Everything is illuminated17 May 2019
Traditional photonic devices have been fabricated from rigid materials on rigid substrates, limiting their applicability in the healthcare sector. Now, researchers at the Massachusetts Institute of Technology (MIT) have developed a method for making photonic devices that can bend and stretch without damage and without compromising optical performance. Patrick Kingsland speaks to MIT associate professor Juejun Hu about the application of this technology for medical device manufacturing.
It is a word that is rarely used in common parlance, but over the past few decades, photonics – the science of generating, controlling and detecting light – has become an integral part of our everyday lives.
In the manufacturing sector, laser-based technologies are enabling cleaner, higher-volume and lower-cost methods of production. In the field of communications, photonic technology is driving advances in optical fibres – the backbone of today’s internet.
Perhaps the most life-changing – or saving – field of photonics can be found in the medical device sector. From oil candles to electricity, light sources have been used in healthcare throughout human history. But with modern optic and photonic technologies, new possibilities are emerging in the diagnosis, treatment and prevention of disease.
“Light is an extremely important probe that people can use to investigate biomedical phenomena,” says Juejun Hu, associate professor of materials science and engineering at the Massachusetts Institute of Technology (MIT).
At the cutting edge of the technology are new designs and materials for flexible photonic sensors, which can be bent, folded, rolled, twisted and stretched to conform to human skin without compromising optical performance.
Hu, and other researchers at MIT, say such devices could be used in a range of skin-mounted, wearable health monitors, potentially revolutionising the medical device manufacturing sector.
A key enabling technology
One of the key benefits of using light for healthcare is that it is non-invasive. Procedures involving photonic devices – which use systems of mirrors, lenses and LEDs to process light beams directly – do not require needles or knives or other instruments that penetrate the skin. This means less pain for patients and less complicated operations for medical practitioners.
“Instead of using a physical probe you use light to penetrate human skin and get images or chemical information about the human body,” says Hu.
Photonic technology has been particularly central in the development of modern imaging techniques such as optical coherence tomography – a noninvasive test that uses light to capture highresolution images of tissue. Optical coherence tomography is now used in a wide range of medical applications, including ophthalmology, dentistry, gastrointestinal endoscopy and dermatology.
“With optical coherence tomography, people use light to generate and reconstruct three-dimensional images of tissue that would be difficult to obtain using other techniques,” Hu explains. “It does no damage to human tissue and has been used very widely in eye surgery as well in examining skin conditions.”
Projected value of the photonics industry in 2020.
German Federal Ministry of Education and Research
The ability of light to carry information in multiple dimensions means photonics is also used for medical sensing. Different wavelengths interact with biological tissues in different ways – some are strongly scattered, others weakly scattered – providing detailed information about biological activities, such as oxygen concentration, heart rate and blood pressure.
“A simple example is oximetry,” Hu says. “This is a machine that allows a doctor or nurse to read out the oxygen level in the blood. The basic idea is that you are sending different wavelengths of light to penetrate through a human finger, which is actually translucent. By looking at the absorption of light at these different wavelengths you can work out various different things.”
Photons are also used to directly affect biological tissue, serving a number of therapeutic and surgical purposes. Light-based therapy is used to treat a range of dermatological conditions; laser procedures are used to correct near and farsightedness in vision; in oncology, photonics immunotherapy has been used to treat cancer.
The photonics field is making such strong progress that, last year, it was named as one of six key enabling technologies by the European Commission’s Horizon 2020 – a research and innovation programme designed to help the EU become a front runner in “market-creating innovation”. The programme’s proposed budget allocation between 2021 and 2027 is a mouthwatering €100 billion.
“Medical photonics research and innovation is leading towards the development of easy-to-use, low-cost screening methods that can be carried out at a general doctors premises, or even at home,” said a European Commission Horizon 2020 press release. “These photonic point-of-care technologies can provide a risk assessment of age and lifestylerelated diseases within a few minutes.”
Unsurprisingly, photonics has been identified as one of the fastest-growing areas within healthcare.
The global market is projected to increase from approximately $32 billion in 2011 to $56 billion in 2020 according to recent market research published jointly by the German Federal Ministry of Education and Research, the Mechanical Engineering Industry Association, the German Electrical and Electronic Manufacturers’ Association and the German High Tech Industry Association.
“Photonic devices are penetrating into many different fields and biomedical applications,” says Hu.
Photonics flexing its muscles
While many photonic devices have already been clinically validated and are widely used in the medical sector, there is still a great deal of work to be done according to Hu. The researcher, who is a principal investigator at the Photonic Materials Group, is particularly interested in developing integrated photonic structures that are smaller and more practical than traditional devices.
“Current optical coherence tomography instruments are pretty big, bulky machines and can really only be used in a dedicated laboratory or a doctors clinic,” he says. “We are looking into essentially trying to miniaturise this and make it more accessible. This is the emerging area within photonics.”
Hu’s group is also focused on making new materials for integrated photonics that are flexible and can be mechanically deformed. Photonic devices that could stretch and bend like human skin could be used in a range of medical applications, such as skin-mounted monitoring devices to detect heart rate, blood oxygen levels and blood pressure.
“We anticipate that the identification of new application fields, where mechanical flexibility either constitutes the key feature enabling the target applications or contributes to significantly improved device performance, will certainly accelerate penetration of the technology into diverse market sectors in the future,” Hu’s team said in a recent paper titled ‘Flexible integrated photonics: where materials, mechanics and optics meet’.
Flexible devices of this kind are not a novel idea. The concept of flexible electronics dates back to the 1960s and is now a $5.13 billion global industry. Lighter and more durable than what came before, flexible electronic devices have seen widespread adoption in the consumer electronics and healthcare market, where they are used in lab-on-chip devices, X-ray detectors and health monitors.
But bringing this technology to the biomedical field has proved challenging. This is because the vast majority of integrated photonic devices are fabricated on rigid substrates, such as semiconductors or glass, which are deemed inappropriate for soft, curvilinear human bodies.
Proposed budget allocation, between 2021–27, of the European Commission’s Horizon 2020.
“If you look at how people conventionally make these miniaturised, integrated photonic devices, it is on some kind of rigid substrate,” says Hu. “But if you look at biological tissues in human beings, we are made of this soft, squishy stuff. That means there is a very large elastic mismatch with traditional photonic devices.”
The obvious candidates to overcome this elastic mismatch are organic polymers, from which most flexible photonic devices have traditionally been fabricated. But while these polymers have inherent mechanical flexibility, they also present their own problem: a low refractive index and poor ability to confine light beams.
“Most of the polymers have more or less the same refractive index,” Hu explains. “This could be problematic because for a lot of cases you want to be able to manipulate the light probe with freedom. Polymers do not allow you to do that.”
Hu and other researchers have therefore been searching for a method to make photonic devices that can do two things simultaneously – bend, twist, stretch and seamlessly integrate with biological tissues such as human skin while also maintaining strong optical performance.
“My group has been working on that for the past six to seven years, and we have developed a whole array of different components along the line,” Hu says. “Components must exhibit good mechanical ruggedness and flexibility that does not compromise the optical properties.”
A hybrid platform
Searching for a material that does this, Hu’s group has turned to an unusual source – a specialised kind of glass called chalcogenide. Though regarded as a fragile substrate, the glass material is widely used in the microelectronics industry and possess a high refractive index with strong optical confinement.
“The glass materials’ success in the microelectronics industry, coupled with their superior optical performance, point to chalcogenide glass micro-photonics as the natural next step of technology evolution,” Hu’s research group said in the paper ‘Chalcogenide glass micro-photonics: stepping into the spotlight’.
To overcome the inherently brittle nature of glass, Hu’s team has formed the stiff material into a springlike coil. The process has been compared with steel, which can be made to stretch and bend when formed into a spring.
“The architecture of this glass coil allows it to stretch and bend freely while maintaining its desirable optical properties,” said MIT in a press release. “Tests have shown that such spring-like configurations, made directly on a polymer substrate, can undergo thousands of stretching cycles with no detectable degradation in their optical performance.”
With this breakthrough, Hu believes his team now has a device that offers versatile light manipulation and transmission, and excellent mechanical flexibility. “I think what is new about our research here is that we developed this kind of hybrid platform that leverages most polymers and this inorganic material to take the best from both worlds,” he says.
While this breakthrough is significant, Hu believes there is still a great deal of work to be done. So far his team has demonstrated a device that allows them to transmit light and exhibit desirable mechanical properties, “but it doesn’t really carry any specific function per se”, he says.
“That is something we are working towards,” he adds. “We are trying to integrate this component, specifically either for biomedical applications or for high-speed data communication.”
It is estimated the technology could be commercially applied within the next two to three years. While Hu may spend most of his time in laboratories and at his desk, drafting academic papers, he says this is his real ambition. “My dream is to see some of the things I develop at MIT have an impact in the real world and benefit the society as a whole,” Hu said in an interview with MIT’s news service. “That’s why I am interested in entrepreneurship, to push them to application.”