Porous metals for biomedical use

Question 1

Cells defined in terms of space

Answer 1

Cells are enclosed/ semienclosed regions of space

Question 2

A pourous metal

Answer 2

Any combination of metal and air

Question 3

Open cell

Answer 3

Pores are interconnected and connect to the outside world - sponge

Question 4

Sponge

Answer 4

Open cell

Question 5

Closed cell

Answer 5

Pores are isolated

Question 6

Foam

Answer 6

Closed cell

Question 7

What affects the behaviour of foams

definitions

Answer 7

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

Question 8

By influencing the structure and base material of a pourous metal we can….

Answer 8

tailor the properties

Question 9

Lattices

Answer 9

Regular, well controlled structures. Creating them has been made alot easier by modern technology

Question 10

Shape of bone internally

Answer 10

Irregular honeycomb shapes

Question 11

Reasons to use porous metals

Answer 11

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

Question 12

Problems with the use of porous metals

Answer 12

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

Question 13

Whats are the ways of foaming metals:

Answer 13

Powder
Casting in a mold
Powder Bed Methods (electron beam melting)

Question 14

Whats are the ways of foaming metals: Powder

Answer 14

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

Question 15

Whats are the ways of foaming metals: Casting in a mold

Answer 15

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

Question 16

Whats are the ways of foaming metals: Powder bed methods

Answer 16

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

Question 17

What metals are used in pourous metals

Answer 17

Normally stainless steel or titanium alloys.

Some implants have a base material and are covered in a coating

Question 18

example of where these can be used in bioengineering

Answer 18

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

Question 19

what is minimum pore size based on

Answer 19

Cell size
Migration requirments
Transport 
enhanced new bone formation
Growth of capillaries

Question 20

Suggested pore size ranges

Answer 20

> 100µm

Question 21

Range of percentage porosity

Answer 21

50-85%

Question 22

Commercially available pourous metals

Answer 22

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

Question 23

Titanium Foam via Gas expansion

Answer 23

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.

Question 24

Titanium foam via space holder method

Answer 24

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

Question 25

Magnetic stimultion in pourous metal implants

Answer 25

If magnetic field is imposed on an implant the mesh will deform, loading the surrounding tissue which is thought to stimulate bone growth

Question 26

Magnesium alloys

Answer 26

Can break down in the bodies environment- a biodegradable metal
The dissolution rate is fast
To control the rate coatings are used and changes in the alloy composition are made
The alloys could be made into foams but they are not always suitible

Question 27

Layer Manufacturing of foam -examples

Answer 27

Ti alloys have been used to make foams based on bone scans. surface treatmenst are used to ensure best osseointegration
The same design as bone may not be the best as different material

Question 28

Layer Manufacturing of Lattices

Answer 28

investigations into lattice structures are being carried out. But such a multitude of options makes this hard
Very specificable

Question 29

The Future of Pourous metals

Answer 29

Designs are becoming better. Different materials and structure combinations are being introduced
New metallic materials may become available
Tailorability to the additive manufacturing process is becoming more accessable
Lots of testing still needs to take place