Rheology

Question 1

Explain how Fick’s law can be used to calculate drug diffusion through the skin:

Answer 1

• J - solute flux
  
  • [amount/unit area/unit time – μg/cm2/sec]
• D - solute diffusion coefficient in the stratum corneum
  
  • [area/unit time – cm2/sec]
• P - solute partition coefficient between skin and vehicle
• δ - thickness of the stratum corneum
  
  • (10 μm=0.001cm)
• ∆CV - difference in solute concentration between vehicle and tissue
• A = area of skin treated
  
  • [cm2]
• KP permeability coefficient
  
  • [distance/unit time – cm/sec]

*assume

• ∆CV ≡ CV because CV >> CT (sink conditions)
• Infinite dosing (drug in vehicle is not depleted over time)

Question 2

What are the three most important parameters in determining the potency of topical corticosteroids.

Answer 2

Chemical structure of the compound, concentration and vehicle

Question 3

Define rheology and the significance of studying the flow behavior of liquids and semisolid dosage forms?

Answer 3

Viscosity = internal friction of fluid which makes it resist flowing past a solid surface or other layers of fluid

• viscosity and flow properties affect products stability, manufacturing and can affect use

*imp in suspensions, emulsions, creams, ointments, etc.

Question 4

define states of matter from viscosity point of view: solid, elastic, viscoelastic, viscous, liquid

Answer 4

Solid: application of force won’t change the shape

Elastic: application of force→will return to original shape

Viscoelastic: viscous properties from liquids +elastic properties from solids (semisolids)

Viscous thicker liquid

Liquid: looses shape immediately upon standing

Question 5

Describe Newtonian flow behavior.

Answer 5

flow in which viscosity is a constant value — it doesn’t change with the force applied to it

Question 6

describe non-Newtonian flow behavior.

Answer 6

Flow in which viscosity is not a constant value → viscosity changes with the force applied to it.

• Variation of shear rate does not cause proportionally change on the shear stress
• Viscosity changes with shear rate (i.e., how fast one is stirring)

Question 7

what are the types of non newtonian systems

Answer 7

Plastic flow (Bingham flow)

Pseudoplastic flow

Dilatant flow

Thixotropic flow

Question 9

Describe the viscosity of Newtonian fluids.

Define shear rate, shear stress and viscosity

Answer 9

Question 10

what is expression of viscosity in Poise.

Answer 10

A poise the tangential force required per unit area to maintain a velocity difference of 1 cm/s between two parallel plates separated by distance of 1 cm

Question 11

What is a rheogram and why it is the only way to understand non-Newtoniam flow behavior?

Answer 11

graph of flow properties

for non-Newtonian fluids it is not sufficient to measure viscosity at one shear rate since viscosity changes with different shear rates, therefore a graph of shear rate vs shear stress is required

Question 12

rheograms corresponding to non-Newtonian (plastic, pseudoplastic and dilatant) materials.

Answer 12

Question 14

Give examples for Newtonian fluids

Answer 14

water, alcohol, glycerol, propylene glycol, mineral oil, true solutions

Question 15

Give examples for non-Newtonian (plastic, pseudoplastic and dilatant) materials.

Answer 15

Plastic: flocculated suspensions, ointments, creams, tomato ketchup

Pseudoplastic: polymers in solution, dispersions of natural and synthetic gums, tragacanth, alginates, gelatin, methylcellulose

Dilatant: defloculated suspensions >50% conc.; cornstarch in water

Question 16

Define thixotropy and corresponding flow curves to plastic thixotropy, pseudoplastic thixotropy and dilatant thixotropy.

Answer 16

Thixotropy = a reversible, time-dependent decrease in apparent viscosity ⇒ gel—sol—gel transformation

occurs upon applied shear and rebuilding of the internal structure over time after removal of shear

ex: bentonite magma

Question 17

describe viscosity in a thixotropic system

Answer 17

• viscosity is dependent on the length of time shear stress is applied
• structure of the liquid doesn’t immediately reform after altering its original flow properties

the time required for the original structure to reform depends on the product

Subsequent replicate tests would not produce the same viscosity

Question 18

How do you determine viscosity and flow behavior of Newtoniam fluids, and non- Newtonian (plastic, pseudoplastic and dilatant) materials.

Answer 18

Newtonian: capillary methods

Non newtonian: need a full rheogram showing plastic fluids, pseudoplastic fluids and dilant fluids

*need to determine at more than one sheer rate