New design lets mini-microscope observe cells in vivo

Two eyes are better than one. Or more precisely, two lines of sight are better than one. That's the word from researchers at the Stanford School of Medicine, who have applied this adage to the design of a tiny microscope that is about the size of an M&M.

"This type of microscope can help us study the biology of cells in their natural environment, reveal early steps in disease development, assess treatment response and determine whether healing is occurring," said Thomas Wang, MD, PhD, an instructor of medicine, who has been involved in the design of the scope. "This is currently hard to do without a biopsy, which often destroys the tissue."

The microscope that Wang has been working on is confocal, meaning that it uses the convergence of two focal points. Such microscopes are usually big and bulky and are most commonly used in the lab to look at cultured cells and layers of tissues. The new design enables miniaturization, letting doctors look at tissue inside a living body.

"When you're doing surgery, it's much better to have smaller tools," Wang said. Tiny microscopes might be used at the tip of an endoscope to view such hollow organs as blood vessels, the gastrointestinal tract and the bladder.

Christopher Contag, PhD, associate professor of pediatrics and principal investigator on this project, said, "In the future, this type of tool could be implanted in the body and, in conjunction with molecular probes, be used to watch how cells communicate through 'talking' and 'touching' each other." Contag, whose PhD is in microbiology, has been developing the tools to watch biology in the whole living animal and teamed up with Wang, whose PhD is in electrical engineering, to assemble a team of investigators to advance the emerging field of in vivo microscopy.

The work was supported by National Cancer Institute funds earmarked for projects that might be used soon for patient care. Wang and Contag joined with Gordon Kino, PhD, professor emeritus of electrical engineering; Olav Solgaard, PhD, associate professor of electrical engineering; chemists at Vanderbilt University; pathologists at the University of Florida, and the firm OBTI to develop the technology.

Thanks to the innovative, "dual-axes" design, the new microscope can focus through relatively thick layers of tissue. Most confocal microscopes have a single-axis design. With single-axis microscopy, the light passes from the light source and into the detector along the same path and through the same lens. While this type of imaging is fine for cells and tissues removed from the body, it is not as useful for studying cells inside the body.

By contrast, the dual-axes design uses separate paths and lenses for the light coming into the tissue being imaged and the light coming out of it. This design lets the microscope focus anywhere in the tissue cross section, much as having two independently focusing eyes allows us to see depth.

Two axes will also make it possible to shrink future microscopes down to millimeter sizes without sacrificing performance. The team's microscale engineering technology, pioneered by Solgaard, has already allowed the current version of the microscope to span just 1 centimeter across.

A key to the new microscope's tiny size is micro-electro-mechanical systems, or MEMS, technology, developed a decade ago through a collaboration between the School of Medicine and the School of Engineering. Like computer chips, MEMS devices often have electronic circuits etched on their silicon surfaces. But unlike computer chips, they also have moving parts.

The team used MEMS to construct the microscope's tiny heart: its mirror. It consists of a silicon wafer polished to a reflective sheen and minuscule actuators to adjust the mirror's position.

The scope could facilitate new kinds of research, and offer tremendous advantages in the clinic. For example, surgeons could use it to help identify the borders of aggressive tumors, easing the removal of only the diseased cells. Surgeons could also use the microscope to ensure all the cancerous cells are removed, reducing the risk of a relapse.

One application is to use the centimeter-wide microscope to assist with a specialized micrographic surgery technique for removing skin cancer lesions. The procedure is geared to minimize damage to nearby healthy skin, a challenge the microscope should be ideally suited to help with, Wang said.

An even smaller prototype - about 5 millimeters in diameter, or half the size of the current one - is the team's current project. This one will be small enough to fit into an endoscope to study diseases of the colon and esophagus, including colorectal cancer and a condition known as Barrett's esophagus.

Wang, who is also a practicing clinical gastroenterologist, enjoys seeing how tools such as the dual-axes MEMS microscope can help those who need it most: his patients.

"Few people have the opportunity to see it from both sides like this," Wang said. "I'm very lucky."