Lecture 7 - Fibrous Scaffold Fabrication

Question 1

Nanofiber

Answer 1

‘Continuous’ fiber which has diameter in range of billionths of meter

Question 2

Applications of Nanofibers

Answer 2

Superlative characteristics of high surface area-volume ratio, smaller size, and superior mechanical strength (stiffness and tensile)

Question 3

Electrospinning

Answer 3

• Apply high electrostatic field to capillary, droplet of polymer solution at tip is deformed into conical shape
• When voltage exceeds threshold, electrostatic forces overcome surface tension and small diameter charged jet ejected
• Solvent in ejected jet begins to evaporate to form polymer fibers and travel toward negatively charged collector

Question 4

Electrospinning Characterisitics

Answer 4

• Can fabricate nonwoven (random network of fibers) and ultrafine fibers with diameters ranging from several microns to 100 nm or less
• Porosity can be >90%
• Fiber diameter can be controlled by polymer concentration, polymer solution flow rate, voltage, distance between tip and collecting plate, solvent type

Question 5

Requirements for Electrospinning: Polymer

Answer 5

• Presence of sufficient intermolecular interactions (e.g., entanglement)
• Usually high MW (could break into particles if too low)

Question 6

Requirements for Electrospinning: Solvent

Answer 6

Controls polymer solution, surface tension, electrical conductivity, and viscosity

• Should be able to dissolve polymer to form solution with appropriate concentration and viscosity
• Should be sufficiently volatile so it can evaporate to large extent before nanofibers collect on deposition (too volatile not good)
• Solvent (or solution) must have ability to carry electrical charge

Question 7

Effect of Polymer Concentration

Answer 7

• Critical concentration: C*
• lower than C*, electrospray (beads)
• higher than C*, electrospinning
• Increase of polymer concentration increases fiber diamter

Question 8

Effect of Conductivity/Solution Charge Density

Answer 8

• Polar solvent needed
• If solution conductivity too low, yields beading (not sufficient charge on surface)
• If solution conductivity high, yields fibers
• Increased solution conductivity/charge density produces more uniform fibers
• Adding cationic surfactants can increase conductivity
• Additions of salt can increase conductivity

Question 9

Effect of Flow Rate

Answer 9

• Low flow rate, yields smaller fiber diameter
• High flow rate, yields bigger fiber diameter
• Too high flow rate, produces beads since fibers cannot completely dry

Question 10

Effect of Voltage

Answer 10

• Need critical applied voltage
• Lower voltages produce bead-free fibers
• Very high voltages, volume of tip decreases resulting in more beading

Question 11

Effect of Electrical Field

Answer 11

• Collecting fibers across the void gap formed between pair of conducting substrates
• ## “Bounces” back and forth

Question 12

Effect of Distance Between Tip and Collector

Answer 12

• Minimal distance required to allow fibers to have sufficient time to dry
• If too close, yields beads (no chance to eject into streams)

Question 13

Effect of Collector

Answer 13

• Plate collector produces random alignment

- Cylinder collector (high-speed rotations) produces aligned structure

Question 14

Disc Collector

Answer 14

Highly aligned fibers

Question 15

Drum Collector

Answer 15

Less aligned fibers

Question 16

Mandrel Collector

Answer 16

Tubular scaffold with less alignment

Question 17

Porous Nanofibers

Answer 17

• Electrospin solution into liquid nitrogen, followed by sublimation under vacuum