Diagnosing a muscular disorder, disease or infection often requires a sample of the tissue to be extracted, but these biopsies can be painful and difficult to perform. Researchers at the Rehabilitation Institute of Chicago (RIC) have developed a less invasive alternative that uses a thin fiber optic probe to quickly scan and measure the health of muscle tissue. For the first time, the team has now tested the system on living muscles.

Sarcomeres are the bundles of filaments that make up muscle fibers, and are the main component that allows muscles to expand and contract. Measuring the length of sarcomeres, and how they move, is currently one of the most accurate methods for diagnosing muscle impairment, but it's a fiddly and invasive process.

Laser diffraction is one way to size up sarcomeres, by shining a beam through the muscle fiber and measuring how the light bounces back. But according to the RIC researchers, it's a surgical process that can actually damage the tissue, and it can't measure how the sarcomeres change during movement. The study also points out that other techniques, like microendoscopy – where optical fibers are used to obtain microscopic images of tissue – are less precise and can be too complicated for regular clinical use.

The researchers' new method, which they call resonant reflection spectroscopy (RSS), uses a principle similar to laser diffraction, but measures the sarcomere length less invasively, more accurately, and can do so while the muscles are moving. A fiber optic probe, measuring just 250 micrometers wide, is inserted into the muscle, running parallel to the sarcomeres. The probe sweeps a laser across several millimeters of muscle and measures the pattern of how the light bounces back, which allows the length of the sarcomeres to be calculated.

The fiber optic sarcomere length profiler is a minimally invasive alternative to a biopsy, scanning the muscle with a sweeping laser light and studying the returning signals to calculate the length of sarcomeres(Credit: Young et al./Biophysical Journal 2017)

"This approach enables measurement of previously unobtainable muscle properties by combining advances in telecommunications technology with a deep understanding of muscle structure, biomechanics, and pathology," says Richard Lieber, senior author of the study. "This bioengineering innovation will permit new studies of human muscle function and pathology and permit efficacy testing of muscle treatments."

In the first tests of RSS in living muscle tissue, the researchers tested the technique on rabbits. In a fraction of a millisecond, the probe scanned 4,200 sarcomeres in several millimeters of muscle. Additionally, the device was able to detect tiny changes in the sarcomere length on the scale of nanometers as the rabbits' leg muscles moved from a passive stretch to contractions induced by electrical signals.

"Our findings demonstrate a new method to measure protein-scale interactions during muscle movement," says Lieber. "To our knowledge, this method achieves sample sizes, resolutions, and compatibility with human movements that no other current or proposed technique can match."

The researchers believe that, eventually, the technique could be used to make 3D models of sarcomeres, allowing scientists to more clearly visualize problems in muscles and plot better courses of treatment. In the meantime, the team is continuing to develop the device, to make it more affordable and effective for use in clinics.

"We plan to use this technology in both fundamental and clinical studies of human movement and movement disorders," says Lieber. "We hope that these new experiments lead to better understanding and maintenance of human muscle health."

The research was published in the Biophysics Journal.

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