA (PDLA), and racemic mixture of D,L-PLA (PDLLA)
PDLLA: amorphous
PLLA + PDLA: semi-crystalline polymers
PLA is a bidegradable polyester that can range from amorphous to crystalline
Poly(lactic acid) (PLA) applications
3D-printing filaments
Surgical screws,pins and meshes
Drug delivery
Degradation steps of semi-crystalline polymers
1)water diffusion into the amorphous regions
2)bond cleavage
3)exposure of the crystalline regions + hydrolytic and enzymatic attack
4)collapse of crystalline regions
5)polymer chain dissolution
PLA and PGA copolymers applications
-sutures
-skin substitures
Polycaprolactone (PCL)
Easy processing
rubbery state at body temperature
Much lower degradation rate than PLA, PGA, and PLGA
higher crystallinity and hydrophobicity
long-termimplants
Degraded by lipase enzymes
Polycaprolactone (PCL) applications
drug delivery
tissue engineering
dentistry
wound dressings
sutures
Which of the following polyesters has the highest melting TºC?
PGA;PLLA;PDLLA;PLGA;PCL
PGA
Tm(ºC)=220-230
Which of the following polyesters has the highest tensile strength?
PGA;PLLA;PDLLA;PLGA;PCL
PLLA (30-80MPa)
Which of the following polyesters are amorphous?
PGA;PLLA;PDLLA;PLGA;PCL
PDLLA
PLGA
Poly(ethylene) (PE)
exist in high and low density
Poly(ethylene) (high density) PE application
tubing for drains and ctheters
prosthetic joints
Poly(vinyl chloride) PVC
PVS is plasticized to make flexible materials
Used for short-term application since plasticizers can be leached resulting in enbrittlement of the material
Poly(vinyl chloride) PVC applications
tubing
blood storage bags
Poly(propylene) PP
isotactic PP is semicrystalline
has high rigidity
high tensile strength
good biostability
Poly(propylene) PP applications
nondegradable sutures
hernia repair
PEEK (Polyether ether ketone)
semicrystalline
polyaromatic
thermoplastic
chemically resistant
high thermal transitions
Used as a composite material for biomedical applications
PEEK (Polyether ether ketone) applications
engineering plastics
cranial defect repair
dental implants
Natural polymers key features
-biocompatibility
-ECM mimic
-can be biodegradable and bioresorbable
-might contain bioactive cues
-multitude of functinoal groups for modification
Main applications for natural polymers
temporary implants
drug delivery systems
Natural challenges
-complex interplay between chemistry, biochemical and biophysical properties
-batch-to-batach variability
-extensive purification steps
-biomechanical properties
Natural polymers: classification
proteins
Polysaccharides
Nucleic acids
Natural polymers: classification by origin
plants
animals (xenogenic)
humans (allogenic and autologous)
Key features of proteins
- Functional sequences of amino acids synthetized by the cells
- Processed from purified or recombinant ECM proteins
- Able to interact with cells via specific recognition domains in their native structure
- Sensitive to environmental conditions (e.g., temperature, pH)
Collagen
- Most abundant ECM protein in mammals
- Excellent biocompatibility, biodegradability, and low antigenicity
- Multiple sources (bovine, porcine, marine) and functions
