A wearable microscope for in vivo investigations of human sarcomere lengths and contractile dynamics [electronic resource]
Keywords: Mechanical Engineering

ABSTRACT Sarcomeres are the fundamental force generating units in skeletal muscle. Biopsies of human muscle and animal models of neuromuscular diseases suggest that various neuromuscular diseases exhibit altered sarcomere structure and contractile dynamics. However, there is no minimally invasive tool capable of measuring individual sarcomere lengths and dynamics in humans that could provide much needed information for the diagnosis, monitoring, and treatment of diseases. This dissertation introduces the first clinically deployable sarcomere imaging system. The system includes a wearable microscope and minimally invasive microendoscope probe to provide direct visualization of sarcomeres in nearly any muscle in the body. The invention of several novel optical and mechanical features was necessary to enable successful sarcomere imaging. Our multifunction microendoscope probe utilizes small diameter gradient refractive index (GRIN) lenses to access deep muscle minimally invasively and collect images of the underlying sarcomeres. We employed optical strategies such as infinity corrected GRIN lenses with telecentric imaging that are the first of their kind for GRIN lens microendoscope probes. We further designed a remote focusing mechanism and associated optical arrangement that yield near constant resolution, magnification, and power at the specimen through the entire focal range. We used our "sarcoscope" to produce the first images of sarcomeres in the posterior deltoid, brachioradialis, vastus lateralis, tibialis anterior, medial gastrocnemius, and soleus in multiple human subjects through the intrinsic generation of second harmonic signals. Subjects wear the microscope head while positioned in a range of postures, permitting systematic analysis of sarcomere lengths at different body postures. We demonstrated this feature by examining the mean sarcomere lengths in the medial gastrocnemius and soleus muscles in subjects positioned in a well-defined posture. Measurements obtained with the sarcoscope were within 3% and 15% of measurements obtained from cadavers in the medial gastrocnemius and soleus, respectively. Our sarcoscope employs a multifunction microendoscope probe that can electrically stimulate muscle. We utilized electrical pulses delivered through the microendoscope probe to record the first images of contracting sarcomeres in live human subjects. We used our microscope to compare twitch contraction timing characteristics in the medial gastrocnemius and soleus muscles in three subjects. We observed a wide range of contraction times in the medial gastrocnemius (54 -- 162 ms) that spanned times typical of fast and slow type fibers tested from human biopsies. The contraction times in the soleus were slower in all three subjects (136 -- 224 ms), and the slowest times we recorded were slower than previously reported values measured from human biopsies or in bulk muscle. We further studied the frequency dependent fusion of separate twitch contractions in the posterior deltoid muscle of a single subject. With a contraction time of 70.65 ms, we observed the beginning of fusion at 3.3 Hz of stimulation, and near complete fusion at 10 Hz. The mobile sarcomere imaging system described in this dissertation represents an important technological advancement. The ability to measure sarcomere lengths and contractile dynamics in nearly any muscle from live humans will have vast clinical implications and lead to a wide range of in vivo discoveries in healthy and diseased muscle that substantially impact human health and movement.