Structuring of biodegradable polymers Flashcards by Georgina Wood

What are the 4 different structuring methods for biodegradable polymers?

Construction of random structures
Fibres
Construction of ordered structures
More controlled (random) structure

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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 well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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 well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

What are some of the advantages of randoms porous materials

Experimentally easy technique

- Can make interconnected porous structures

How well did you know this?

Not at all

Perfectly

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 well did you know this?

Not at all

Perfectly

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 well did you know this?

Not at all

Perfectly

How are polymer fibres treated once made. What is this particularly useful for

Knitting and Weaving

This is often used for vascular implants

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

What are some advantages and disadvantages of stereolithography

+These are the highest resolution techniques available, pore size can be down to 10micrometers

The monomers themselves can be costly
Limited by the development of photopolymerisable liquid monomer materials

What are some advantages and disadvantages of selective laser sintering

\+ Good compressive strength 
\+ Wide range of materials 
\+ Solvent free
- High processing temperatures
- Pore size is dependent on the powder particulate size

What is fused deposition modelling

This is a type of 3D Bioprinting practice

The material is either thermally or chemically treated as it passes through a nozzle
The nozzle moved to extrude a fibre of polymeric material from which the physical model is built layer by layer by lowering the table upon which the model sits
The nozzle also creates a support structure to support overhanging or unconnected features which are manually removed post processing

What examples have there been of fused deposition modelling

It has been used in osteochondral tissue engineering to grow cartilage and bone together
It has also been used to help fix vascular networks by using a sacraficial structure that can easily be dissolved in water. Within the 3D scaffold prototype vasculature is incorperated
Hydrogels have also been used as tracks for vasculature

What are the positives and negatives of fused deposition modelling

+ This method can easily be used to make different morphologies and densities which can then be layered for different tissue growth
+ Good compressive strength
+ Solvent Free
- High Processing temperatures
- Limited material range
- Inconsistent pore openings
- Requires support structure for irregular shapes

How do we carry out bioprinting

also known as Organ Printing

Similar process to fused deposition modelling and inkjet printing
Thermo responsive Gels are printed and then cells are sprayed onto the solidifying thin layer of polymer solution. However in the process of spraying the cells may be damaged or cell aggregation could occur within the droplets. Tissue spheroids can be used as building blocks for the organs

The material choices for this method are limited

Why would we used additive manufacturing over other options

Foams are normally much softer thane either soft or hard tissue. This can be over come by additive manufacturing because we can design precise mechanical properties,
More controllable materials also mean better tissue growth

Electrospinning 3D objects

The combination of what?

Occurs when we we combine additive manufacturing with electrospinning
Gives us the opportunity to create 3D manufactured electrospun objects
You electrospin on a construct and replicate the underlying pattern by changing the height of the collectors
Field lines curve towards objects that stick out so this can be used in the process
This process enables electrospun mats- within these electrospun mats stem cell niches can be replicated. Microfibrous mats with added relief cam be replicated easily

PolyHIPEs

High Internal Phase Emulsions

Water droplets are surrounded by a very thin film of polymer. Said polymer is then cured and the remainder is dried forming polyHIPE
Porosity and connectivity can be tuned and structured by combining them with stereo-lithography

Define Hydrogel

Insoluble network of polymer chains that swell in aqueous solutions

Hydrogels undergo swelling in analogy to…

… dilution of free polymer chains in solution

This swelling depends on temperature and pH

What are hydrogel responses to temperature and pH

They impct the swelling behaviour
We often use hydrogels that collapse at body temp
Ionic hydrogels with acidic side groups collapse at a low pH

What use could a pH sensitive hydrogel have

Could be used for glucose sensing

Boronic acid binds to glucose
Quencher releases fluorescent tag in the hydrogel

Brainscape's Knowledge GenomeTM

Structuring of biodegradable polymers Flashcards

Brainscape's Knowledge Genome^TM