Porous metals for biomedical use Flashcards
Cells defined in terms of space
Cells are enclosed/ semienclosed regions of space
A pourous metal
Any combination of metal and air
Open cell
Pores are interconnected and connect to the outside world - sponge
Sponge
Open cell
Closed cell
Pores are isolated
Foam
Closed cell
What affects the behaviour of foams
definitions
Base Material- the basic properties of this influence those of the foam
Structure - properties are influenced by the
- amount of porosity
- cell shape and size (open or closed)
- ordered or not ordered
-isotropic or oriented
By influencing the structure and base material of a pourous metal we can….
tailor the properties
Lattices
Regular, well controlled structures. Creating them has been made alot easier by modern technology
Shape of bone internally
Irregular honeycomb shapes
Reasons to use porous metals
Better scaffolds High strength compared to polymers Fracture resistant compared to ceramics Porosity allows stiffness matching bone Porosity timulated bone ingrowth Increases the shear strength of a joint which improves fixation Large surface area and permeability
Problems with the use of porous metals
Fatigue resistance is reduced due to fabrication and struture
Corrosion may occur so there is risk of toxicity from by products
Not bioactive- but can be made suitable by coating
Whats are the ways of foaming metals:
Powder
Casting in a mold
Powder Bed Methods (electron beam melting)
Whats are the ways of foaming metals: Powder
A metal powder is mixed with a removable space holder (e.g salt). It is then shaped and processed using sintering and pressing. The space holder is then revoved either by disolving or thermally treated
Whats are the ways of foaming metals: Casting in a mold
Polymer foams can be surrounded by ceramic powder and then burnt away leaving a ceramic mold to cast the metal. The mold is then removed using water jets and shaking- this leaves and open cell mold
Whats are the ways of foaming metals: Powder bed methods
Additive layer manufacturing process
Uses a laser to melt powder in certain areas only. This is repeated over many layers and the unmelted powder is removed leaving a 3D structure
Needs large pieces of equipment
But make regular latices and can be most controlled
Limited by CAD file size and part size in the machine
What metals are used in pourous metals
Normally stainless steel or titanium alloys.
Some implants have a base material and are covered in a coating
example of where these can be used in bioengineering
Spinal fixations Fracture plates Wires, pins and screws Artificial ligament anchors craniofacial and mixillofacial implants dental implants joint replacment Where high strength is needed could always add a dense core
what is minimum pore size based on
Cell size Migration requirments Transport enhanced new bone formation Growth of capillaries
Suggested pore size ranges
> 100µm
Range of percentage porosity
50-85%
Commercially available pourous metals
Regenerex- Porous titanium for orthopaedic and acetabular augments
GeoStructures-Porous Titanium used for vertibrae replacement
Hedrocel - porous Tantalum used for joint replacement coating and soft tissue attachment
Titanium Foam via Gas expansion
powder pressed together under argon
This is then heated in a vacuum so the trapped gas expands the structure
Pores gradually interconnect leading to a mostly open structure
Can be made bioactive by infiltrating with peptide amphilphile nano fibre gels.
Titanium foam via space holder method
Sintering powderwith removable phase if often used for titanium
Regularly used for spinal fusion devices
Cell culture tests find that cells will grow through if perfusion is used
