Review Development and application of piezoelectric materials for ultrasound generation and detection Amir Manbachi and Richard S C Cobbold Institute of Biomaterials and Biomedical Engineering Ultrasound Group, 164 College Street, Toronto, Ontario, Canada M5S 3G9 Corresponding author: Richard S C Cobbold. Email: Cobbold@ecf.utoronto.ca Abstract The piezoelectric effect and its converse are the primary means used in biomedical ultrasound for converting acoustical energy into electrical energy and vice versa. Piezoelectricity has found many bioengineering applications ranging from ultrasound imaging and therapeutics, to piezoelectric surgery and microelectromechanical systems, and to biomedical implants with associated energy harvesting. Because of its fundamental importance to the proper functioning of most medical ultrasound systems, it is important to gain a general understanding of the effect, the history of its development and from this, an appreciation of its limitations and advantages in the generation and detection of ultrasound. This article describes the historical evolvement associated with its use in relation to most medical ultrasound applications and is intended to serve as an introduction for non-expert readers. Keywords: Piezoelectricity, history, ultrasound, piezoelectric surgery Ultrasound 2011: 1–10. DOI: 10.1258/ult.2011.011027 Imagine when your cell-phone or laptop battery dies exactly when you need it the most! How convenient it would be if you could tap on a small surface on that device for few times and get some more power. This is an active field of research, thanks to the piezoelectricity phenomenon, which refers to the ability of some material to replace mechanical displacement to electrical energy and vice versa. The primary aim of this article is to describe piezoelectri- city, the history of its development and applications of the piezoelectric effect, particularly as related to medical ultra- sound. We begin with some definitions and by briefly explaining the principles underlying how piezoelectric media provide a means for converting energy between elec- tric and acoustic, and vice versa. This is followed by a broad account of the history of piezoelectric developments, a description of various phases it has gone through, together with brief mention of the individuals primarily responsible for this work. A major application of piezoelectric media over the past 50 years has been for the formation of medical diagnostic images with increasing contrast and res- olution. A sketch of this development is followed by a review of some examples of recent advancements in the field and potential future alternatives. While this review is fairly brief and oriented towards the non-specialist reader, more complete reviews are available in the literature. For example, see Hunt, 1 Mason 2 and Duck. 3 What is piezoelectricity and why is it called so? Piezoelectricity literally means: pressure-driven electricity. The online etymology dictionary 4 states that ‘piezo-’ comes from the Greek root ‘piezin’ meaning ‘to press, squeeze’; and ‘electricity’ comes from Elektron, the Greek word for ‘amber’, an ancient source of electric charge. In practice, it was observed that some solid materials produce electricity under pressure and vice versa, and exhibit slight movements when an electric field is applied. The first phenomenon is called ‘piezoelectric effect’ and the reverse is called ‘converse piezoelectric effect’. Some of these materials are shown in Figure 1. How does it work? (Mechanism) The separation of positive and negative electrical charges in many molecules results in what is known as a dipole moment and these are typically shown as a vector extend- ing from the negative charge to the positive charge. The dipole density for a medium (also known as the electric polarization, with units of C.m m 23 ) is the vector sum- mation of all the dipole moments per volume of the crystal. 5 In piezoelectric crystals, when a mechanical stress is applied (the crystal being compressed, twisted or pulled) the molecular dipole moments re-orient themselves and thus cause a variation in surface charge density and thus a voltage. 6 This effect is illustrated in Figure 2 for Ultrasound 2011: 1–10 Published online on 3 November 2011 Ultrasound, doi: 10.1258/ult.2011.011027 Copyright 2011 by the British Medical Ultrasound Society