Electromechanical Coupling of Murine Lung Tissues Probed by Piezoresponse Force Microscopy

Elastin is a major constituent of lung that makes up approximately 30% of lung's dry weight, and the piezoelectricity of elastin is expected to be exhibited in lung tissues. Because hundreds of millions of cycles of inhalation and exhalation occur in one's lifetime, such piezoelectric effect leads to hundreds of millions of cycles of charge generations in lung tissues, suggesting possible physiological significance. Using piezoresponse force microscopy (PFM), we show that the murine lung tissues are indeed piezoelectric, exhibiting predominantly first harmonic piezoresponse in both vertical and lateral modes. The second harmonic response, which could arise from ionic motions, electrochemical dipoles, and electrostatic interactions, is found to be small. The mappings of amplitude, phase, resonance frequency, and quality factor of both vertical and lateral PFM are also obtained, showing small fluctuation in frequency, but larger variation in quality factor, and thus energy dissipation. The phase mapping is confined in a small range, indicating a polar distribution with preferred orientation. It is also found that the polarity of the electromechanical coupling in lung tissues can be switched by an external electric field, resulting in characteristic hysteresis and butterfly loops, with a presence of internal bias in the polar structure. It is hypothesized that the piezoelectric charge generation during inhalation and exhalation could play a role in binding of oxygen to hemoglobin, and the polarity switching can help damp out the possible sudden increase in air pressure. We hope such observation of piezoelectricity and its polarity switching in lung lay the foundation for the subsequent studies of its physiological significance.