1. Polymers for Medical Applications
1. Polymers for Artificial Joints 2. Bioabsorbable Polymers for surgical applications 3. Adhesives for medical applications

2. 1. Polymers for Artificial Joints
Figure (a) Normal joint
Figure (b) replacement of the joint is required
There are several regenerative treatments, but joint replacement with an artificial joint is the most common and effective treatment

3. Modern Total Arthoplasty
The artificial joint has a sliding interface using a combination of
a hard material against a soft material.
Hard material: Metallic femoral head
Soft material: Polytetrafluoroethylene (PTFE) shell
Cement material: cold-curing acrylic cement
(polymethylmethacrylate)- to fix the components and
to transfer the stress more uniformly

4. SOFT MATERIALS
The soft material made of polytetrafluoroethylene (PTFE), then it was replaced by high density polyethylene (HDPE) and later by ultra high molecular weight polyethylene (UHMWPE).
UHMWPE was chosen because of its low friction coefficient, high resistance to wear, high impact resistance, high ductility and stability in the body

5. Problems of Total Joint Replacement

6. What are the differences between HDPE and UHMWPE???

7. The differences between HDPE and UHMWPE
Morphology – the chain length of the tie molecules in UHMWPE is much higher than that in HDPE
Molecular weight – UHMWPE shows extremely high molecular weight (two to six million), in contrast to 20,000 – 30,000 for regular HDPE, LDPE & LLDPE.
Crystallinity & density of UHMWPE are lower than that of HDPE, due to the high molecular weight and chain structure

8. Chemical Structure

9. Properties of UHMWPE

10. Properties of UHMWPE

11. Processing of UHMWPE
Fabrication methods for thermoplastics cannot be applied for processing of UHMWPE
When UHMWPE is melted (> crystalline melting temp.), the resin becomes rubbery but does not flow
Why the processing of UHMWPE is complicated???

12. Processing of UHMWPE
Require a combination of temperature, high pressure and time.
Methods are ram extrusion, compression molding and direct compression molding.
The objective of the methods is to apply enough temperature and pressure to fully sinter the UHMWPE particles

13. Processing of UHMWPE

14. Wear properties of UHMWPE
The abrasion resistance of UHMWPE is
The highest of the various materials
Relative weight loss vs.
type of polymers
Relationship between molecular weight
of polyethylene and abrasion weight loss
Abrasive wear resistance is increased
With linearly increasing molecular weight

15. Wear properties of UHMWPE
Problems with UHMWPE in the application of artificial joints- wear particle produced at sliding surfaces- accelerates the loosening
Wear particles
Formation of particles
with diameter < than 1 micron
Solution: Use of transfer film
Lubrication; a very thin film of
Polymer is transferred to the
Opposing surface, lead to the
Resultant coefficient of friction
Being very low
Schematic illustration of an artificial hip joint

16. New processing of UHMWPE
To obtain high performance implants-alter the properties of UHMWPE
Increased the crystallinity (without causing degradation)- by using temperature greater than 250C and pressure greter than 2,800 atm.- obtained crystallinity over 80%
Crosslinking UHMWPE by low-dose γ-ray sterilization in a vacum or inert gas, then stored in oxygen free environment or heat-treated at temp. below the melting point in a vacuun or inert gas
Addition of vitamin E- prevent the crack formation

17. BONE CEMENT
Acrylic cement is used for the fixation of total joint prosthesis
The cements used in orthopedic surgery are combination of prepolymerized PMMA solid particle and the liquid monomer
The powder particles are sphere (30 to 150 µm in diameter), molecular weight of 20,000 to 2 million
For the reaction to occur,prepolymerized PMMA needs to contain an initiator, dibenzoyl perioxide (BP)

18. BONE CEMENT
The liquid monomer contains the activator N,N-dimethyl-p-toluidine (DMPT)
The monomer will polymerized on its own when exposed to light or heat.
To prevent to monomer from polymerizing, the liquid generally contain an inhibitor or retardant, hydroquinone- function to absorb free radical that may occur and initiating the polymerization

19. Preparation of Bone Cement
The prepolymerized + liquid monomer (mixed together), chemical reaction begin with activator (DMPT) and the initiator (BP) combining and releasing a benzoyl peroxide free radical, and react with the monomer. Polymerization begin.
Chains with double bond converted to single bond, heat is generated as an exotherm, and the cement cure

20. Problems of PMMA Bone Cement
Strong exothermic setting reaction
Toxic effect of the monomer
Inability to bond directly to bone - caused loosening at the interface
Brittle nature
- To overcome these problems, many types of bioactive bone cements have been developed.

21. To improve the biochemical properties of PMMA bone cement, many types of bioactive particle fillers have been added into the cement
Example of particle fillers are glass ceramic, titania (anatase & rutile), etc

22. Recent studies on Bone Cement + titania particles (K. Goto et al
Recent studies on Bone Cement + titania particles (K. Goto et al., Biomaterials 26 (2005))
Figure (c)
Shows direct
Contact
Between bone (B)
And Cement (C),
while Figure (b)
Shows soft
Tissue layer
Less than
10 um. The soft
In (a) and (d)
Is thicker
Than (b) and (c)

23. 2. Bioabsorbable Polymers for surgical applications
Polymeric materials and composites have been used in medical applications; tissue replacement, support of tissues and delivery of drugs
Based on their behavior in living tissue, polymeric biomaterials can be divided into;
1)Biostable
2) Bioabsorbable (biodegradable/bioresorbable)

24. Biostable Polymers Are inert
Cause minimal response in surrounding tissue
Retain their properties for years
Example: polyethylene, polypropylene; used for endoprostheses and sutures

25. Bioabsorbable (biodegradable/bioresorbable)
Temporary internal fixation, can be partially and fully bioabsorbable material
Bioabsorbable implant preserve the structure of tissue at the early stage of the healing, example in bone, tendon and tissue
After that, the implant decomposes, and stress are gradually transferred to the healing tissue
Bioabsorption of the materials induced by the metabolism of the organism

26. Bioabsorbable (biodegradable/bioresorbable)
Bioabsorbable surgical devices need no removal
Requirements for bioabsorbable materials;
noncarcinogenic, tissue compatible, nontoxic, etc
Should not cause morbidity
Must provide adequate mechanical strength and stiffness
Degradation should occur by hydrolysis in aqueous media

27. Bioabsorbable Polymers for surgical applications
Suture Materials
Polyglycolid acid (PGA) and Polylactic acid (PLA) have been used as synthetic bioabsorbable sutures
Bioabsorbable sutures are used in the fixation of bone fractures,closure of soft tissue wound, etc

28. A Typical Suture Line

29. Polyglycolid Acid (PGA) and Polylactic (PLA)
PGA - High molecular weight, hard, tough crystalline polymer, Tm at about ºC, Tg of 36ºC.
PLA – Tm of ºC, Tg of 57ºC.
Such polymers can be processed into fibers, films, rods, screws, plates, clamps, etc
Advantages of polymeric materials compared to metal and ceramics; easy and cheap to make

30. Bioabsorbable Polymers for surgical applications
2) Porous Composites
Combining bioabsorbable polymers in porous and nonporous materials
Hydroxyapatite powders and blocks have applications in the bone surgery, e.g. to fill the defects
Since porous ceramics are brittle, the toughness has been increased by combining them with polymers

31. Bioabsorbable Polymers for surgical applications
3) Drug Delivery System
Polymeric devices for the controlled release of drugs and antibiotic have been studied
These polymers show several advantages over traditional repeated dosage methods
This technique can save patients from being exposed to greater amounts of drug at the desired site of action

32. Bioabsorbable Polymers for surgical applications
4) Partially Bioabsorbable Device
The reinforcement of bioabsorbable polymeric matrices with biostable fibers produce strong, partially bioabsorbable materials
Example; PLA matrix reinforced with carbon fiber, copolymer MMA and N-vinylpyrrolidone reinforced polyamide fibers, etc used for ligaments, tendon, scaffolds, etc.

33. Example of Bioabsorbable materials in artificial skin
Skin damage following severe burns or ulcers, such as diabetic foot ulcers, is notoriously difficult to heal.
This is because the dermis cells will not regenerate in the absence of a matrix on which to grow
Recently the development of tissue engineering and, in particular, artificial skin has presented advances in this area
These artificial skins (keratinocyte seeded IntegraTM, DermagraftTM, and ApligraftTM which contain neonatal cells in combination with matrices formed from bovine collagen or the soluble suture materials polylactic and polyglycolic acids) provide a matrix for dermis growth and the neonatal cells contained in them produce growth factors which promote healing

34. 3. Adhesives for medical applications
The use of surgical tissue adhesives in medicine has developed over 40 years
Traditionally, the area of tissue reattachment or repair following surgery has been dominated by sutures, staples and wiring
Recently, there is a huge potential for tissue adhesives in clinical practice

35. Pressure Sensitive Adhesives (PSAs)
PSAs have been used for adhering wound dressing to skin
PSAs have Tg in the range of -20 to -60ºC, which means they are soft materials at room temp.
These soft polymers are able to flow and wet out on to a surface and are able to adherence to that surface

36. Pressure Sensitive Adhesives (PSAs)
The bond formed between PSA and substrate is not permenant and can be broken with a measurable force
Mid-19th century, the first adhesive plasters were used, the first aid application of dressing become more demanding, and undergone significant development
Early adhesive formulations were based on blends of natural rubber and resin.
Now PSAs were dominated by acrylic copolymer

38. Requirement for PSAs
Should be permanently and aggressively tacky, adhere with only slight finger pressure
Form a strong bond with surfaces
Sufficient cohesiveness that it can be removed without leaving a residue
Need to be chemically and biologically accepted to the skin-no irritation or sensitization

39. Requirement for PSAs
5) Adhesives must have sufficient flow to ensure intimate surface contact
6) Must be able to cope with moisture at the skin without compromising performance
7) PSAs should be easily removed with minimal trauma to the skin

40. Example of First Aid Dressing

41. Adhesive Types Acrylic Polymer
Widely used due to natural adhesive behavior and wide scope of formulation/property tailoring
PSAs are typically copolymer composed of ‘hard’ monomer and ‘soft’ monomer
The Tg of the resultant polymer can be controlled by the ratio of hard and soft monomers

42. The nature of alkyl group, R’, can be used to dictate the adhesives properties, by varying the chain length and hydrophilic/hydrophobic nature of the group
Chain length

43. Rubber-Based PSAs Early medical adhesives were based on natural rubber
Now changed to synthetic elastomers such as polyisoprene and polyisobutylene
Polyisobutylene tend to pack tightly, results in low air and moisture permeability
The low Tg of these materials produce flexible material, that are naturally tacky, allowing the polymer to wet out the skin surface

45. Silicones
- Used since mid 1960, have been utilized for tapes, dressing, bandages
Typically formulated from silicone resins and polydimethyl siloxane gum
To impart cohesive strength, the polymer and resin are crosslink to form a network
The properties of the final adhesives can be controlled by ratio of component and the cross-link density

47. Types of Transdermal Drug Delivery Designs More recently, silicone adhesives were used in transdermal drug delivery system-controlled entry of pharmaceutical into the blood
Delivery of an active ingredient through the skin and
into the blood vessels before delivery into the target organ