Relaxation of competing electromechanical couplings in murine artery

Piezoelectricity and pyroelectricity in biological tissues, which originate from oriented fibrous proteins with a polar axis, have long been suggested to play important roles in physiological functions. The possible manipulation of their polarity by external mechanisms, however, remains unsettled. We revisit this problem here using piezoresponse force microscopy (PFM) as the tool and the intima layer of murine artery as a model system. By carefully examining first and second harmonic piezoresponses at both selected points and through spatial mapping, we establish that electromechanical coupling probed by PFM is predominantly piezoelectric in the intima layer, while the quadratic effect makes only minor contributions. More importantly, we observe competition between the linear and quadratic effects after removal of DC biases applied to the sample surface, revealing not only interesting relaxation dynamics, but also highly asymmetric piezoresponse. Positive DC rotates dipoles in tropoelastin monomers away with reduced alignment, while negative DC aligns dipoles more leading to enhanced piezoresponse. The electric manipulation of biological polarity is thus demonstrated, with the relaxation time constant determined on the order of 0.1 s, much slower than classical ferroelectrics.