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Quelling Fear about the Smallest Nanoparticles

Teensy Nanoparticles: Not so Poisonous
The finest nano-sized silica particles used in biomedical engineering likely won't cause unexpected biological responses due to their size, according to scientists at the Pacific Northwest National Laboratory in Richland, Washington.  This news should calm fears that cells and tissues will react erratically when exposed to the smallest silica nanomaterials.
By using total surface area to measure dose, instead of particle mass or number of particles, and then observing how cultured cells responded biologically, the scientists determined that size won"t generally matter.
•If you consider surface area as the dose metric, then you get similar types of responses independent of the size of the particle,• said nanotoxicologist Brian Thrall. •That suggests the chemistry that drives the biological responses doesn't change when you get down to the smallest nanoparticle.•
What Goes on at the Nanoscale?
Nanoparticles are made up of spherical particles that are 100 to 1,000 times smaller than the width of a human hair. They are used in everything from biomedicine to tires. These tiny spheres are invaluable because their properties offer distinct advantages, such as being able to glide through blood vessels to deliver drugs.
Whether these materials are safe for human consumption is not clear.  Previous work suggested nanoparticles become more toxic to cells the smaller the particles get.
Some researchers measure the dose by total weight, some by the number of particles.  Neither method distinguishes whether a nanomaterial's toxicity is due to the inherent nature of the material or the particle size.
To find out, the scientists measured the dose at which the particles caused a biological response. The biological response was either death of the cell, or a change in which genes the cell turned on and off. They found that when calculating doses by particle number or mass, the amount needed to generate a biological response was erratic.
The best way to pinpoint how toxic the particles are to cells was for them to calculate the dose based on the total surface area of the nanomaterial.  Only when they considered the surface area of the dose could they predict the biological response.
They saw that the biological response was very similar regardless of nanoparticle size. Inside cells, some genes responded to nanoparticles by ramping up or down.  Seventy-six percent of these genes behaved the same for all nanoparticle sizes tested.  This indicated that, for these genes, the nanoparticles didn't pick up weird chemical properties as they shrunk in size.
However, the team found some genes for which size did matter.  These genes fell into two categories: smaller particles affected genes that might be involved in inflammation, and the larger particles appeared to affect genes that transport positively charged atoms into cells.  This latter result could be due to metals contaminating the preparation of the larger particles, Thrall suggested.

By Neil Whitehall
Get Biomedical Engineering Jobs, Contributing Editor

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