Apr 29, 2009
Tapping into bioelectricity
Coupling between electrical stimuli and mechanical motion is ubiquitous in biological systems and inorganic materials alike. In the realm of inorganic materials, a wide range of piezo- and ferroelectric materials are used for MEMS, FeRAM, SONAR and medical imaging applications. Understanding electromechanical coupling and polarization dynamics in these materials requires the ability to probe electromechanical activity at the nanometre level, a feat that is well served by piezoresponse force microscopy, a contact-mode atomic force microscopy technique generally associated with large loading forces. Soft, organic materials, including many biomolecular and cellular systems, however, are typically imaged in non-contact or intermittent (tapping) contact modes in liquid environments to minimize surface damage. While coupling between electrical signals and mechanical motion in biological systems is essential to living systems and many biopolymers are piezoelectric, little is known about the effect of electrical stimuli on the functionality of biological systems, particularly at the nanoscale.
Recently, researchers at UCD, ORNL, UCSF and Asylum Research investigated the feasibility of intermittent contact mode piezoresponse force microscopy (PFM) in a liquid environment based on simultaneous mechanical and electrical probe modulation to image electromechanical coupling in a mode consistent with reduced surface damage of soft materials.
The measured signal in a PFM experiment is generally a combination of electromechanical and electric forces and, at first glance, the signal in an intermittent contact mode version of PFM will be dominated by long-range electric or electrostatic forces. The electromechanical contribution to the displacement signal is expected to dominate only if (1) the electrical modulation frequency corresponds to the contact resonance, (2) the electrostatic interactions are effectively screened and (3) imaging is performed at small amplitude set-points, maximizing the residence time of the probe in contact with the surface.
The researchers reported in Nanotechnology that these conditions can be met by imaging at frequencies corresponding to the first contact resonance in liquid. The ions in the liquid are found to effectively screen the electrostatic interactions, allowing contrast consistent with the electromechanical signal to be obtained on model ferroelectric materials and piezoelectric tooth dentin.
The authors noted that further control of the screening can be implemented through the choice of solvent and that additional improvements may be attainable with the use of shielded probes, which allow precise control over the application and measurement of local response in solution. The ability to measure contrast consistent with electromechanical response in an intermittent contact mode provides a pathway to measure electromechanical coupling in a wide range of soft biosystems in liquid environments.
About the author
The work was performed at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory and was supported by the ORNL LDRD program and NIH/NIDCR Grant RO1-DE16849. Stefan Habelitz is a professor in the Division of Biomaterials and Bioengineering of the Department of Preventive and Restorative Dental Sciences at the University of California, San Francisco. Roger Proksch is co-founder and president of Asylum Research. Stephen Jesse is a staff member at CNMS and Sergei Kalinin is co-theme leader for functional imaging on the nanoscale at CNMS. Brian Rodriguez is a lecturer in nanoscience and member of the Nanoscale Function Group at the Conway Institute of Biomolecular and Biomedical Research at University College Dublin.