8. Rheology of Topical Products Flashcards
Ideal properties of topical formulations - Physical properties
- Smooth texture
- Elegant in appearance
- Non dehydrating
- Non gritty & non staining’
- Non hydroscopic
- Easy to apply (easy dispersion)
Ideal properties of topical formulations - Physiological properties
- Non-irritating
- Does not alter membrane / skin function
- Miscible with skin secretion
- Have low sensitisation effect
- Prolonged retention time & release
What is Rheology?
“Science of deformation & flow”
Flow: Measures viscosity (resistance to flow)
Deformation: Quantifies viscoelasticity & predicts sample properties
Rheology graphs:
- Shear stress vs Shear rate
- Viscosity vs Shear rate
- Dilatant -> Newtonian (y=x) -> Shear thinning
- Dilatant: +ve deflection
Newtonian: No deflection
Shear-thinning: -ve deflection
Most products are Pseudoplastic - shear thinning
Power law describes flow behaviour:
τ= (kγ)^n
τ= shear stress k = consistency index γ = shear rate n = flow index
Important of Rheology for semisolids
- Spreading & adherence to skin
- Removal from jars or extrusion from tubes
- Capacity of solids to mix with miscible liquids
- Release of drug from the base
- To ensure long shelf-life
Viscometer vs Rheometer
Viscometer:
- Measures viscosity over a limited shear rate range
- Low cost solution for quick comparative testing of products
Rheometer:
- Measures viscosity over a wide range of shear rates & shear strains
- Measures viscoelasticity
Viscometer cannot differentiate between semisolids of similar viscosity - requires a rheometer
Major properties of topical products that the viscometer cannot measure
- Yield stress: Measure of the stress that must be applied to elicit significant flow
- High shear viscosity: Gain a reliable understanding of the spread ability & ease of application
- Zero-shear viscosity: Viscosity of a formulation when effectively at rest
- Thixotropy: Rate at which a material undergoes structural breakdown
Viscoelasticity
- Topical products are semisolids (not completely solid nor completely liquid like)
- E.g. Suspensions when stored behave like a solid (prevents sedimentation) but needs to behave like a liquid when poured
Oscillation testing
- Oscillatory (sinusoidal) deformation (stress/strain) is applied to a sample
- Phase angle/shift between the deformation & response is measured
- Elastic: In phase
- Viscous: Out of phase (by 90 degrees)
Viscoelastic parameters (4)
The complex Modulus:
- Measure of materials overall resistance to deformation
- G* = Stress*/Strain
The elastic (storage) Modulus:
- Measure of elasticity of material
- The ability of the material to store energy
- G’ = (Stress*/Strain) Cos θ
The viscous (loss) Modulus:
- The ability of the material to dissipate energy
- Energy lost as heat
- G” = (Stress*/Strain) Sin θ
Tan θ:
- Measure of material damping
- Tan δ = G”/G
If…
- G’ > G”, phase angle less than 45 degrees - SOLID LIKE
- If G” > G’, phase angle greater than 45 degrees - LIQUID LIKE
- G - modulus - still a measure of toughness
Mechanical properties (measured by Texture Analyser) (6)
- Firmness
- Adhesiveness
- Cohesiveness
- Consistency
- Spreadability
- Stiffness / elasticity
Component of texture analysis - Young’s modulus (E)
E = Stress / Strain
Stress
- F/A in N
- Pa (1Pa = 1Nm^2)
- Shear stress acts in the direction PARALLEL to the materials surface they deform
- Normal stress act in a direction PERPENDICULAR to the surface of material they deform
Strain:
- Measure of relative displacement between the particle of a material
- Strain is the change in length per unit length (in mm) - dimensionless
Force displacement curve: F1: A1: A2/A1: A3:
F1: FIRMNESS - max peak force during the first compression
A1: CONSISTENCY- related to firmness, thickness or viscosity
A2/A1: COHESIVENESS- rate at which the material disintegrates under mechanical action
A3: ADHESIVENESS - work required to overcome the attractive forces between the surface of the sample & the surface of other materials with which the sample comes into contact
Refer to lecture notes
Consistency of creams: Assessed by
- Forward extrusion test - stimulating force required to extrude the sample by patient
- Backward extrusion test - indication of product physical failure & viscosity
