Introduction:: This investigation explores the possibility of using laser light to non-invasively monitor muscle tetanus by sending the laser light through the skin and detecting changes in the characteristics of the light scattered off the underlying muscle tissue. Such a capability would be highly useful for a number of applications, including muscle rehabilitation for patients who have suffered from strokes, atrophy, sports injuries, and other muscle disorders, and computer-control of muscle movement, which may help paralyzed individuals regain usage of their limbs or amputees to control electrically-powered artificial prostheses. Two processes were studied: detection of changes during muscle tetanus of the polarization of the backscattered light or changes in the refractive index. For the latter, initial ex-situ results show an optical response to the presence of Ca++ ions in saline solution. A possible complex formation may explain the large light deflection, which deviates from the free carrier dispersion effect theory. An attempt to apply optical probing to detect muscle activity in earthworms was also performed. Preliminary results indicate light deflection correlated with the earthworm movement. Furthermore, we attempted to measure birefringence changes due to the muscle movement of the earthworm. The results indicated an apparent polarization change; however, a more reliable method with a stronger signal is still needed to confirm the effect.


Materials and Methods:: In an earlier paper [10], we proposed two mechanisms to quantify muscle activity. The first is to detect refractive index changes caused by the creation of Ca²⁺ ions in the muscle sarcomeres [11]. The subsequent ion concentration change produces a refractive index change which further alters the diffracted optical beam (Drude-Zener effect) [12]. The second is based upon birefringence changes along the muscle tissue filament axis, which induces a polarization rotation of a polarized laser beam [9], [13]. A more detailed description of the theories can be found in our previous paper and will not be repeated here [10].

3. Experimental Setup

The experimental setup was designed to study the optical response of Ca⁺⁺ ions in a saline solution. 0.15M NaCl was used as the saline solution. Different concentrations of CaCl₂ were made using CaCl₂.2H₂O and DI water. The NaCl solution was put into a cuvette, and CaCl₂ solution was added to the NaCl solution. A laser beam (625 nm wavelength) passed through the NaCl solution, which was detected from the other side by an optical detector. The detector incorporated a modulator to generate a square pulse, and the detector was connected to an oscilloscope and a computer. Figure 1a shows the overall system setup. To increase the sensitivity of the light detection, a convex lens was added in between the sample and the modulator, as shown in Figure 1b. The setup for the birefringence experiment is depicted in Fig.3a.






Results, Conclusions, and Discussions::

Results and Discussion

Titration Result

Figure 4 shows the result of the CaCl₂ titration in the setup shown in Fig. 1a. The initial increase of CaCl₂ results in a linear increase in light intensity, which seems to be consistent with the literature [14], [15]. This is expected from the free carrier dispersion effect or optical absorption from the dispersion equation. In further analyzing the data,

Earthworm Refractive Index Results

The earthworm experiment was set up as shown in Fig. 1b, with the sample replaced by an earthworm inside a case. With the earthworm tranquil most of the time, the earthworm’s body blocked the laser beam (we used a 630-nm laser). The signal pattern resembles the one from the earthworm’s end and is shown in Fig. 6a. The spikes sync with the optical modulator frequency, indicating light passing through. Figure 6b is the experiment control without any worms.

Birefringence Experiment Result

Figure 7 shows examples of the PMT signal obtained when using the pulsed Nd: YAG laser beam (1064 nm) probing the earthworm. In Fig. 7a, the earthworm is stationary, and the peak signal is approximately 1.08 V. In Fig. 7b, the earthworm is moving, and the peak signal decreases to approximately 0.63 V

Conclusion

Experiments have been conducted to demonstrate the principle of optical probing in muscle activity. Preliminary results show light deflection and possible birefringence effects correlated with the worm movement. However, more rigorous experiments are needed to validate the optical responses.




Acknowledgements (Optional): : Acknowledgments The authors would like to thank the University of Washington - Bothell (UWB) STEM staff and administration's help, and the STEM division for letting us use the facility.




References (Optional): :
References



1. Z. Puthucheary, S. Harridge, and N. Hart, “Skeletal muscle dysfunction in critical care: Wasting, weakness, and rehabilitation strategies,” Critical Care Med. 38, S676-S682 (2010).

2. D. M. Needham, A. Truong, E. Fan, “Technology to enhance physical rehabilitation of critically ill patients,” Critical Care Med. 37, S436-S441 (2009).

3. T. G. Sugar, J. He, E. J. Koeneman, J. B. Koeneman, R. Herman, H. Huang, R. S. Schultz, D. E. Herring, J. Wanberg, S. Balasubramanian, P. Swenson, and J. A. Ward, “Design and control of RUPERT: A device for robotic upper extremity repetitive therapy,” IEEE Trans. Neural Sys. Rehab. Eng. 15, 336-346 (2007).

4. C. G. Burgar, P. S. Lum, P. C. Shor, and H. F. Machiel Van der Loos, “Development of robots for rehabilitation therapy: The Palo Alto VA/Stanford experience,” J. Rehab. Res. Dev. 37, 663-673 (2000).

5. H. Hummelsheim, M. L. Maier-Loth, C. Eickhof, “The functional value of electrical muscle stimulation for the rehabilitation of the hand in stroke patients,” Scandinavian J. Rehab. Med. 29, 3-10 (1997).

6. T. Lloyd, G. De Domenico, G. R. Strauss, and K. Singer, “A Review of the use of electro-motor stimulation in human muscle,” Australian J. Physiotherapy 32, 18-30 (1986).

7. A. M. Dollar and H Herr, “Lower extremity exoskeletons and active orthoses: Challenges and state-of-the-art,” IEEE Transactions on Robotics 24, 144-158 (2008).

8. M. Belau, M. Ninck, G. Hering, L. Spinelli, D. Contini, A. Torricelli, and T. Gisler., “Noninvasive observation of skeletal muscle contraction using near-infrared time-resolved reflectance and diffusing-wave spectroscopy,” J. Biomedical Optics 15, 057007 (2010).

9. M. Irving, “Birefringence changes associated with isometric contraction and rapid shortening steps in frog skeletal muscle fibres,” J. Physiology 472, 127-156 (1993).

10. L. K. Lam and W. D. Kimura, “Computer-Interfacing with Noninvasive Muscle Activity Diagnostic,” in Proceedings of 22nd International Conference on Human-Computer Interaction (HCI), Copenhagen, Denmark, July 19-24, 2020.

11. S. Gehlert, W. Block, and F. Suhr, “Ca2+ -dependent regulations and signaling in skeletal muscle: from electro-mechanical coupling to adaptation,” Int. J. Mol. Sci. 16, 1066-1095 (2015).

12. K. Seeger, Semiconductor Physics, (Springer-Verlag, NY, 1985).

13. R. C. Haskell, F. D. Carlson, and P. S. Blank, “Form birefringence of muscle,” Biophys. J. 56, 401-413 (1989).

14. C. C. Wang, J. Y. Tan, L. H. Liu, “Wavelength and concentration-dependent optical constants of NaCl, KCl, MgCl2, CaCl2, and Na2SO4 multi-component mixed-salt solutions.” Applied Optics 56(27), 7662-7671, 2017.

15. T. G. Mayerhofer, A. Dabrowska, A. Schwaighofer, “beyond Beer’s law: why the index of refraction depends (almost) linearly on Concentration.” ChemPysChem 21(8), 707-711, 2020.

16. N. Manonmani, C. K. Mahadevan et. al., “Growth and Studies of the New Crystal Formed with NaCl and CaCl2.” Materials and manufacturing processes 22(3), 388-391, 2007.

17. D. B. Sirdcshmukh, L. Sirdchmukh, K.G. Subhadra, Alkali Halides – A Handbook of Physical Properties, Springer Series in Materials Sciences, vol. 49, Springer, Berlin, 2001.

18. H. Kimura, “Impurity effect on growth rates of CaCl2.6H2O crystals.” Journal of Crystal Growth 73, 53-62, 1985.