Page 187 - Biomedical Engineering and Design Handbook Volume 2, Applications

166 MEDICAL DEVICE DESIGN

than light and small particles. Viscosity of the environment is another important parameter in diffusion,
since the rate of molecular movement is associated with the viscosity of the environment. Diffusion is
fastest in the gas phase, slower in the liquid phase, and slowest in the solid phase.
Mathematically, the rate of drug delivery in diffusion-controlled delivery systems can be
described by Fick’s laws. Fick’s first law of diffusion is expressed as 9
dC
j =− D (6.3)
dx
where J = flux of diffusion
D = the diffusivity of drug molecule
dC/dx = the concentration gradient of the drug molecule across diffusional barrier with
thickness dx
According to the diffusion principle, controlled-release drug delivery systems can be designed as
a reservoir system or a matrix system. Drug released from both reservoir- and matrix-type devices
follow the principle of diffusion, but they show two different release patterns as shown in Fig. 6.3.

Drug in reservoir
Backing layer
Rate-controlling membrane

Epidermis

Blood supply (B) Dermis

A

Reservoir Matrix Moving boundary
Membrane Diffusion Diffusion
layer (epidermis) layer (epidermis)
C p C p
C
R
C R C d

C d Sink (blood) Sink (blood)

h M h D h + d hp h D
p
B C
FIGURE 6.3 Schematic illustration of a transdermal drug delivery system in use (a), concentration
profiles of reservoir (b), and matrix type of system (c).