BENG 186A Lecture 8: Structures of Biomaterials

What are modern biomaterial fibers and textiles made of?

Polymer based fiber

What are properties of medical fibers?

• Intermediate to high molecular weight

• Ability to form ordered crystalline regions quickly

• Linear polymers with minimal number of side groups or crosslinks

• Low to intermediate interchain bonding so it can be stretched

What are examples of medical fibers?

Dacron and Gore-Tex

What is melt spinning?

• Used to extrude thermoplastic polymers, polymers which can be heated reversibly

• Polymer is heated above melting temperature and extruded through spinneret

• Cools as it is extruded, draw, entangled, and wound

What is wet spinning?

• Used for non thermoplastic polymers or when do not want to heat polymer

• Spinneret is extruded through solvent bath to precipitate polymer

What is electrospinning?

• Good for finer fibers

• Can mimic natural scales of biology

• Versatile, can use a variety of different polymers and composites and achieve different pore size and stiffness

• Involves ejection of charged polymer solution onto oppositely charged surface

Impacts on electrospinning?

• Increasing concentration increases fiber diameter

• Increasing diameter decreases conductivity

• Thicker leads to higher flow rate

• Random alignment is a stationary flat collector and aligned is a rapidly rotating drum

What are the uses of fiber alignment?

• On anisotropic tissue: nerve regeneration, tendon engineering, muscle and cardiac tissue

What are porous and textured materials good for?

• Tissue ingrowth

• Promote new vessel formation

• Disrupt excessive fibriotic capsule formation

What is sintering?

• Heat

• Pore size depends on size of initial particle, Tm of materials, and time

What is salt leaching?

• Salt crystals put into mold, polymer hardens, salt dissolved using solvent and a pore is left

• Creates interconnected porous structure

What is gas foaming?

• No need for solvents, good for making materials with bioactive agents

• High temperature compression molding into solid disc, incubate in a high pressure gas, release of pressure leads to nucleation of bubbles

• Nature of gas bubbles means pores are poorly connected

• Connectivity of pores matters as it determines if cells, nutrients, and waste can make it through scaffold

How to 3D print metals?

• Powder layer of metal deposited

• High power laser scans to melt or sinter metal particles at cross section

• New layer deposited

• Restricted to certain printable metals

How to 3D print ceramics?

• Typically printed or sintered

• Common materials are alumina and zirconia oxides

What is a bottom up and top down approach?

• A bottom-up approach involves nano-scale and self-assembly strategy when it

is introduced to the body

• A top-down approach starts with a bulk material that is patterned

Does the synthetic implant have a larger or smaller Young’s modulus than the native

artery? What is the practical implication?

• Larger Young’s modulus than the native artery

• This means that it is more stiff, less compliant than the native artery

• This could lead to compliance mismatch and altered hemodynamics, disturbed

shear stress, poor endothelialization, increased thrombosis risk, and intimal

hyperplasia at the graft junction

How could fibers be fabricated as random vs. aligned fibers in electrospinning?

• Fiber orientation can be dictated by changing the collector design

• For random fibers a stationary flat plate collector can be used

• For aligned fibers a collector that forces fibers to orient in one direction can be used such as a rotating mandrel/drum collector, a parallel electrode collector, or

magnetic/electric field-assisted collector

How were 2 layers of vascular graft created? Why might you want 2 layers for this graft?

• Could be fabricated using sequential electrospinning – one polymer deposited on

rotating mandrel followed by second polymer for layered tubular structure

• 2 layers to mimic different functions of inner (luminal) and outer layers of natural

  blood vessel – 2 layers allow independent control of mechanical properties,

  degradation rate, biological response

  -Outer layer provide mechanical strength and prevent rupture

  -Inner layer can improve blood compatibility and promote endothelialization

Which has higher printing resolution?

Light-based printing – light can be precisely patterned and polymerization can occur in

defined regions

Nozzle-based printing resolution is limited by nozzle diameter, material viscosity,

and spreading of the deposited filament