System can sterilize medical tools using solar heat
Device could provide pressurized steam to run autoclaves without the need for electricity in off-grid areas.
IMAGE: AUTOCLAVES, WHICH ARE USED TO STERILIZE MEDICAL TOOLS, REQUIRE A STEADY SUPPLY OF HOT, PRESSURIZED STEAM. RESEARCHERS AT MIT AND THE INDIAN INSTITUTE OF TECHNOLOGY HAVE COME UP WITH A... view more
CREDIT: COURTESY OF THE RESEARCHERS. EDITED BY MIT NEWS.
Autoclaves, the devices used to sterilize medical tools in hospitals, clinics, and doctors' and dentists' offices, require a steady supply of pressurized steam at a temperature of about 125 degrees Celsius. This is usually provided by electrical or fuel-powered boilers, but in many rural areas, especially in the developing world, power can be unreliable or unavailable, and fuel is expensive.
Now, a team of researchers at MIT and the Indian Institute of Technology has come up with a way to generate the needed steam passively, using just the power of sunlight, with no need for fuel or electricity. The device, which would require a solar collector of about 2 square meters (or yards) to power a typical small-clinic autoclave, could maintain safe, sterile equipment at low cost in remote locations. A prototype was successfully tested in Mumbai, India.
The system is described in the journal Joule, in a paper by MIT graduate student Lin Zhao, MIT Professor Evelyn Wang, MIT Professor Gang Chen, and 10 others at MIT and IIT Bombay.
The key to the new system is the use of optically transparent aerogel, a material developed over the last few years by Wang and her collaborators. The material is essentially a lightweight foam made of silica, the material of beach sand, and consists mostly of air. Light as it is, the material provides effective thermal insulation, reducing the rate of heat loss by tenfold.
This transparent insulating material is bonded onto the top of what is essentially off-the-shelf equipment for producing solar hot water, which consists of a copper plate with a heat-absorbing black coating, bonded to a set of pipes on the underside. As the sun heats the plate, water flowing through the pipes underneath picks up that heat. But with the addition of the transparent insulating layer on top, plus polished aluminum mirrors on each side of the plate to direct extra sunlight at the plate, the system can generate high-temperature steam instead of just hot water. The system uses gravity to feed water from a tank into the plate; the steam then rises to the top of the enclosure and is fed out through another pipe, which carries the pressurized steam to the autoclave. A steady supply of steam must be maintained for 30 minutes to achieve proper sterilization.
Since much of the developing world faces limited availability of reliable electricity or affordable fuel, "we saw this as an opportunity to think about how we can potentially create a low-cost, passive, solar-driven system to generate steam, at the conditions that are necessary for autoclaving or for medical sterilization," explains Wang, who is the Gail E. Kendall Professor of Mechanical Engineering and head of the mechanical engineering department.
Being able to test the system in Mumbai was a bonus, she says, because of the city's "relevance and importance" as the type of location that might benefit from such low-cost steam-generation equipment.
In the Mumbai tests, even though the sky was hazy and cloudy, providing only 70 percent insolation compared to a sunny day, the device succeeded in producing the saturated steam needed for sterilization for the required half hour period.
The test was carried out with a small-scale unit, only about a quarter of a square meter, about the size of a hand towel, but it showed that the steam production rate was sufficient that a similar unit of somewhere between 1 and 3 square meters would be sufficient to power a benchtop autoclave of the kind typically used in a doctor's office, Zhao says.
The main limiting factor for practical deployment of such devices is the availability of the aerogel material. One company, founded by Elise Strobach PhD '20, who is a co-author of this paper, is already attempting to scale up the production of transparent aerogel, for use in high thermal efficiency windows. But so far the material is only produced in small amounts using relatively expensive laboratory-grade supercritical drying equipment, so widespread adoption of such a sterilization system is likely still a few years off, the researchers say.
Since the other components, except for the aerogel itself, are already widely available at low cost throughout the developing world, fabrication and maintenance of such systems may ultimately be practical in the areas where they would be used. The parts needed for the quarter-square-meter prototype came to less than $40, Zhao says, so a system sufficient for a typical small autoclave would be likely to cost $160 or so, once the necessary aerogel material becomes commercialized. "If we can get the supply of aerogel, the whole thing can be built locally, with local suppliers," he says.
The process could also be used for a variety of other purposes, the team says. For example, many food and beverage processing systems rely on high-temperature steam, which is typically provided by fossil-fuel powered boilers. Passive solar-powered systems to deliver that steam would eliminate the fuel costs, and so could be an attractive option in many industries, they say.
Ultimately, such systems should be much more cost-effective than systems that concentrate sunlight by tenfold or more to generate steam, because those require expensive mirrors and mountings, as opposed to the simplicity of this aerogel-based approach.
"This is a significant advance," says Ravi Prasher, a professor of mechanical engineering at the University of California at Berkeley and an associate director at Lawrence Berkeley National Laboratory, who was not involved in this work. "Generating high-temperature steam with high energy efficiency has been a challenge. Here the authors have achieved both."
"The quality of the research is very high," Prasher adds. "Access to passive sterilization techniques for low-income communities who do not have access to reliable electricity is a big deal. Therefore, the passive solar device developed by the MIT team is very significant in that regard."
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The research team also included Bikram Bhatia, Lenan Zhang, Arny Leroy, Sungwoo Yang, Thomas Cooper, and Lee Weinstein at MIT, and Manoj Yadav, Anish Modi, and Shireesh Kedsare at IIT Bombay. The work received support from the Tata Center at MIT and from the U.S. Department of Energy.
Written by David L. Chandler, MIT News Office
Portable solar-powered device for sterilizing medical equipment in the field
VIDEO: THE VIDEO SHOWS THE FIELD TEST SETUP IN MUMBAI, INDIA, AND HOW STEAM WAS GENERATED CONTINUOUSLY DURING EXPERIMENTS view more
CREDIT: LIN ZHAO
By integrating a transparent, cloud-like aerogel with a solar heater, scientists can now efficiently trap solar energy to generate steam that is hot enough and at high enough pressure for sterilizing medical instruments even under hazy and partly cloudy weather. The solar thermal device, reported November 18 in the journal Joule, may help alleviate infection-related healthcare burden and facilitate the adoption of solar energy as a potential power source in resource-limited regions.
Almost like a pressure cooker with high temperature and high pressure, devices called autoclaves are used to cook germs at 121°C (249.8°F) and 205 kPa as a standard medical equipment sterilization procedure. However, due to the heat loss to the environment, traditional solar heaters have difficulties reaching temperatures above boiling point. In the new study, the research team incorporated a thermal insulating aerogel into the solar thermal device to reduce the escaping heat, allowing the device to generate 128°C (262.4°F) and 250 kPa steam for sterilization.
"When we were doing the field research in India, many healthcare providers said that they can operate an autoclave if there is reliable energy, but there are certain occasions where the electricity is unavailable," said first author Lin Zhao of Massachusetts Institute of Technology, who conducted a field study in Mumbai, India. "They are then forced to use maybe unsterilized equipment, or they have to use some rudimentary method like boiling water to try to sterilize things. A solar-powered device, in that case, would certainly be helpful to them."
Unlike other steam-generating solar thermal devices, which rely on solar tracking that requires operators to concentrate sunlight, the team's device is stationary, more user friendly, and energy efficient. In the field test, the aerogel-aided device generated steam at 100°C (212°F) with more than two times higher energy efficiency than other solar thermal devices reported in the lab. It also successfully performed standard sterilization under the Centers for Disease Control and Prevention's guidelines.
"As long as you have sunlight, you can use this device. It doesn't have to rely on well-built infrastructure. You can use it off-grid in remote areas," said Zhao. But like other solar-powered gadgets, it also has a limit--cloudy and rainy days. "It really depends on your location. If you're in a sunny area, our device may offer more benefits."
Zhao and his colleagues' device also has a lower price tag, and most of its components are commercially available in the solar water heater industry. Compared to benchtop autoclaves on the market, which can cost up to $3,000, the researchers' device costs about $35 per unit. Three to six units of the prototype device can sufficiently power a benchtop autoclave, demonstrating its potential as an inexpensive and accessible approach to reduce infection in remote and developing regions. The next step for the research team is to manufacture the aerogel in high quantity and quality.
"The real 'secret sauce' or the enabling factor is the transparent aerogel. Once we have the aerogel scaled up, we can think about providing the device to people who need it, such as channels like NGOs and other organizations," said Zhao. "We can also think about using the same technology for larger-scale applications. For example, food processing plants and chemical plants, they use boilers steam generators too."
CAPTION
This image shows scientists from MIT evaluating the device's performance in "real world" situation, set up in a field research site in Mumbai, India.
This work was supported by the MIT Tata Center for Technology + Design, IIT Bombay, and the US Department of Energy, Office of Science, Basic Energy Sciences.
Joule, Zhao et al.: "A passive high-temperature high-pressure solar steam generator for medical sterilization"
https://www.cell.com/joule/fulltext/S2542-4351(20)30496
CAPTION
This image shows the transparent, cloud-like, thermal aerogel insulating the solar-driven autoclave.
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