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Diffusion Principles in Biological Systems

Diffusion, Types, Passive diffusion, Facilitated (carrier-mediated ) diffusion, Active transport, Filtration, Laws of diffusion, Fick's theory etc.


Molecules move in mass, usually due to gradients of concentration when they are subjected to random motion."
Molecular movement occurs when their concentration gradient is influenced, such that molecules move from high to low concentrations."
Pharmaceutical sciences apply diffusion phenomena such as:
  • Dissolving of a drug from its dose form
  • There are four types of dialysis: ultrafiltration, microfiltration, dialysis, hemodialysis.
  • Drug distribution and permeation in biological tissue
  • Determination of polymer molecular weight
  • Predicting absorption & elimination of drugs.
Molecular and particle diffusion occurs when they move passively from high concentration to low concentration areas. Concentration gradients must exist for diffusion to occur. Due to the differences in amounts of particles, molecules, or solutes between the two regions, they will move between them.


1. Passive diffusion

Moving from a high-to-low concentration zone, materials are being moved. Concentration gradients occur when one concentration and one concentration differ. The diffusion process continues until the gradient has disappeared.

2. Facilitated (carrier-mediated ) diffusion

Cellular membrane molecules are transported across the membrane by special transport proteins.

3. Active transport

A progression of molecules along a membrane against a gradient of concentration from a region of low concentration to a region of high concentration

4. Filtration

Influence of hydraulic pressure on the movement of solvent molecules.

Laws of diffusion

It was derived in 1856 by Adolf Fick.

Fick's first theory of diffusion

If steady-state diffusion is assumed, diffusion flux has a direct relationship with concentration gradients."

J= -D dc/dx


J = diffusion flux (grams per square centimeter/second)

Whenever D is given, it is the diffusion coefficient or diffusivity (cm squared per second)

dc= change in concentration of material ( g/cubic cm)

Distance change (cm) = dx

Mass is transferred across a unit cross-section area per unit time by diffusion flux (J).

J= dM/S dt

Second Law of Diffusion according to FICK:

"Changes in concentration in one region with time are proportional to increases or decreases in concentration gradients at that particular point in the system."

dc/dt = -dJ/dx


Franz cell apparatus is the device are composed of two connected chambers separated by a membrane. There are known concentrations of solute in the donor chamber. Receptor chambers contain a fluid, sampling of which takes place regularly for analysis. A 37-degree internal temperature is maintained in the chamber. There are various membrane materials, from excised tissue to tissues constructed from cadaver tissue.

During experimentation, solute diffuses from the donor chamber into the recipient chamber through the membrane. During the experiment, the solution from the receptor chamber is periodically removed and analyzed. This test determines whether the diffusant has penetrated the membrane. Each sample removes the chamber solution and replaces it with a new solution.


Reservoir (laminated matrix) device

There is an inner core surrounded by an insoluble membrane inside a hollow system. Coatings and encapsulations can be used to apply polymers. Upon partitioning into membranes, drugs diffuse with surrounding fluids through diffusion. Upon entering the membrane, the drug diffuses to the periphery & exchanges with the surrounding fluid. A coating's polymer content, coating thickness, and microcapsule hardness are factors that determine its rate of drying. The diffusion law determines release.

Matrix (Monolithic) devices

Insoluble matrixes disperse solid drugs. A first solution dissolves the outer layer of the drug. Then it will be exposed to a bathing solution. Once the drug diffuses through the membrane, it can affect the surrounding environment. A diffusion matrix can come in two configurations:
  • Rigid matrix
  • Swellable matrix
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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