Building a future on tiny spheres
Colloidal silica reinforces structural stability in 3‑D printing.
Credit: Getty Images

Colloidal silica is helping create a world with on‑demand 3‑D printing, cleaner water, and medications with
reduced side effects

By Michael Eisenstein, C&EN BrandLab contributing writer

Over the past century, colloidal silica has given the world a versatile tool for construction, aviation, industrial manufacturing, and a host of other applications. But even after so many years, these humble solutions of tiny particles have not run out of surprises.

Today’s scientists continue to rely on stable, versatile, and well-characterized colloidal silica to facilitate innovative technologies that would have been unimaginable a generation ago. Powerful new manufacturing processes require robust and affordable raw materials to fulfill their promise. As industrialization continues to spread throughout the world, global societies will need more sophisticated tools to control pollution and preserve clean water supplies. And for every promising new drug, pharmaceutical companies need to determine the ideal vehicle for delivering the drug to the right place at the right time.

For these and other cutting-edge applications, countless industries are continuing to take advantage of colloidal silica’s distinctive physical and chemical properties.

A universal building block

Over the past decade, 3‑D printing has begun to transition from a high‑tech toy for hobbyists to a mainstream manufacturing tool. For example, the aerospace and medical prosthetics industries now routinely use this method for the precision crafting of metal parts.

The Shepherd group at Cornell is printing flexible 3‑D structures, such as this rendition of Touchdown, the unofficial bear mascot of Cornell. The structures are also conductive, which is a step towards custom-built ‘soft electronics’ components.
Credit: Shepherd Group at Cornell University

Over the past decade, 3‑D printing has begun to transition from a high-tech toy for hobbyists to a mainstream manufacturing tool. For example, the aerospace and medical prosthetics industries now routinely use this method for the precision crafting of metal parts.

Aside from metal, numerous other materials can serve as a medium for 3‑D printing, including glass, ceramics, and a wide range of polymers. Now colloidal silica is finding its way into many 3‑D inks. According to Natalia Krupkin, technical services manager at W. R. Grace & Co., manufacturer of LUDOX® colloidal silica, this is fitting from a historical perspective. “LUDOX® colloidal silica was used in printing applications before. In fact, since day 1 of the digital printing era, colloidal silica was used in inks and paper coatings,” she says. While colloidal silica is typically used to improve the flow of toner for 2‑D printers, its role in 3‑D printing is to instead act as a binder that reinforces printed materials. “Ceramic structures that are being printed are typically porous,” Krupkin says. “When colloidal silica is added to the formulation, it basically blocks the holes, or pores, and improves the integrity and stability of the structure.”

Colloidal silica is also an effective binding ingredient when the intended product is soft and squishy rather than rigid and robust. For example, André Studart and colleagues at the Swiss Federal Institute of Technology (ETH), Zurich, prepared a 3‑D ink from oil‑and‑water emulsions containing a mixture of colloidal silica and a carbohydrate known as chitosan. The soft and flexible structures produced in this printing process could potentially be used in biomedical implants or as a vehicle for drug delivery. Robert Shepherd and Emmanuel Giannelis of Cornell University likewise used colloidal silica to 3‑D print flexible hydrogel-based structures that can conduct ionic currents and execute simple mechanical functions.

Colloidal silica serves as a binder for flexible hydrogels (left), which can then be 3‑D printed (middle) into a wide variety of shapes, such as this model of the Eiffel Tower (right).
Credit: Shepherd group at Cornell University
Colloidal silica serves as a binder for flexible hydrogels (left), which can then be 3‑D printed (middle) into a wide variety of shapes, such as this model of the Eiffel Tower (right).
Credit: Shepherd group at Cornell University

In addition to the biomedical applications envisioned by Studart’s team, Shepherd and Giannelis foresee the potential for custom printing soft, hydrogel‑based robots that can be used in applications that are out of reach of their clunky, metallic counterparts.

A solution for salinity

In the early days of the industrial era, economic development was the priority. Companies gave little thought to environmental impacts. Today, the consequences of unfettered industrialization are all too clear. For example, the release of toxic wastewater from the hydraulic fracturing, or fracking, process that extracts natural gas from shale deposits seriously endangers the health of nearby communities. The damage has led to popular opposition and bans on fracking in many jurisdictions.

As a result, many governments are moving to ensure that future growth is coupled with efforts to minimize pollution and waste as much as possible. “There’s a concept called zero liquid discharge, which is being talked about a lot,” says Shihong Lin, assistant professor of civil and environmental engineering at Vanderbilt University. “The idea is that you are not allowed to discharge any liquid waste into the environment.”

To achieve this goal, factories need mechanisms for removing the various contaminants in their wastewater, yielding clean processed water that can safely be reused. Reverse osmosis is a commonly used purification technique in which pressure is applied to contaminated water to achieve filtration through a semipermeable membrane. However, it is not an energy‑efficient solution for treating the hypersaline wastewater resulting from many industrial manufacturing processes—including fracking—because of the high pressure required to achieve effective purification.

As an alternative, Lin’s group is developing specialized materials for a purification method known as membrane distillation. One approach his group is exploring is omniphobic membranes, which repel both water and oil. These membranes are rough and porous, but wastewater is unable to pass through their openings. The liquid is subjected to heat and pressure, which produce pure water vapor that can pass through the pores while leaving unwanted salt behind. “If you have waste heat from power plants or chemical processes or even geothermal energy, then you can basically tap into that as the driving force,” Lin explains.

Hydrophobic membranes (left) may not be up to the task of membrane distillation, as they allow oils in wastewater found at water treatment plants to penetrate and foul the membrane. Colloidal silica–coated ‘omniphobic’ membranes (right) block both oil and liquid water, allowing only clean water vapor to pass through. Schematic from Environ. Sci. Technol. 2016, 50, 3866-3874.

Colloidal silica is a core component of the membranes manufactured by Lin’s group. These tiny nanoparticles increase the roughness and surface area of the membrane, creating conditions that help prevent untreated water from penetrating the pores. Colloidal silica can be easily modified to confer surface properties that contribute to the omniphobic behavior. “It generally seems that membrane distillation is only viable if the membrane surface is textured with nanomaterials,” notes Cagri Üzüm, technical services manager for colloidal silica at Grace. “Colloidal silica in particular is the most promising nanomaterial for this thanks to its availability, environmentally friendly nature, and well-known surface chemistry.”

Lin’s group and others have demonstrated in the laboratory the conceptual feasibility of using such membrane distillation units for the long-term treatment of hypersaline wastewater. This approach has not yet been deployed in an industrial-scale setting, but companies have recognized the technology’s potential. “The number of scientific publications combining membrane distillation and nanoparticles has increased five-fold since 2010, and more companies are building pilot-scale plants each year,” Üzüm says.

Lin further envisions that membrane distillation could potentially expand access to drinking water in resource‑limited settings, where the technique could be incorporated into low‑cost passive solar thermal desalination units in which heat from the sun powers water purification. “That’s a really promising direction,” he says.

Shielding sensitive skin

Many people survive their adolescence with a little help from benzoyl peroxide, a popular compound for treating acne. It is also an effective treatment for another skin condition, rosacea, which can cause redness, swelling, and sores. Unfortunately, many people with rosacea experience uncomfortable burning and irritation when applying benzoyl peroxide to their skin.

Sol‑Gel Technologies, a pharmaceutical company based in Israel, has a solution for these patients. Sol-Gel employs an encapsulation approach first developed in the 1990s by Hebrew University of Jerusalem chemist David Avnir, the company’s cofounder. Colloidal silica is a critical component of this process, according to Ofer Toledano, Sol-Gel’s vice president of research and development. “The colloidal particles migrate to the surface of the capsule’s core, which is the active ingredient, and form a silica shell that tightly surrounds it,” he says. “Our encapsulation approach enables us to control the characteristics of that silica shell, such as thickness, surface area, pore size, and pore volume.” Tuning these features allows control over the drug’s rate of release from the capsules and limits the skin’s exposure to this potentially irritating compound.

The company’s first product, EPSOLAY® dermatologic cream, is a cream laden with silica-encapsulated benzoyl peroxide, which is currently undergoing a phase III clinical trial in the US as a treatment for rosacea. Preliminary data from that study indicate that this product can effectively reduce inflammation in people with rosacea while causing minimal irritation. The company expects to have final results by the end of 2019.

In parallel, Sol-Gel has collected promising data from a second encapsulated therapy, termed Twin, that combines benzoyl peroxide with another acne drug, tretinoin. This represents a breakthrough on multiple fronts, according to Toledano. “First, these two ingredients are incompatible, and there are no fixed dose combinations on the market,” he says. “Secondly, the two active ingredients are irritants, and a combined product of these two irritant ingredients could be unbearable.” Sol-Gel encapsulated these two agents separately and then combined them at a defined ratio in Twin, which can potentially deliver the full benefits of dual treatment while minimizing side effects and maintaining the potency of both drugs.

Sol-Gel is focused primarily on dermatologic indications, and the company has several other silica-encapsulated formulations in the clinical pipeline, including antiviral and antiparasitic drugs. However, Toledano believes that a similar encapsulation process could readily be tailored for controlled release of a wide range of drugs, including orally administered therapies. “The silica shell is stable in diverse conditions, such as elevated temperatures and extreme pH. Silica is also approved by the US Food and Drug Administration for use in oral dosage forms,” he says. Indeed, a number of academic research groups are testing silica nanoparticles as a means of delivering drugs orally or intravenously; they are even exploring strategies for modifying these particles with molecular tags that essentially serve as address labels for specific target tissues or cells. This targeting could potentially lead to more potent and tumor‑specific cancer therapy, for example, sparing healthy cells the toxic effects of therapy.

These applications remain beyond the horizon for now. But they illustrate the versatility and robustness of colloidal silica, which has given scientists and engineers a foundation for more than 100 years of innovation—and suggest that this material may still have many more surprises in store.

GRACE® and LUDOX® are trademarks, registered in the United States and/or other countries, of W. R. Grace & Co.-Conn. EPSOLAY® is a trademark, registered in the United States and/or other countries, of SOL-GEL Technologies Ltd. This article is an independent publication and is not affiliated with, nor has it been authorized, sponsored, or otherwise approved by Cornell University, Vanderbilt University, U.S. Food and Drug Administration, and Hebrew University. This trademark list has been compiled using available published information as of the publication date of this brochure and may not accurately reflect current trademark ownership or status. © Copyright 2019 W. R. Grace & Co.-Conn. All rights reserved.

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