Porous Metals for Biomedical Use Flashcards
What makes a material porous?
Cells are enclosed or semi-enclosed regions of space
Porous metals: any combination of metal and air
What is the difference between open and closed cells?
- Open cell = pores are interconnected and connect to the outside environment (sponge)
- Closed cell = pores are isolated (foam)
What are lattices?
have regular (or at least well-controlled) structures
What is foam behaviour affected by?
- Base material – basic properties influence those of the foam
- Structure – properties influenced by amount of porosity, cell size and shape (open or closed), ordered / not ordered, isotropic or oriented, etc.
What are the reasons to used porous metals?
- Evident advantages as scaffolds
- High strength scaffolds – compare to polymers (PCL 300 MPa –Ti 100 GPa)
- Fracture resistant – compare to ceramics
- Porosity allows stiffness matching to bone
- Porosity stimulates bone ingrowth
- Increases shear strength of joint – fixation
- Large surface area and permeability
What are the problems with porous metals?
- Fatigue resistance reduced (due to fabrication and structure) Same as for all metal implants
- No degradation* – permanent implants, risk of toxicity from corrosion by-products
- Not bioactive – although suitable for coating, e.g. with organically modified apatite
How is a foaming metal- powder formed?
- A removable, space holder material is combined with metal powder (e.g. salt)
- Powder is shaped and processed (e.g. pressing and sintering)
- Space holder removed thermally or by dissolving
What is a foaming metal – casting into mould?
- Polymer foams can be surrounded by ceramic powder, then burnt out at high T
- This leaves a mould to cast metal in
- Finally, the mould must be removed (water jets, shaking, etc.)
- Gives high quality, open celled foams
- Foams of this type are sold under the name Duocel
What are powder bed methods?
- In Additive Layer Manufacturing processes, a laser or electron beam is used to melt powder in certain areas only
- An additional layer of powder is added and the process repeated
- After processing, unmelted powder removed and recycled
- 3D structures, including regular lattices, can be built up
What equipment / manufacturers are used?
Manufacturers include Renishaw, Aconity3D, Arcam (shown)
• Heat source is an electron beam or laser
• Techniques sometimes called Electron Beam Melting (EBM)= for lattices, Selective Laser Melting (SLM), etc
• Often used for Ti, Stainless steel
What are the biomaterial applications?
• Spinal fixation • Fracture plates • Wires, pins and screws •Artificial ligament anchors • Cranio-facial and maxillofacial implants • Dental implants • Joint replacements Where high strength is needed, a dense core may be used
What metals are often used?
• Normally stainless steel or titanium alloys (titanium has a lower modulus),
sometimes Co-Cr or tantalum.
Suggestions that NiTi shape memory alloy may have desirable mechanical properties
• Often applied as a surface layer or coating, sometimes as bulk porous material for a large part of an implant
What is minimum pore size likely to be based on?
- Cell size
- Migration requirements
- Transport
- Enhanced new bone formation
- Growth of capillaries
What are the commercially available materials?
- Tritanium (Stryker)
- Regenerex (Biomet)
- Stiktite (Smith and Nephew)
- Geo Structures
- Trabecular Metal (Zimmer)
- Hedrocel (Implex Corp)
How does titanium foam formed via gas ecpansion?
- Powders pressed together (wires can be used to give an anisotropic material) under argon
- Heated in vacuum – gas trapped expands the structure
- Pores gradually interconnect, leading to a mostly open structure (~70% of pores open, ~30% closed)
- Peptide Amphiphile nanofibre gels have been infiltrated into these foams – to make them bioactive
What is titanium via space holder method?
- Sintering powder with a removable phase is often used for Ti – particularly for spinal fusion devices
- Cell culture tests find that cells will grow throughout if perfusion is used
What is magnetic stimulation?
Further utility of metal implants has been proposed for sintered fibre arrays
• If a magnetic field is imposed, the mesh will deform
• This will load surrounding tissue – stimulate bone growth
What is Mg foam?
• Mg alloys (e.g. Mg-2Y-1Zn) can break down in body environment*
• But dissolution rate is fast and hydrogen evolved; change alloy composition or use coatings / surface treatments to control rate
27 days 91 days
• These alloys can be made into foams (by replication)
What is layer manufacturing of foams?
- Ti-6Al-4V foams based on scans of trabecular bone
- Surface treatments used to get best osseointegration in studies
- But no reason why real bone shapes in metal should work best
What is the future for porous metal implants?
- “Design” of required properties is getting better
- Also, the idea of using different materials / structures for different parts is more common
- New metallic materials may become available
- Individual tailorability of additive processes is attractive; but will this be affordable for all?
- Many good ideas, but rigorous testing needed before they become real
What are the suggested ranges of pore size?
>100 µm, 325 µm, 200-500 µm, 500-1500 µm Porosity can be 50-85%, with some evidence that larger pores are needed for higher porosity
What is tritanium (Stryker)?
vapour deposition of titanium on a polymer foam, acetabular components
Also used in Additively manufactured form; Tritanium PL Posterior Lumbar Cage – vertebral fusion device, FDA 510(k) clearance
What is Regenerex (Biomet)?
Porous Ti, acetabular and orthopaedic augments, FDA Approved
What is Stikit (Smith and Newphew)?
Porous Ti coating
