Structuring of biodegradable polymers Flashcards
What are the 4 different structuring methods for biodegradable polymers?
- Construction of random structures
- Fibres
- Construction of ordered structures
- More controlled (random) structure
What are the importance of scaffold dimensions ?
- The pore size is dependent of the tissue/cell type
- The pore structure needs to be known
- Pores need to be interconnected
What about pore structure determines whether the tissue carries out certain functions
SPECIFIC EXAMPLES
- Vascularisation needs a pore size of at least 250micro meters
- Fibroblast in-growth requires a scaffold pore size between 5 and 15 micro meters
- Hapatocytes (liver cells) need a pore size of 20micrometers
- Osteoid cells require a pore size between 40-100micrometers
What is the construction of random structures method and how to we carry it out
This is similar to the technique we would use to introduce porosity into metals
It makes micrometer scale pores
It can be done by
- Effervescent salt
- High pressure CO2 gas
- Freeze drying
- Solvent casting and particulate leaching (Salt/ Sugar Particles)
How do we use effervescent salts to create random porous structures in bidegradable polymers
This is a type of gas foaming
- These create foams
Gas is used as a porogan via the use of effervescent salt
COME BACK
What is the meaning of the word porogen
Any of a mass of particles, of a specified shape and size, used to make pores in moulded structures used for tissue engineering (they are dissolved away after the structure has set)
How do we use solvent casting and particulate leaching to create porous biomaterials ?
What controlled the porosity and connectivity of the biomaterial?
Salt/ sugar particles
The polymer is mixed with solvent (e.g. PLA) and a salt
It is then cast and vacuum dried
The salt is then washed out
The overall porosity is controlled by the salt size
The ratio or polymer to salt particulates controlled the connectivity
In creating random porous biomaterials what is one thing all the methods have in common
They all carryout a combination of 2 immiscible( the materials in these different phases do not mix) phases
solid-liquid-gas
What is Gas Foaming and how do we carry it out
Gas (normally CO2) or effervescent salts are mixed with a polymer to create a polymer matrix
The equipment for this is expensive but once paid for the CO2 is cheap
Freeze drying. What is it and how do we carry it out?
Method of Random porous material manufacturing
A solvent polymer mixture which phase separates at low temperatures
The solvent then sublimes (transitioning from solid- gas state without passing through the liquid state) , leaving spaces(pores) in the freeze dries material where it once was
This method is time consuming. Takes about 4 days per sample
What are some of the advantages of randoms porous materials
- Experimentally easy technique
- Can make interconnected porous structures
What are some of the disadvantages of random porous materials
- They don’t have good structural stability
- The porosity and morphology are hard to control independently
- Poor mechanical integrity
How do we make polymer fibres by spinning
A polymer solution is filtered and then passed through a spinnerette. The fibre created is then stretched, washed and a fibre finish is applied.
Once dries this long stretch of fibres is either packaged up or cut into usable lengths.
There are different variations to this method
- Wet spinning in a chemical bath
- Dry spinning using solvent evaporation
- Melt spinning using solidification
Industrially the fibres range from 10-100 micro meters in diameter
This is used for Silk-PLA-PCL
How are polymer fibres treated once made. What is this particularly useful for
Knitting and Weaving
This is often used for vascular implants
What is electrospinning
Electrospinning uses an electrical charge to form a mat of fine fibres. When the electrical force at the surface of a polymer solution or polymer melt over comes the surface tension, a charge jet is ejected
Additive manufacturing techniques , what are they?
Example
Family of fabrication processes that were developed to make engineering prototypes in minimum lead time based on CAD models.
In May 2013 a 3D bioresorbable splint was printed to keep a new born babies airways open
What are some of the advantages and disadvantages of additive manufacturing techniques
- Reduced lead time to produce prototype component or to manufacture parts
- Increased ability to compute manifacturing properties or components and assemblies
- Manufacturing of bespoke parts
- This might help develope a distributed economy so people/ hospitals can print their own parts.
What are the two main types of additive manufacturing techniques
Printing techniques- 3D printing and wax printing
Laser based techniques- STereolithography and selected laser sintering
Fused deposition modelling
How does 3D printing work?
- A layer of powder is spread on a platform
- Inkjet printer head deposits drops of binder of selected parts of the cross section
- The binder dissolves and joins the adjacent powder particles
- The table is then lowered by a layer thickness
- New layer of powder is deposited and the system repeats over and over again until the finished shape is created
- The shape is then shaken to remove powder and get part
This can currently be carried out with the material being printed directly
What are some of the advantages and disadvantaged of 3D printing
\+ Easy process \+ Patient specific - The binder can be toxic - The pore size is limited by the size of the powder - Not great mechanical strength
Laser based techniques
Laser based machines that either photo-polymerise (polymer changes properties when exposed to light) liquid monomers or sinter(the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction) powdered material
Stereolithography is an example of a laser based technique how is it carried out
- A photocurable monomer is used
- Layer by layer this is hit by a laser beam, with the table moving a layer lower each new layer. This UV laser solidifies part of the cross section.
- Once the shape is completed it is removed and the supporting structure is broken off and the part is cured in an oven.
Give examples of where Stereolithography (SL) scaffold have been used
They have been used in bone tissue engineering
They have also been used to create nerve guide conduits- Here the structure of the nerve/ axon schwann cells are mimicked using PEG, PLA and PCL. The contact guidance is created through the use of grooves with a depth of about 20micrometers
Selective laser sintering is another example of a laser based technique
A moving laser beam sinters heat fusible powders in areas corresponding to the CAD design layer at a time. After each layer is completed, the table moved downwards one layer and a new layer of powder is spread. The areas not sintered, the powder is shaken loose and removed
