7. semi-solids

rheology

study of flow and deformation of materials

viscosity

measure of resistance to flow

shear stress

force/area

shear rate (Y)

rate of deformation

Newtonian fluids

viscosity is constant regardless of shear rate

shear stress and rate relationships

linear

straight line through origin

non-Newtonian fluids

viscosity changes with shear rate

types of non-Newtonian fluids: plastic

no flow until yield stress reached then flows linearly

e.g. toothpaste

doesn’t pass through origin

pseudoplastic (shear thinning)

viscosity decreases as shear rate increases

e.g. ketchup

dilatant (shear thickening)

viscosity increases as shear rate increases

thixotropy

time-dependent decrease in viscosity under shear

structure breaks down on shaking

rebuilds over time

rotational viscometers

measures resistance to rotation in fluid

advantages: wide range of shear rates, can produce flow curves

types of rotational viscometers

concentric cylinder: fluid between 2 cylinders

cone and plate: small sample, precise measurement

flow curve

shear stress vs shear rate

straight line: Newtonian

curve: non-Newtonian

advantages of semi-solid

avoid first-pass metabolism

provide local or systemic effect

non-invasive

dermatological semi solid dosage form

creams, ointments, gels

rectal/vaginal semi solid dosage form

suppositories, pessaries

what are creams

viscous semi-solid emulsions for external use

water in oil cream

greasy, moisturising

uses: dry skin

oil in water cream

non-greasy, easily washed off

typical ingredients in creams

water, oil, emulsifying agents, active drug

pharmaceutical use of cream

deliver drug: NSAIDs, antibiotics

cosmetic uses

moisturising, cleansing

key stability factors of creams

emulsifier system: must stabilise droplets

droplet interactions: prevent coalescence

unique structure in O/W creams

1. oil droplets

2. crystalline gel phase

3. hydrate phase

4. aqueous phase

liquid crystalline phases

increase viscosity

improve stability

trituration

mixing powders/liquids into base

Levigation

wet grinding of coarse powders

water-soluble ointment

PEG-based

non-occlusive

easily washed off

water-insoluble ointment

emulsifying:

can absorb water, moisturising

non-emulsifying:

highly greasy, strong occlusion (trap moisture)

uses: emollients

what is a paste

semi solid with high solid content

powder dispersed in fatty base or hydrophilic base

key properties of pastes

thick and stiff

less penetrating

uses of pastes

protective dressings, nappy rash

types of pastes

fatty pastes, non-greasy pastes

what is a gel

semi solid with liquid trapped in 3D polymer network

composition of gel

high water content

gelling agents

one phase gels

uniform system

2 phase gels: suspended particles

properties of gels

non-greasy, high drug release

advantage of gels

good appearance, stable, easy application

uses of gels

drug delivery

lubrication

suppositories

solid, single dose preparations for rectal administration

key properties

solid at room temp

melt or dissolve at body temp

pessaries

solid, single dose preparations for vaginal use

uses: antifungals

advantages

avoids first pas metabolism

useful for vomiting, unconscious patients

avoid GIT irritation

disadvantages

poor patient acceptability

variable absorption

manufacturing steps of suppositories

melt base

add drug

pour into mould

cool and solidify

suppository bases: oleaginous (fatty base)

e.g. cocoa butter

melt at body temp, good spreading, comfortable

synthetic triglycerides

more stable

no polymorphism (stable when heated)

better water absorption

water-miscible bases

don’t melt, dissolve instead

longer drug release

displacement values

problem: drug replaces some of the base

definition: amount of drug that displaces 1g of base

patient counselling

• insert after bowel movement

• lie down after insertion

• store below 15 degrees to prevent melting

• moisten before use