SBA-15 Mesoporous Silica for Drug Tribo-Charge Control - University of Huddersfield, 2018

Afzal, M. S. et al. (2018). The effect of mesoporous silica impregnation on tribo-electrification characteristics of flurbiprofen. *International Journal of Pharmaceutics*. https://doi.org/10.1016/j.ijpharm.2018.03.059

International Journal of Pharmaceutics · 2018

Researchers used ACS Material SBA-15 mesoporous silica to suppress tribo-electrification of flurbiprofen, cutting charge from -226 to -10 nC/g.

About this research

Researchers at the University of Huddersfield used SBA-15 mesoporous silica from ACS Material to suppress the tribo-electrification of flurbiprofen (FBP), reducing the drug's saturation charge from -226 nC/g down to as little as -2 nC/g when loaded into the silica's pore network. This 2018 study, published in the International Journal of Pharmaceutics by Afzal, Zanin, Ghori, Granollers, and Šupuk, demonstrates that embedding a highly charging active pharmaceutical ingredient inside a low-charging porous carrier is an effective route to controlling unwanted electrostatic build-up during powder processing. The authors compared two loading routes - solvent impregnation with acetone and physical powder mixing - and quantified the effect of silica-to-drug ratio on charge, adhesion, crystallinity, and pore filling.

Tribo-electrification is a persistent problem in pharmaceutical manufacturing. When powders contact equipment walls during mixing, conveying, granulation, or capsule filling, charge transfer can cause segregation, blockages, content-uniformity failures, and in extreme cases dust explosions. Flurbiprofen, a non-steroidal anti-inflammatory drug, is notorious for its sticky, highly charging crystalline particles, making it a useful model for the worst case. Conventional charge mitigation strategies often require additional processing steps that compromise the API's physicochemical profile. The authors instead test whether confining the charging crystals inside the cylindrical pore channels of SBA-15 can intrinsically lower the powder's charging tendency, and whether the host itself contributes negligibly to charge under shaking.

The ACS Material SBA-15 was sieved to a 150-250 µm particle-size fraction, while flurbiprofen was fractionated to 38-63 µm. Binary mixtures were prepared at 5, 10, 15 and 20 wt% silica using two routes. In the solvent route, 1.5 g of FBP was dissolved in 5 mL acetone, SBA-15 was added, the slurry stirred for five minutes, then dried at room temperature for 24 h and at 40 °C for a further 24 h. In the powder route, 5 g of drug and the appropriate silica fraction were tumbled in a Turbula mixer at 49 rpm for 10 minutes. Mixtures were characterised by DSC, TGA, powder XRD, UV-Vis content uniformity, and BET/BJH nitrogen sorption to confirm pore filling. Tribo-electric charge was measured by shaking ~0.1 g of powder in a stainless steel capsule at 20 Hz inside a Retsch MM 400 and dropping the charged powder into a Faraday cup connected to a Keithley 6514 electrometer.

The BET specific surface area of pristine SBA-15 was 744.6 m²/g with a mesoporous volume of 0.79 cm³/g. After loading, the surface area collapsed to 6.4 m²/g (5 wt% silica, solvent impregnation) and 11.9 m²/g (physical mixing), confirming that the drug filled the pore network rather than coating exterior surfaces. DSC traces showed the FBP melting enthalpy fall from 116.5 J/g to 71.5 J/g for 20% silica mixtures, indicating a 40-50% reduction in apparent crystallinity at maximum loading. TGA confirmed successful uptake with no hydrate/solvate formation. Pristine flurbiprofen reached a saturation tribo-charge of -226.4 nC/g and stuck ~45% of its mass to the shaker walls after 5 minutes. SBA-15 alone barely charged, reaching only -1.2 nC/g after 10 minutes with ~18% adhesion. Solvent-impregnated binary mixtures charged at only -2 to -10 nC/g across all ratios, and adhesion dropped below 14%. Physical mixtures also reduced charge and adhesion, though less dramatically, because larger agglomerates were less able to enter the pores. Content uniformity stayed within the 95-105% British Pharmacopoeia range for every formulation.

These results have direct relevance to pharmaceutical powder handling, dry powder inhaler design, blending, milling, and tablet compaction, where electrostatic build-up degrades flow and dosing accuracy. The same low-charging-carrier strategy could be transferred to other sticky APIs, and the SBA-15 platform is already of interest for solubility enhancement of poorly water-soluble drugs, suggesting a dual benefit of dissolution improvement and electrostatic control. The authors propose extending the approach to additional model APIs and to evaluating the impact on downstream compaction and dose uniformity.

For formulation scientists and process engineers working on electrostatic charge mitigation, dry powder inhalers, or controlled-release solid dispersions, SBA-15 mesoporous silica is available from ACS Material in the same grade used in this study. The paper provides a quantitative benchmark - more than an order-of-magnitude reduction in charge-to-mass ratio - against which other low-charging carrier candidates can be measured.

How ACS Material products were used

• SBA-15 Mesoporous Silica (Molecular Sieves)  — “Mesoporous silica (SBA-15) was obtained from ACS Material (California, USA).”

Product Performance in this Study

SBA-15 functioned as a low-charging mesoporous carrier that dramatically suppressed the electrostatic charge of flurbiprofen (from -226 nC/g to roughly -2 to -10 nC/g) and reduced wall adhesion from ~45% to below 14%, confirming the silica's effectiveness as a tribo-charge moderating host.

Related product categories

• Molecular Sieves

Frequently asked questions

How does SBA-15 mesoporous silica reduce tribo-electrification of pharmaceutical powders?

SBA-15 acts as a low-charging carrier that confines the highly charging API inside its cylindrical mesopores. With the drug embedded in the pore network, drug-drug and drug-wall contacts during shaking are reduced, lowering the probability of electron transfer. In this study, loading flurbiprofen into SBA-15 cut the charge-to-mass ratio from -226 nC/g to between -2 and -10 nC/g, and wall adhesion from about 45% to under 14%.

What is the difference between solvent impregnation and physical mixing for loading drugs into SBA-15?

Solvent impregnation dissolves the drug in acetone before adding SBA-15, allowing molecular-scale penetration into mesopores. Physical mixing tumbles the dry powders together, which is limited by particle agglomeration. BET data showed solvent-loaded mixtures had lower residual surface area (6.4 m²/g vs 11.9 m²/g at 5 wt% silica), indicating better pore filling and consequently a larger reduction in tribo-electric charge.

Why is flurbiprofen a useful model for studying powder electrostatic charging?

Flurbiprofen is a crystalline NSAID with poor compaction, low solubility, and strong adhesion to metal surfaces, all traits associated with high tribo-charging propensity. It reaches a saturation charge of around -226 nC/g and sticks to about 45% of the shaker wall mass after 5 minutes of shaking. These extreme values make it an ideal worst-case test material for evaluating charge-mitigation strategies.