lecture 18: tissue engineering - novel biomaterials Flashcards
1
Q
What is tissue engineering?
A
- the loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in human health care
- a new field, tissue engineering, applies the principles of biology and engineering to the development of functional substitutes for damaged tissue
- Langer and Vacanti credited with founding the field
- aims to generate new functional tissue to repair or replace tissues missing due to disease, genetic defects or trauma
- promise of:
- alleciating tissue shortages
- superior results
- customised implants
- new treatments where none currently suffice
- great promise but limited outcomes to date… why?
2
Q
What are different types of hurdles of TE success?
A
- technical
- commercial → can you get the funding you need to carry out the research required / develop it etc
- regulatory
3
Q
What are components of tissues?
A
- cells
- matrix
- blood supply
4
Q
What is the traditional TE approach?
A
- take some kind of support material
- e.g. degradable polymer
- make it into the shape needed
- add cells
- growth factors to encourage cells to do what we want
- culture in the lab for some time
- hopefully cells will attach to the support
- grow, flourish, form a piece of tissue
- after some time put construct into the patient
- if the original scaffold was designed appropriately it should fit exactly
- tried in a number of simple tissue engineering attempts and often it doesn’t work
- it’s simple, sounds conceptually reasonable, but has many challenges
- some of these challenges arise from the selection of the biomaterial
- your body does not typically respond well to a large foreign object being placed within it
5
Q
How do strategies depend on tissue type?
A
- design criteria based on specific tissue properties
- in vitro construct development feasible for avascular/small/2D tissues
- 3D human scale tissues need blood supply early
- in vivo bioreactors allow construct development concurrently with vascularisation
6
Q
What are challenges for tissue engineering?
A
- blood supply for 3D vascularised tissues
- suitable biomimetic matrix materials
- delivery of biological signals
- infection control
- → tailored design of systems for tissue engineering
7
Q
What are the roles and significance of biomaterials in tissue engineering?
A
- scaffolds, surfaces and microenvironments for cell growth
- ideally mimic native tissue ECM
- provide space and biochemical environment to allow new tissue to grow
- mechanics important as well as chemical and biochemical interactions
8
Q
What are design criteria and processes used to select, fabricate and tailor biomaterials for tissue engineering?
A
- many criteria apply – good design may require compromise/optimisation
- designs depend on tissue target
- “biocompatibility” oft claimed but ill defined
- novel materials developing e.g. “smart” polymers, bioactive glasses, macroporous 3D hydrogels, composites, hierarchiaclly porous scaffolds, 3D printed materials
9
Q
What are criteria for biomaterials in TE?
A
- biocompatibility → need to know our materials won’t cause an adverse reaction
- mechanical properties for target tissue and implantation site → e.g. in femur will need to be strong
- biodegradability profile (time, strength and by-products) → ideally biomaterial will degrade as tissue develops, want these two rate processes matching, strength for long enough but allowing space for tissue to grow
- suitable in vivo responses e.g. inflammation, FBR
- ability to be fabricated into desired structures
- cost-effective, available, regulatory approval
- ability to be sterilised safely
- adequate stability and shelf-life
- promote desired cellular responses e.g. proliferation differentiation, gene expression
10
Q
What are limitations and challenges of using biomaterials in tissue engineering?
A
- potential problems include foreign body reaction, acid release, toxicity, supply, cost and reproducibility
- lack of knowledge of design criteria
- lack of predictability of in vivo behaviour and reponses
11
Q
What are tailored porous biomaterials?
A
- polymers
- hydrogels
- ceramics
- composites
need to think about:
- chemical and physical properties
- architecture
- stiffness
- degradation
12
Q
What is an example of scaffold fabrication?
A
- solid free-form fabrication
- steriolithography
- 3D printing
- many biomaterials
- sometimes cells
13
Q
What is the foreign body reaction (FBR) to implanted synthetic materials?
A
- FBR is the normal reaction of a higher organism to an implanted synthetic material
- limits biomaterial implant performance and tissue regeneration
- schematic of FBR stages…
- surgeon implants biomaterial
- the biomaterial absorbs a layer of proteins
- cells (neutrophils and macrophages) interrogate the biomaterial
- cells fuse to form giant cells and secrete protein signalling agents (cytokines)
- in response to the cytokines, fibroblasts arrive and begin synthesizing collagen
- the biomaterial is encapsulated in an acellular, collagenous bag
- this is what will happen if you use straight PLGA
14
Q
What are surface engineering strategies?
A
- layer-by-layer (LbL) assemblies (Tristan Croll, Dewi Go)
- hide the surface so we don’t get foreign body reaction
- layer different chemicals on it
- hydrolysis and layering of different amine groups on the surface to make it functional
- bring in polyelectrolites (hyaluronic acid, chitosan)
- have opposite charges so can layer them up one after another
- cross link these layers with carbodiimide (EDC)
- this will hopefully prevent the body from recognising the foreign object in the same way
- PLGA scaffolds with LbL coating to provide a “blank slate” → little FBR
15
Q
What are examples of delivery of bioactive molecules?
A
- growth factor delivery
- gelatin microspheres
- size/shape easy to control
- cross-linking allows control of chemical and mechanical properties
- electrostatically bind many GFs
- achieved GF release over ~ 3 weeks
- mouse chamber adipogenesis groups
- I. collagen + free FGF-2
- II. collagen + buffer-loaded microspheres
- III. collagen + FGF-2 loaded microspheres → controlled release of growth factor lead to greater tissue growth
16
Q
What are potential delivery vehicles?
A
- scaffolds
- microspheres, nanospheres, nanofibres, nanoporous materials
- other 2D or 3D biomaterials
17
Q
How are porous polymer microspheres produced?
A
- combining inkjetting with thermally induced phase separation:
- about 50 micron in diameter
- pores inside them where molecules can be loaded
- resevoir of polymer solution and jet that out into liquid nitrogen → freezes them very quickly
- inkjet technique allows for the production of uniformly sized droplets
- take out solvent
- left with nice porous beads
18
Q
What is the multilayered polyelectrolyte biomolecule delivery strategy?
A
- FGF2
- aminolysed microspheres → HA → Chi → HA → Chi → Heparin → bFGF → Heparin → Chi → HA → Chi → HA
- crosslink during the buildup to ensure stability (EDAC/NHS)
- so covered the surface and added a growth factor
- heparin is a natural binding site for the growth factor, helps to preserve bioreactivity
- controlled release
19
Q
What are the in vitro release kinetics for FGF2?
A
- release conditions: 0.01M PBS, pH 7.4, 37 ± 0.5C, rotation at 90 rpm sink conditions
- rather than losing growth factor in less than one day we can get a long slow release out to more than thirty days
- may be more suitable for delivering growth factor in a tissue engineering construct
- long release times
- steady rate of release
- tunable via microsphere and LbL properties
20
Q
What is controlled release to modulate inflammation?
A
- alpha-MSH anti inflammatory peptide
- alpha-MSH can be uniformly absorved on PLGA microspheres
- 94% of alpha-MSH was released in 3 days
- coated the sample through hydrophobic interactions → simple strategy of binding was not sufficient to control the release of this molecule
- insight into peptide-surface interactions:
- using molecular dynamic stimulation
- modelling alpha-MSH peptide interaction with various surfaces
- e.g. hyrophobic, negatively charged, positively charged
- alpha-MSH binds strongly to hydrophobic surface
- positive residue Arginine pointing upwards at pH 9
- tells us something about how to bind that biomaterials surface
- in vivo response to alpha-MSH coated PLGA microspheres once optimised
- looked at by histology after 3 days and after 14
- natural reaction to collagen
- PLGA alone has big immune response
- PLGA + alpha-MSH → smaller immune response
21
Q
What is dual biomolecule delivery?
A
- to incorporate both growth factor and anti-inflammatory hormone
- basic Fibroblast Growth Factor - 17.2 kDa
- induces angiogenesis, cell division, chemo-attraction
- promotes the regeneration of adipose tissue, cartilage and nerves
- specific binding between heparin and bFGF help stabilise the GF
- could control the release of the two molecules relatively independently
- biomolecule size affects release rate
- number of residues: alpha-MSH = 13, bFGF = 154
- MW (kDa): 1.6 vs 17.2
- Siza (A): 40 x 12 x 4 vs 30 x 30 x 45
- increasing number of layers: much less vs less
- increasing crosslinker concentration: no change vs much less
22
Q
What are antimicrobial composites for medical devices?
A
- simple sol-gel process → allows us to coat nanoparticles of inorganic material of things like silver onto different kinds of constructs, looking at it for orthopoedic
- composites can be coated on or used to make devices
- need to prevent bacteria forming biofilms
- particularly important in constructs that are designed to last for longer
- long slow release
- increasing the amount of silver nanoparticles we get a decreasing number of bacteria
23
Q
How can we scale up towards clinical application?
A
- 3D rapid prototyping to fabricate customisable constructs
- in vivo bioreactors for soft tissue engineering
- were able to engineer a new piece of tissue
- kept connection to blood supply
- developing this type of technology for reconstructive surgery
