Lectures 8-11 Flashcards
What are five biological properties of naturally derived ECM?
chemotactic properties, angiogencity, antimicrobial, anti-inflammatory/immune mediators, low immunogenicity and immunoregulation
chemotactic properties
natural ECM has peptides/growth factors which help recruit cells
angiogenecity property
Some ECM can induce formation of blood vessels
antimicrobial property
many ECM are resistant to bacterial infections due to antimicrobial proteins
anti-inflammatory property
some ECM contains cytokines which are immunomodulatory; prevent large immune responses
low immunogenicity property
some ECM exhibit low or reduce expression of surface antigens
What are 3 forms of natural ECM biomaterials?
whole decellularized organs, micronized ECM tissues, separated/extracted ECM
Micronized ECM tissues
ECM that is micronized into tiny pieces using cryogenic mill or blender; samples can be injected instead of surgically implanted
Separated/Extracted ECM
liquid/gel/solids, derive ECM via various separations techniques
What are some naturally derived biomaterials not derived from mammals?
- Silk Fibroin: produced by spiders and has poor resorption properties
- Chitosan: derived from arthropod exoskeletons and crustacean shells and is biologically renewable, biodegradable and biocompatible
- Cellulose: makes up plant cell walls; wood is most important source
What is matrigel?
ECM biomaterial secreted by EHS mouse sarcoma cells; It is angiogenic (forms blood vessels) but is derived from a rat tumor so has little clinical applicability
What are some limitations to using naturally derived materials?
cost, can result in variable products, can still induce inflammatory response
What are the 3 important goals for using naturally derived biomaterials for tissue engineering applications?
Remove all the immunogenic components, sterilize the ECM, and retain the native architecture
What is decellularization?
remove the cells from the tissue
What are the 3 main categories of decellularization agents?
chemical agents, biological agents, physical and miscellaneous agents
When selecting a decellularization agent, what are the 4 factors to consider?
Tissue cell type, tissue density, tissue lipid content, and tissue thinkness
What are some types of chemical decellularization agents?
acids and bases, hypotonic/hypertonic solutions, detergents (surfactants), alcohol, solvents
What do detergents/surfactants do?
solubilize cell membranes and dissociate DNA from proteins
What are the 2 most common detergents/surfactants for decellularization?
SDS (sodium dodecyl sulfate) and Triton X-100
What is the major CON of detergents/surfactants?
the cytotoxic agents can penetrate deep into tissues and so residual detergent must be completely flushed out before cell death
What are the 2 main classes of biological decellularization agents?
enzymes (like trypsin) and non-enzymatic agents like chelating agents that aid in cell dissociation from ECM
What are some physical decellularization techniques?
temperature, force and pressure, non thermal irreversible electroporation
What are 4 sterilization techniques?
simple treatment with acid/solvent, ethylene oxide (gas) exposure, gamma irradiation, electron beam irradiation
Simple Treatment with acid/solvent
sterilization technique; acid may not penetrate full ECM and acid/solvent residues must be removed after
Ethylene Oxide Exposure
sterilization technique; changes mechanical properties and can cause immune response
Gamma irradiation
sterilization technique; can cause residual lipids to be cytotoxic and gamma irradiated materials can be hazardous to humans in large quantities
Electron beam irradiation
sterilization technique; friendlier to plastics; higher per item cost in long term
What are the advantages of using synthetic biomaterials?
reproducible processing, great engineering flexibility, can be cheap to produce on mass scale
What is the disadvantage of using synthetic biomaterials?
don’t mimic native tissue as well as naturally derived biomaterials
What are the desirable features of synthetic bioscaffolds? (5)
- maintain 3D space for tissue formation
- If biodegradable/bioresorbable, breakdown products need to be non-toxic and degradation matches time scale for new growth
- needs to have biological activity incorporated
- long term needs to facilitate nutrient diffusion
- mimic native ECM of proposed site
What is a monomer?
individual subunits that can be linked in various ways to give linear, branched, and crosslinked polymers
What is a polymer?
a large molecule composed of many repeated subunits (monomers)
What is chain growth polymerization?
requires the presence of a free radical initiator and monomers react only with active center (radical) of growing chain
What is step growth polymerization?
requires bi- or poly- functional monomers where all functional groups can react with one another
What are the 3 steps to Free radical (chain growth) polymerization?
initiation, propagation, termination
What are biodegradable polymers?
biodegradable polymers are comprised of monomers linked to one another through functional groups that have unstable backbone links; The unstable links allow them to degrade over time
What are 3 biodegradation mechanisms?
- enzymatic degradation
- hydrolysis
- combination of 1&2
What is Mn, Mw, and Mv?
Mn - number average MW (total weight of chains / # of chains)
Mw - Weight average MW (always larger than Mn)
Mv - viscosity average (determined by viscosity measurements; closer to Mw)
What are 4 things that affect the rate of hydrolysis?
- chemical reactivity of material
- amount of crystallinity
- amount of water available
- material surface area to volume ratio
What is bioerosion and what types occur in polymers?
Bioerosion is the physical process that results in weight loss of a polymer scaffold (degradation)
Bulk erosion: materials fracture then dissolve
Surface erosion: polymer only dissolves at surface
What is PGA and PLA?
PGA = Poly (glycolic acid)
PLA = Poly (lactic acid)
What is PGLA?
When PGA and PLA are combined into a co-polymer and depending on the ratio, degradation rate can be tuned
What is a ceramic?
inorganic, non-metallic solid material compromising atoms primarily held in ionic and covalent bonds
What determines if it is glass or ceramic?
depends on what side of the scale they fall on; amorphous = glass; crystalline = ceramic
What is bioactive glass (BG)?
Si-based glasses that make strong bonds to bone
What properties need to be considered when creating a scaffold? (5)
- material composition
- surface properties
- degradation properties and products
- mechanical properties
- architecture of pores
What is a pore and porosity?
pore is a void space within a scaffold and porosity is a measure of the void space in a material
Macro-pores
> 50 microns; influence tissue function
Micro-pores
<50 microns; influence cell function
What are the characteristics of pores? (3)
size, shape, and interconnectivity
Porogen Leaching
polymer solution is infused with a template material and after polymerization, the template/sacrificial material is leached out by soaking in solution
RESULT: scaffold with random pore connectivity
Phase Separated Scaffolds
mix polymer with solvent, remove or dilute the solvent, materials solidify into different microstructures
Gas Forming
polymers are pressurized and saturated with a gas and pressure is released causing air bubbles in the material
Electrospinning
use a polymer solution, apply high voltage to the solution that is pumped into a syringe, the droplet elongates and stretches into a fiber that is ejected after critical voltage is reached
Extrusion
continuously extrude polymer melt to form a scaffold and by controlling extrusion rate, fiber diameter can be tuned along with crystallinity
3D printing: Sterolithography
U laser is scanned over a bath of resin which photopolymerizes where the beam strikes and each layer is moved to make room for the next
3D printing: plaster based
use ink-jet printer to print an adhesive onto layers or powder, use glue to form shape, sinter material in an oven
3D printing: Fused deposition modeling
plastic filament is fed through extruder which heats up the plastic and patters it onto a platform
Selective Laser Sintering
begin with powder, laser is applied to heat and sinter powder into a solid, layer by layer addition
Laser Etching
begin with solid scaffold and use laser to vaporize scaffold pores but can only make straight channels with no interconnectivity
Which are the conventional Pore fabrication techniques?
porogen leaching, phase separated scaffolds, gas forming
