Structuring of (biodegradable) polymers

Question 1

What is the importance of scaffold dimensions?

Answer 1

• Scaffolds need to have a pore structure
• Pores need to be interconnected
• Pore size dependent on tissue/cell type:
• Fibroblast ingrowth occurs with scaffold pore sizes of 5-15 µm, hepatocytes and osteoid cells need pore sizes of 20 µm and 40–100 µm
• Vascularisation: needs pore sizes of at least 250 µm

Question 2

What can you make micrometer scale pores?

Answer 2

Salt/sugar particles
Effervescent salt
High pressure gas (CO2)
Freeze drying

Question 3

What is solvent casting and particulate leaching?

Answer 3

Mix polymer in solvent (e.g. PLA/chloroform) with salt (NaCl, porogen), cast, vacuum dry, wash out the salt. Overall porosity and connectivity can be controlled by salt size and ratio polymer salt particulates.
- Solid salt/sugar particles in polymer matrix, particles washed out

Question 4

How can random porous materials be made?

Answer 4

• Solvent casting and particulate leaching
• Gas foaming
• Freeze drying

Question 5

What is gas foaming?

Answer 5

Gas (CO2) or gas forming particles (evanescent salt, e.g. CaCO3) in polymer matrix
- Using gas as porogen via using effervescent salt. This technique includes both melt/solvent molding and particulate leaching aspects. Porosities as high as 90% with pore sizes from 200-500 µm are obtained using this technique

Question 6

What is freeze drying?

Answer 6

Solvent (water) polymer mixture which phase separates at low temperature, solvent sublimes

• High-pressure processing/supercritical fluid technology: applying a gas, e.g. CO2, to dry polymer at high P, forms a single phase polymer/gas solution. High Pressure Gas Foaming
• The pressure is reduced and results in nucleation of dissolved CO2 and growth of gas cells to generate pores within the polymer matrix.

Question 7

What are the advantages of Solvent casting and particulate leaching, gas foaming and freeze drying?

Answer 7

Experimentally easy techniques.

Can make very porous materials that are interconnected.

Question 8

What are the disadvantages of Solvent casting and particulate leaching, gas foaming and freeze drying?

Answer 8

Poor mechanical integrity, lack of structural stability, morphology and porosity is difficult to control independently

Question 9

How are fibres made?

Answer 9

• Spinning
• Knitting and Weaving
• Electrospinning: non-woven fabrics

Question 10

What is spinning?

Answer 10

Polymer solution coagulates after spinnerette to produce fibres Variants: wet spinning (chemical bath) – dry spinning (solvent evaporation) – melt spinning (solidification) Continuous industrial production of typically 10-100 µm diameter fibres that can be woven Used for Silk-PLA-PCL

Question 11

What is knitting and weaving?

Answer 11

Popular choice for vascular implants

Question 12

What is electrospinning?

Answer 12

Electro-spinning uses an electrical charge to form a mat of fine fibers. When the electrical force at the surface of a polymer solution or polymer melt overcomes the surface tension, a charged jets is ejected

Question 13

How can user controlled structures be created?

Answer 13

Additive Manufacturing techniques

Question 14

What is Additive Manufacturing?

Answer 14

family of fabrication processes developed to make engineering prototypes in minimum lead time based on a CAD model of the item

Question 15

What are the benefits and disadvantages of Additive Manufacturing?

Answer 15

1) Reduced lead times to produce prototype components or to manufacture parts.
2) Increased capability to compute manufacturing properties of components and assemblies.
3) Manufacturing of bespoke parts, e.g. Patient specific implants
4) Possibility of developing a distributed economy (print parts at home/in hospital)

Question 16

How can Additive Manufacturing save lives?

Answer 16

3D printed splint to keep newborn’s airways open

Question 17

What are examples of additive manufacturing techniques?

Answer 17

can produce user controlled scaffolds for tissue engineering via using CADCAM
• Printing techniques: 3D printing an wax printing
• Laser based techniques: stereolithography and selected laser sintering
• Fused deposition modelling and bioplotting techniques

Question 18

What is 3D printing?

Answer 18

Prints material, including printing a chemical binder onto powdered material or directly printing wax

1. Layer of powder spread on platform
2. Ink-jet printer head deposits drops of binder on part crosssection
3. Binder dissolves and joins adjacent powder particles
4. Table lowered by layer thickness
5. new layer of powder deposited above previous layer 6. Repeat steps 2-4 till part is built
6. Shake powder to get part

Question 19

What are laser based techniques?

Answer 19

Including laser-based machines that either photopolymerise liquid monomer or sinter powdered material

Question 20

What is stereolithography?

Answer 20

1. Raw material: photocurable monomer by a laser beam
2. Part constructed layer-by-layer
3. Supporting platform → in container at depth.
4. UV laser solidifies part crosssection
5. Platform lowered by
6. Part cross-section computed at current height
7. Repeat Steps 4, 5
8. Removed completed part, 8. Break off supporting structures
   (9. Cure the part in oven.)

Question 21

What are some examples of SL scaffolds: bone?

Answer 21

PLA Stereolithographic scaffolds for bone tissue engineering

1) UV-lamp/laser focused in liquid photocurable material
2) Material polymerises in focus
3) Via scanning 3D object can be built

Question 22

What are nerve guidance conduits?

Answer 22

Produce NGCs in PEG, PLA and PCL via projection µSL
• Mimic the structure of the nerve / axons-Schwann cells
• Contact guidance via grooves (20 µm)

Question 23

What are selective laser sintering?

Answer 23

• Moving laser beam sinters heat-fusable powders in areas corresponding to the CAD geometry model one layer at a time to build the solid part
• After each layer is completed, a new layer of loose powders is spread across the surface
• Layer-by-layer, the powders are gradually bonded by the laser beam into a solid mass that forms the 3-D part geometry
• In areas not sintered, the powders are loose and can be poured out of completed part

Question 24

What are nozzle based systems?

Answer 24

3d printing

which process material either thermally or chemically as it passes through a nozzle

Question 25

What is fused deposition modelling?

Answer 25

• FDM uses a moving nozzle to extrude a fibre of polymeric material (x- and y-axis control) from which the physical model is built layer-by-layer. • The model is lowered (z-axis control) and the procedure repeated. • Although the fibre must also produce external structures to support overhanging or unconnected features that need to be manually removed - Easily produce different morphologies and densities - Can layer these for different tissues - Medical-grade polycaprolactone and tricalcium phosphate with Recombinant human bone morphogenetic protein 7 (rhBMP-7) - Implanted in critical bone defect in sheep

Question 26

What is bioprinting?

Answer 26

Similar to ink jet printer or FDM Print gels that are thermo responsive Cells are sprayed onto the solidifying thin layer of polymer solution • Organ printing: Tissue spheroids as building blocks Scaffold-free vascular tissue engineering using bioprinting

Question 27

What are the advantages and disadvantages of 3D printing?

Answer 27

45 µm pore size, ADV: easy process DISADV: Pore size dependent on powder, binder can be toxic, lack of mechanical strength

Question 28

What are the advantages and disadvantages of stereolithography?

Answer 28

10 µm pore size Advantages: Highest resolution technique available. Disadvantages: Monomers can be costly. Limited by the development of photopolymerisable liquid monomer material

Question 29

What are the advantages and disadvantages of Selective laser sintering?

Answer 29

Achievable pore size: 45–200 µm Advantages: Good compressive strengths, Wide range of materials, Solvent free Disadvantages: High processing temperatures, Pore size dependent on the powder particulate size

Question 30

What are the advantages and disadvantages of Fused deposition modelling?

Answer 30

Achievable pore size:100–1000 µm, Advantages: Good compressive strengths, Solvent free Disadvantages: High processing temperatures, Limited material range, Inconsistent pore opening in x-, y- and zdirections, Requires support structures for irregular shapes

Question 31

Why choose additive manufacturing over other techniques?

Answer 31

* Foams are normally much softer than either soft or hard tissue (E < 0.4 MPa) * AM based materials can be built with precise mechanical properties * More controllable materials manufacture and better tissue ingrowth

Question 32

Why use electrospinning for 3D objects?

Answer 32

provides possibility for patterning 3D object manufacture

Question 33

How are additive techniques combined with electrospinning?

Answer 33

Electrospinning on construct and replication of underlying pattern can be tuned by changing height of the collectors Electrostatically governed /field lines curve towards object that sticks out Enables patterned electrospun mats

Question 34

What are electrospun corneal rings?

Answer 34

* Stem cell niches can be replicated within electrospun mats * Microfibrous mats with added relief, can be replicated easily

Question 35

Why combine additive mandufacturing with HIPEs?

Answer 35

High Internal Phase Emulsions, water droplets surrounded by very thin film of organic material (polymer), cure the polymer and dry remainder→ polyHIPE Porosity and connectivity can be tuned, can be structured

Question 36

What are polyHIPEs?

Answer 36

Can be structured via combining them with stereolithography

Question 37

What are hydrogels?

Answer 37

Insoluble network of polymer chains that swell in aqueous solutions Gels can be qualified by crosslinker: Covalent: covalent junctions Physical: non-covalent junctions

Question 38

What is the structure and swelling behaviour of hydrogels?

Answer 38

undergo swelling in analogy to dilution of free polymer chains in solution • ΔGmix= ΔHmix – TΔSmix (generally enthalpically favorable and enthropically unfavorable) • depends on T and pH (charged residues)

Question 39

What is hydrogels dependence on temperature and pH?

Answer 39

Hydrogels can have a temperature dependent or pH dependent swelling behaviour Temperature: hydrogels that collapse at body temp pH: ionic hydrogels with acidic side groups collapse at low pH

Question 40

What is the temperature response of a hydrogel?

Answer 40

* poly(N-isopropylacrylamide), PEO/PPO block copolymers (pluronics) * Materials phase separate from solvent with increasing T (Lower Critical Solution Temperature LCST – Transition temperature) * ΔGcol = ΔHcol – TΔScol (collapse ΔH unfavorable/ ΔS favorable) • poly(N-isopropylacrylamide) PNIPAAM (Lower Critical Solution Temperature): 32ºC • Transmittance: collapsed form opaque • Can be done with a hydrogel in solution and with a surface immobilised hydrogel

Question 41

What is the pH response of a hydrogel?

Answer 41

* pH jump ~5 (pKa COOH) * COOH releases H+ * COO- repel each other * H+ recombines with OH- COOneeds counter ions * Na+ and OH- diffuse into gel * Influx of new ions creates osmotic pressure, drives swelling further

Question 42

What is the used of the ph and temperature response of a hydrogel?

Answer 42

* Injectable hydrogel delivery for tissue engineering | * T and pH dependent drug delivery / drug release

Question 43

How can a PH sensitive hydrogel be used for sensing?

Answer 43

Used for pH sensing, could be used for glucose sensing

Question 44

What is a glucose responsive hydrogel?

Answer 44

* A bit more complicated * Boronic acid binds to glucose * Quencher releases fluorescent tag in hydrogel

Question 45

What is the main advantage of using high pressure gas foaming?

Answer 45

The main advantage of this method is that it excludes the use of organic solvents. Since the technique does not include a heating process, it is also useful for incorporation of heat sensitive biomolecules.