Non-metallic biomaterials Flashcards
What are polymers?
- Polymers are made up of large numbers of similar repeat units linked to each other by covalent bonding
- Repeat units – monomers – most likely liquid form and then go through polymerisation to become a polymer – usually monomers are toxic (have to ensure no monomer residue)
Describe the two groups of polymers
Thermoplastics - polymers with linear molecules (substances that soften upon heating and can be remoulded and recycled)
Thermosets - substances that do not soften under heat and pressure and must be re-machined or incinerated to remove from the environment
Why do we use polymers for medical devices?
Biocompatible Easy to produce Often cheaper Designed to mimic Designed to prevent requirement for revision surgery
Name some inert polymers.
PMMA
Acrylic
Silicone
Name some thermoplastic polymers for biomaterials.
PMMA, Polyolefin, Teflon, Polyvinyl chloride, polycarbonate, polyester, nylon
Name some thermosetting polymers for biomaterials.
Butyl rubber, chlorosulfonated polyethylene, epichlorohydrin rubber, polyurethane, natural rubber & silicon rubber
Name some thermosetting polymers for biomaterials.
Butyl rubber, chlorosulfonated polyethylene, epichlorohydrin rubber, polyurethane, natural rubber & silicon rubber
What is PTFE commonly used for?
For a heart valve, it serves as a sewing ring / receptor for sutures
Shunts to carry cerebral spinal fluid from hydrocephalic patient
Middle ear drain tubes, sutures – heart valves
What are acrylic polymers used for?
e.g., PMMA
o Used for contact lenses, bone cement, dentures, maxillofacial prostheses
What are polyurethanes used for?
Vascular tubes, artificial heart assist devices
What are polyamides used for?
Applications in intracardiac catheters, components in dialysis device, sutures
What is silicone used for and what is the most widely used type?
o The most widely used is polydimethylsiloxane
o Used for:
Catheters, lines
Silicone prostheses: finger, toe, mammary, maxillofacial surgery
List the natural bioresorbable polymers.
o Fibrin – blood clotting o Collagen o Chitosan o Gelatin o Hyaluronan
List the synthetic bioresorbable polymers.
o PLA, PGA, PLGA,PCL (Polyorthoesters) o Poly(dioxanone) o Poly(anhydrides) o Poly(hydroxybuterates) o Polyphosphazenes
List the properties of PCL.
- Biodegradable polymer
- Semi-crystalline
- Modulus = 0.5 GPa, Strength = 16 MPa
- Low melting point (60 °C)
List the properties for PLA.
- Biodegradable polymer
- Two forms:
Semi-crystalline P-L-LA
Amorphous P-DL-LA - Modulus = 1.8 GPa, Strength = 50 MP
What is PGA?
Polyglycolic acid - relatively very fast resorbing polymer
What is PLGA?
Polylactic-co-glycolic acid - one of the most widely investigated biodegradable polymers for drug delivery
What are the two types of polymerisation?
Chain-reaction (or addition) and step-reaction (or condensation) polymerisation
Explain the chain reaction.
It is a three-step process (initiation, propagation and termination) involving two chemical entities (monomer and catalyst)
Explain what is meant by linear, branched and cross-linked polymers.
o Linear polymers are made up of one long continuous chain
o Branched polymers have a chain structure that consists of one main chain with smaller molecular chains branching from it.
o Cross-linking in polymers occurs when primary valence bonds are formed between separate polymer chain molecules.
What two distinct physical structures can segments of polymers exist in?
Crystalline or Amorphous
Crystalline is ordered geometry – amorphous is random. Most polymers are a combination of tangled and disordered regions surrounding the crystalline areas
Which polymers undergo surface erosion and what is the process?
Poly(ortho)esters and polyanhydrides
o Sample is eroded from the surface
o Mass loss is faster than the ingress of water into the bulk
Which polymers undergo bulk degradation and what is the process?
PLA, PGA, PLGA, PCL
o Degradation takes place throughout the whole of the sample
o Ingress of water is faster than the rate of degradation
What do bioresorbable polymers do in the body?
Materials and/or devices used in repair procedures that:
o Breakdown over time to materials that can be eliminated from the body via natural pathways
o Ideally leave no evidence of the repair injury
What are the advantages of bioresorbable polymers?
o Eliminated from the body and replaced by host tissue
o No need for second surgical procedure to remove implant
o Avoid complications of metal implants – stress shielding, corrosion, release of metal ions
o Allow transfer of loads to healing bone
o Revision surgery not complicated by presence of implant
o Compatible with MRI imaging
o Can be used to deliver bioactive agents etc.
How do PLA and PGA degrade?
From inside out (autocatalysis)
Release acidic degradation products – in an “acid burst”
PLLA-based materials slow to be resorbed (3-5 years)
Compared PLA and PGA.
PGA degrades more quickly PGA has higher acidity/toxicity of products but PGA response seen within 8-16 weeks PLA degrades in 1.5 – 3/5 years
What are the efficacy and speed of action of a drug dependent on?
o Site of administration
o Method of administration
What is the duration of action of a drug dependent on?
o Metabolic conversion
o Excretion in kidney, GI tract, skin and respiratory system
What polymer would you use for fast functioning NSAIDs?
PDA
List the forms of sustained release.
- Zero-order - Continuous administration of constant dose
- Positioned - Absorption of drug at predetermined position in GI tract
- Quick-Slow - Fast, concentrated dose, followed by controlled release
- Slow-Quick - Concentrated dose with a fast bolus after a time delay
- Accelerated - Starts slow and gets faster
- Time Delay - Drug released after a predetermined delay
- Binary - Two different drugs from one formulation
- Multiple Pulse (clinical trial)
What is iontophoresis?
- Electrically charged drug pushed through epidermis by electrostatic repulsion
What is iontophoresis controlled by and used to administer?
- Delivery rate controlled by current density, pulsed voltage, drug concentration, ionic strength
- Used to deliver
o Lidocaine (topical anaesthetic)
o Pilocarpine (diagnosis of cystic fibrosis)
o Dexamethasone (anti-inflammatory into joints)
Explain phonophoresis.
- Combines ultrasound therapy and topical drug therapy
- Ultrasonic vibrations enhance transport of drug through dermal layer
- Typically used to administer hydrocortisone in conjunction with physiotherapy
o Drug is mixed with ultrasound gel
Define a monolithic drug delivery system.
Therapeutic agent dispersed in polymer matrix – release controlled by diffusion from matrix
List four other types of drug delivery system.
Membrane Controlled - Membrane limits rate of release
Osmotically Controlled - Water is osmotically driven into compartment containing osmotic agent. Increase in volume pushes moveable partition, forcing agent out through delivery orifice
Chemically Controlled - Polymer Backbone (B-B-B-B), covalently bonded (A) to drug. Drug is released as bond A is hydrolysed. Polymer is broken down and excreted (hydrolysis of bond B). Reactivity A>B
Swelling Controlled
Liposome drug delivery
o Water soluble drug contained within phospholipid bilayer. Administered as an emulsion into the bloodstream
o 0.02 to 10μm
o Site-specific, targeted drug delivery
o Easily taken up by reticulo-endothelial macrophages (common in liver, spleen, bone marrow).
- Stealth Liposomes
o Liposome coated with polymer – avoids detection by immune system
