Silica is the most common mineral in the earth's crust and is found in rocks, sand and, in minimal quantities, in mineral water.

It appears in the form of granules, a stable white powder. Commercially it is found in the form of 

• nanoparticles, mesoporous, particle size 200 nm, pore size 4 nm
• nanopowder, particle size 10-20 nm (BET)

Depending on its characteristics, it takes the names 

• Colloidal silica (white powder with a native particle size of 5nm to 40nm with filling, thickening, reinforcing and thixotropic functions in various materials).
• Precipitated silica (white powder produced by a solution of precipitated silicate salts with anti-caking, matting, viscosity and abrasive properties).  It is used in animal feed in feed additives, functions as an anti-acidity agent, flavouring agent, mould inhibitor. It has a pH of 6.0 ~ 7.5. Soluble in sodium sulphate.
• Pyrogenic silica (a white powder of very low specific weight with thickening and thixotropic properties, i.e. it can vary viscosity when added as a thickener to liquids).
• Fumed silica (white powder used as a reinforcing material in silicone rubber and as a moisture absorber in the electronics industry, paints, fibre optics, coatings).

What it is used for and where

Medical

From the medical point of view, this study is interesting:

"There has been a plausible link between human exposure to aluminum and Alzheimer's disease for several decades. We contend that the only direct and ethically acceptable experimental test of the 'aluminum hypothesis', which would provide unequivocal data specific to the link, is to test the null hypothesis that a reduction in the body burden of aluminum to its lowest practical limit would have no influence upon the incidence, progression, or severity of Alzheimer's disease. Herein we are testing the hypothesis that silicon-rich mineral waters can be used as non-invasive methods to reduce the body burden of aluminum in individuals with Alzheimer's disease and a control group consisting of their carers and partners. We have shown that drinking up to 1 L of a silicon-rich mineral water each day for 12 weeks facilitated the removal of aluminum via the urine in both patient and control groups without any concomitant affect upon the urinary excretion of the essential metals, iron and copper. We have provided preliminary evidence that over 12 weeks of silicon-rich mineral water therapy the body burden of aluminum fell in individuals with Alzheimer's disease and, concomitantly, cognitive performance showed clinically relevant improvements in at least 3 out of 15 individuals." (1).

The objective was to investigate at baseline the potential association between the composition of drinking water and the level of cognitive function in women taking part in the Epidemiology of Osteoporosis (EPIDOS) Study and to determine during follow-up the effects of the composition of drinking water on the risk of Alzheimer. Conclusions: Silica in drinking water may reduce the risk of developing AD in elderly women. The results corroborate those of another epidemiologic study carried out in France. The potential effect of silica needs to be confirmed in additional investigations (2).

Mesoporous silica nanoparticles (MSNs) have been used in drug delivery systems to improve drug stability and solubility, protect cargo, target specific tissues, and enhance drug circulation time and controlled release (3).

Nitric-silica baths had beneficial myoadaptive and analgesic effect on patients with reflex dorsopathy syndromes. Balneotherapy combined with the treatment by sinusoidal modulated currents, massage, and remedial gymnastics was efficacious in alleviating musculotonic and orthopedic problems in patients with lumbar osteochondrosis. This effect was more pronounced than with the use of gas-bubble baths due to the improvement of trophic processes in the muscular tissue of the affected back and limb regions (4).

In this review, the use of mesoporous silica nanoparticles for photodynamic therapy (PDT) applications is described for the year 2017. Since the pioneering work in 2009, nanosystems involving mesoporous silica nanoparticles have gained in complexity with a sophisticated core-shell system able to perform multi-imaging and multi-therapies, not only for cancer diseases but also for anti-microbial therapy, atherosclerosis, or Alzheimer disease. Near-infrared, excitation light based on up-converting systems, X-rays or persistent luminescent systems are described for deeper tissue treatments (5)

Silica is a very common material that can be found in both crystalline and amorphous forms. Well-known toxicities of the lung can occur after exposure to the crystalline form of silica (6).

• Molecular Formula   (SiO₂)n  SiO₂ or O₂Si
• Molecular Weight   60.083
• Exact Mass   59.966755
• CAS   7631-86-9  

• UNII    ETJ7Z6XBU4
• EC Number   231-545-4
• DSSTox Substance ID  DTXSID1029677     DTXSID9029851     DTXSID4029852     DTXSID3032040
• IUPAC  dioxosilane
• InChI=1S/O2Si/c1-3-2  
• InChl Key      VYPSYNLAJGMNEJ-UHFFFAOYSA-N
• SMILES   O=[Si]=O
• MDL number  MFCD00011232    MFCD00148343
• PubChem Substance ID    24882839
• ChEBI  30563
• NCI    C29853
• ICSC    0248     0808     0809     0807
• RXCUI     9771    314826  
• RTECS   VV7310000     VV7325000     VV7335000
• NACRES     NA.23    

Synonyms

• Silicon dioxide
• dioxosilane

References_______________________________________________________________

(1) Davenward S, Bentham P, Wright J, Crome P, Job D, Polwart A, Exley C. Silicon-rich mineral water as a non-invasive test of the 'aluminum hypothesis' in Alzheimer's disease. J Alzheimers Dis. 2013;33(2):423-30. doi: 10.3233/JAD-2012-121231. 

(2) Gillette-Guyonnet S, Andrieu S, Nourhashemi F, de La Guéronnière V, Grandjean H, Vellas B. Cognitive impairment and composition of drinking water in women: findings of the EPIDOS Study. Am J Clin Nutr. 2005 Apr;81(4):897-902. doi: 10.1093/ajcn/81.4.897. 

(3) Tang, F., Li, L. & Chen, D. Mesoporous silica nanoparticles: synthesis, biocompatibility and drug delivery. Adv Mater 24, 1504–1534,  https://doi.org/10.1002/adma.201104763 (2012).

(4) Kovalenko TS, Miriutova NF, Zhiliakov IV. The use of nitric-silica mineral water in combined rehabilitative therapy of musculotonic dorsopathy syndromes. Vopr Kurortol Fizioter Lech Fiz Kult. 2008 Jul-Aug;(4):8-10.

(5) Bayir S, Barras A, Boukherroub R, Szunerits S, Raehm L, Richeter S, Durand JO. Mesoporous silica nanoparticles in recent photodynamic therapy applications. Photochem Photobiol Sci. 2018 Nov 1;17(11):1651-1674. doi: 10.1039/c8pp00143j.

(6) Ryu HJ, Seong NW, So BJ, Seo HS, Kim JH, Hong JS, Park MK, Kim MS, Kim YR, Cho KB, Seo MY, Kim MK, Maeng EH, Son SW. Evaluation of silica nanoparticle toxicity after topical exposure for 90 days. Int J Nanomedicine. 2014 Dec 15;9 Suppl 2(Suppl 2):127-36. doi: 10.2147/IJN.S57929.