What are colloids?
Colloids are mixtures containing solids in liquid or liquid in liquid and all of these contain distinct solid (and/or liquid) particles which are dispersed to various degrees in a liquid medium. The size of the particles determines the properties of the colloid.
The term colloid also is specific to the size of the individual particles. The particles are larger than atomic dimensions but small enough to exhibit Brownian motion.
Particle size and colloids
If the particles are larger, their dynamic behaviour within the suspension is governed by forces of gravity and sedimentation. For smaller particles in colloids, irregular motion in suspension can be attributed to the collective bombardment of a myriad of thermally agitated molecules in the liquid suspending medium. This was described originally by Albert Einstein in his dissertation.
This erratic particle behaviour could adequately be described using the theory of Brownian motion. This critical size range (or particle diameter) of these particles typically ranges from nanometers (10−9 m) to micrometers (10−6 m).
Nanoparticles in colloids and human diseases
Several neurodegerative diseases are linked to colloidal deposits in the brain containing nanoparticles. These are activated within the brain by colloidal or particulate material of exogenous origin. While the sites of damage and types of aggregate involved can vary widely, the sequence of events triggered by their chronic presence with the brain is more or less common.
These diseases commonly have genetic origins that involve accumulation of complex carbohydrates and lipids, such as galactosidoses and sphingolipidoses. These insoluble deposits are intolerable to the brain and its vital functions.
The growing exposure of the population to nanoparticles from a variety of sources thus is a cause for concern. The major route of contact with such materials is not by way of inhalation of ambient air and fumes, but increasingly by the use of nanoparticles in consumer products, food additives and medical drugs. The final effects of these exposures are unknown.
The ability of colloids and nanoparticles to initiate inflammatory events within the brain may be based on factors like:
- The particulate deposits are chemically inert but if they are of the critical size, they can lead to the initiation of immune responses from glial cells or the defence cells of the brain.
- These colloidal nanoparticles have a large surface area and thus can serve as an attractant of metal ions. When these include transition metals capable of expressing more than one valence state under physiological conditions (for example Fe, Cu, Mn), this forms an effective platform for Fenton-like cycling activity. This can lead to production of reactive oxygen species and cause oxidative damage to the brain.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)