L5 Injectable Biomaterials

Question 1

1. When is there a need to use injectable biomaterials to preplace/repair/regenerate damaged/diseased/missing tissue?

Answer 1

Two main reasons:

• People prefer not to be opened up where possible - injectable biomaterials can be administered in a minimally invasive manner. Benefits include:
  
  • Shorter recovery time
  • Reduced risk of associated complications and infection
  • Saving of billions of dollars (cost matters!)
• People want to be “healed” ASAP. Injectable biomaterials can fill up intricacies of the tissue defect, such that there is:
  
  • No need to wait for surrounding tissue to grow for implant stability
  • Relatively immediate patient mobilisation if the defect site is load-bearing (provided that the injected material is strong enough)
  • Relatively easy application from a surgeon’s perspective

Question 2

2. Where/when would someone use injectable biomaterials in the clinical setting?

Answer 2

Filling a tissue defect (“cavity”) that is mostly surrounded by viable tissue, where we want to maintain integrity of this viable tissue without introducing lesions.

Other:

• Areas that are hard to access without otherwise damaging or surgically removing otherwise perfectly healthy tissue.
• Accessible via a single surface/opening

Question 3

3. What are some of the current/potential clinical uses for injectable biomaterials in tissue engineering?

Answer 3

1. Stabilising and restoring volume in skeletal compression fractures
2. Filling osteoporotic bone for hardware augmentation in fracture treatment
3. Filling surgically induced defects
4. Restoring height of nucleus pulposus in disc degeneration disease
5. Restoring structure of myocardial defects
6. Delivery of cells into highly complex tissue (eg CNS, heart, liver, pancreas) to restore biochemical functions

Detail:

1-3 - injectable cements, 3-6 - injectable hydrogels

2. Need to make sure the bone is strong enough to withstand screw placement, or the bone could fracture further. Eg “cements” provide a base to support screw hardware. Eg areas: tibial plateau, calcaneus, vertebra
3. Putting back material into the hole that which material has been taken out (eg bone). Eg root canal operations on teeth - an injectable material could enhance the tissue regrowth in that mould. Eg areas: femoral head, trochanteric fixation.
4. When a tissue no longer acts in a favourable way mechanically, eg dense collagen patches in damaged heart tissue. An affected area can be injected with a hydrogel that provides mechanical and tissue ingrowth support.
5. Injected cells to purely serve a biomechanical function, however a suitable biomaterial must be used to deliver these cells. The dynamic environment must be survival by the cells, particularly in the injection process. There’s a speed difference between the syringe body to the needle (fluid mechanics) which stretches the cells and may damage/kill them. Certain hydrogel materials are more viscous than water, which reduces the strain on the cells.

Question 4

4. How are the properties of degradation rate and strength of a material are related?

Answer 4

Degradation property often relates to strength; strong materials degrade poorly, those with good degradation rates are often weak. Balance is important, superior degradable materials can enhance fast tissue growth and degenerate at a similar rate as the tissue regenerates to support itself structurally/biochemical. If one was to inject a weak material for eventual degeneration before tissue restoration, then the tissue could collapse. Materials need to be designed in a way that will satisfy the relatively immediate need for both the patient and the surgeon.

Question 5

5. What is required of cements and other composites as injectable biomaterials, particularly for use in bones?

Answer 5

• Starts as a viscous solution (to be able to be injected)
• Becomes strong enough over time (solidification) to support structure (in compression, shear and tensile)
• But also matches stiffness
• Maintains its integrity in 37 degrees
• Sets relatively quickly (<15 min)
• Doesn’t induce pain (ie via toxins, abnormal pH)
• Doesn’t reach temperatures >50 degrees during setting reaction
• Integrates well with bone tissue
• Stimulates new bone growth
• Initial phases don’t separate during and after injection
• Resorbs/dissolves in the body at an appropriate rate
• Interconnected porosity to allow new bone and blood vessel growth
• Radiopaque enough for surgeon to see via x-ray/CT

Question 6

6. What are some of the limitations of cements biologically?

Answer 6

Normal cements are not biologically compatible due to:

• Long setting time (>24h)
• High alkalinity (pH>12… this is good for killing bacteria however…)
• Exothermic (produces heat after mixing, 50-60 degrees)
• Trace toxic materials in the mixtures from manufacturing facilities (eg cement factories)

Question 7

7. Why do we need injectable bone cements?

Answer 7

Bone’s primary function is to withstand mechanical forces.

Diseased bone (eg osteoporosis or due to osteosarcoma removal) will have:

• Less bone mass
• Less ability to withstand mechanical forces
• Increased likelihood to break (particularly in load-bearing regions)

Question 8

8. Discuss the use of PMMA in Bone TE, including advantages and disadvantages.

Answer 8

• Polymethylmethacrylate, ie Perspex
• “Plastic glass”
• First injectable material used for bone “cement” (though a polymer)
• First used in 1950s (still used a lot now)

Various constituents may be introduced into the mix, eg radiopaque particles (visible under x-ray) or antibiotics, for further functional use in tissue/imaging etc (see attached)

Advantages:

• very strong (50 MPa tensile, 120 MPa compression)
• can add components like mentioned antibiotics/radiopaque particles)
• (that’s about it!)

Disadvantages:

• toxic, volatile monomer (bad for both surgical team and patient)
• highly exothermic (50-110 degrees)
• bioinert leading to fibrous tissue formation
• high stiffness leading to adjacent fractures
• can lead to a Bone Cement Implantation Syndrome

Question 9

9. What are some strategies to improve PMMA as a material of maintained use in TE?

Answer 9

• Improving bioactivity - add bioactive particles
• Reducing stiffness - add a gel phase or placticizers
  
  • Need to watch out on effect on polymerisation reaction…
• Reduce exothermic reaction - add ceramic nanoparticles (eg MgO)
  
  • However temperatures are still too high for the body, only reduction in temperatures by 5-10 degrees so far

Question 10

10. Describe the use of Calcium Phosphates in TE.

Answer 10

• Any compounds with calcium and phosphate ions (Ca²⁺, PO₄^3-)
• The mineral phase of bone is a type of calcium phosphate.
  
  • Carbonated hydroxyapatite (HA)
  • CA₁₀(PO₄)₆(OH)₂, where 4-6% of phosphate groups are replaced by carbonates (CO₃^2-)
• Many types of calcium phosphates, each with different physical and chemical properties.
• Some can be formed via a cement reaction. Calcium phosphate phases can be combined to form various cements (CPC’s)

Apatite-forming CPC:

• Stronger
• Have very limited resorption
• Remain at physiological pH
• Longer setting time - disadvantage; certain agents may be added to reduce the setting time.

Brushite-forming CPC:

• Weaker
• Fast initial resorption (with the surrounding environment) (metastable at pH ~7.4; turns into apatite over time)
• Acidic (pH 2~6) - a problem
• Short setting time - too short, if you don’t add other chemicals in the formulation it will set in 10s, which is undesirable in surgery.

Question 11

11. What are some of the advantages and disadvantages of CPCs?

Answer 11

Advantages

• Bioactive - can support bone growth/regeneration
• Apatite-based cements have high compressive strength (can reach 50 MPa)
• Brushite-based cements can be resorbed

Disadvantages

• Far weaker than PMMA
• Brittle - very weak in tensile/shear stress
• Can’t add components without weakening the cement
• No interconnected macro-porosity
• Limited resorption
• Powder/liquid phase separation
• Only really used for bone void fillers in low-load bearing (distal radius) or primarily compression-load bearing (tibial plateau) regions

Question 12

12. What are some of the strategies to improve CPC?

Answer 12

• Improving tensile/shear strengths - add fibres
  
  • But affects injectability and interferes with setting reactions
    ​
• Improving bone ingrowth - increase macroporosity using porogens
  
  • Also through adding bioactive particles (eg PMMA)
  • There is no real adequate mechanism to generate pores big enough for tissue ingrowth however. Examples have used soluble sugar particles (mannitol), gases (CO₂) evolved from cement reaction, foaming solution as liquid phase (like shaving cream).
    
    • But these impact the injectability and mechanical strength of the material.
      ​
• Improve bone ingrowth through other bioactive ions
  
  • However the effect of the ions must be observed on the whole system; of dopant ions on reaction kinetics and cell interactions.
  • May have impacts on injectability and mechanical strength…
    ​
• Improve cement cohesion - use a medium that will prevent particles disassociating from solution (viscosity, ion affinity)
  
  • Eg chitosan, sodium alginate, starch
  • Affects injectability, setting reactions, mechanical strength

Question 13

13. Discuss the use of Calcium Sulphate Cements including improvement strategies.

Answer 13

• Eg plaster of Paris
• Dissolves well in physiological environment
  
  • Sometimes too fast to allow for adequate tissue ingrowth, which leaves the defect remaining, unhealed.
• Not quite ‘bioactive’
• Releases a large amount of calcium ions - this provides a good environment for bone to regrow, but does not actively influence it
• Low mechanical support

Improvement strategies:

• Compositing with less resorbable CPCs
• Compositing with calcium phosphates/silicates for extra mechanical strength

Question 14

14. Discuss the use of Calcium Silicate Cements, including improvement strategies.

Answer 14

• Very similar to Portland cement (without toxic components)
• Often used in dentistry and maxillofacial surgery (antibacterial environment)
• High mechanical strength
• Bioactive, but doesn’t resorb
• Still too high pH (~12)
• Setting time too long (~4h)
  ​

Improvement strategies:

• Decreasing setting time by changing reactivity of calcium silicate phase
• Using a setting accelerator like sodium phosphate or calcium chloride

Question 15

15. What makes soft tissue “soft”?

Answer 15

• Extracellular matrix (ECM) - not calcified like bone
• ECM possesses higher degree of hydration (varying degrees for different tissues)
• Possesses viscoelastic properties

Question 16

16. Describe briefly the basic structure of hydrogels

Answer 16

• Network of physically/chemically cross-linked polymer chains
• Often designed to synthetically mimic the extracellular matrix
• Hydrogels are intrinsically microporous

Question 17

17. List some of the properties of hydrogels that lead to unlimited potentials for tissue engineering

Answer 17

1000s of synthetic polymers, with variations in the number of:

• Molecular weights
• Polymer concentrations
• Potentially polymer composite (co-block or interconnected network) hydrogels
• Crosslinking/gelation mechanisms
• Bioactive particle corporations

Question 18

18. Injectable self-setting hydrogels for soft tissue repair/replacement are not always self-setting in physiologically friendly conditions, nor do they always ideally match the mechanical properties of treated tissue. Describe three classes of these hydrogels used in TE, with reference to this statement.

Answer 18

Thermosensitive hydrogels:

• Lower critical solution temperature of 32 degrees.
• Hydrophilic below this temperature, hydrophobic above it.
• The hydrophobic reactions at body temperature lead to the separation of water and polymer.
• They are injectable at room temperature, and biocompatible, but very weak.

PVA theta gels:

• Aqueous solution of network-forming polymers
• Mix with a “gelling agent” at elevated temperatures
• Reduction of temperature = reduction of solvent quality = PVA is forced together and crystallizes
• Trialled in repositioning of papillary muscle of heart to prevent mitral regurgitation in left ventricle.
• Advantages: high creep resistance, water content, compressive and tensile strength (~100kPa) and strain (tensile ~400%)
• Disadvantages: initial temperature of solution needs to be elevated beyond physiological temperatures (liquid at 80-90 degrees, solidifies around 40; not as damaging as PMMA but still risk of thermonecrosis/damage - use depends on the level of cell viability)
• Poor bioactivity on its own - eg no interaction with cartilage alone, but better interactions with cells and fibrin used together, however it is difficult to incorporate cells into this injectable biomaterial (will die due to reactions; heat)

Injectable Silk Hydrogels:

• Silk fibroin solutions can be “triggered” to initiate gelation (at first just liquid solutions)
• Once you initiate the trigger, you need to inject it before it becomes no longer injectable - a window of application
• Used when mechanical support is not immediately required
• Advantages: low immunogenicity, biodegradable
• Improvements: BMP-2, a protein that encourages bone growth - enhances the endogenous ability of the surrounding cells and tissue to regenerate

Question 19

19. When is targeted cell delivery via injection required and what is the main fall-back?

Answer 19

Required when cells need to be transplanted to tissue defect to restore biochemical functions (additional load bearing is not required)

• Heart, pancreas, central nervous system

We want as many cells to be alive for therapeutic effect, however in reality, very poor cell viability (1-32%). This is usually due to:

• Exposure to hypoxic environment and/or initial immune response
• Fluid dynamics (extreme changes in velocity at the chamber-needle juncture; large diameter to far smaller)

Injectable hydrogels can reduce cell loss during injection

• Increased viscosity - changed fluid dynamics
• “plug flow” phenomenon - instead of the liquid flowing in a continuous stream, parts of the gel travel together (like a plug)
• Cells on the needle/syringe wall will still die, but in the middle they will survive, as they’re not subject to the sheer force stress and changing velocity
• Increases alive:dead ratio of cells

Question 20

20. Can we add cells to cements?

Answer 20

Short answer: no

• Cement reaction is aggressive; a lot of ions and electrons move around, which is damaging to cells.
• Microspheres have been considered in which the cells are protected, but then there are problems in the release of these cells from a rigid and solid structure

Question 21

21. Describe the current state of injectable biomaterials today and the gap that needs to be bridged to better success in the future.

Answer 21

See attached - merging of the Venn diagrams to reach the centre

Question 22

22. What are some of the differences between injectable cements and injectable hydrogels?

Answer 22

(Consider the main points of previous cards and consolidate into a short answer)