1.  Field of Invention
The present invention relates to a piezoelectric sensor for measuring shear and compression. 
2.  Brief Description of the Prior Art
A typical soft-material/tissue mechanical property tester requires an external force (displacement) applicator and an external displacement (force) gauge.1,2 The external force (displacement) generator may be hydraulic or piezoelectric and the external displacement gauge (force gauge) may be optical or piezoelectric.  Regardless of the mechanism of force/displacement generation and displacement/force detection, typical tissue/soft-material mechanical testing is destructive and it requires specimens cut to a disc shape to fit in the tester.  In addition, a compressive elastic modulus tester e.g., an Instron is also different from a shear modulus tester, e.g., a rheometer.3 Currently, no single instrument measures both the compressive Young's modulus and the shear modulus. 
Over the past decades, many techniques have been developed to image tissue structures.4-9 Computer Tomography (CT)10 takes 360 degree X-ray pictures and reconstructs 3D tissue structures using computer software.  Magnetic Resonance Imaging (MRI)11 uses powerful magnetic fields and radio waves to create tissue images for diagnosis.  Ultrasound (US)12 transmits high-frequency waves through tissue and captures the echoes to image tissue structures.  T-scan (TS)13 measures low-level bioelectric currents to produce real-time images of electrical impedance properties of tissues.  Ultrasound elastography (UE)14 evaluates the echo time through tissues under a constant mechanical stress and compares it to that of the same tissue when unstressed.  A tissue strain map is then obtained, from which an image of 2D elastic modulus distribution is created by inversion techniques.  Tactile imaging tools using array pressure sensors probe spatial tissue stiffness variations.  However none of these techniques have the ability to probe tumor interface properties. 
The detection of abnormal tissue as cancerous growth requires improvements in screening technologies.  The key to successful treatment lies in early detection.  Imaging techniques such as mammography in breast cancer screening, detect abnormal tissue by tissue density contrast.  Mammography is the only FDA approved breast cancer screening technique, which has a typical sensitivity of 85% that decreases to 65% in radiodense breasts.10 However, in these screening processes there is a high incidence of false positives.  In fact, only about 15-30% of breast biopsies yield a diagnosis of malignancy.  Changes in tissue stiffness have increasingly become an important characteristic in disease diagnosis.  It is known that breast cancers are calcified tissues that are more than seven times stiffer than normal breast tissues.11-14 Thus, contrasting levels of stiffness within the breast may indicate cancerous tissue.  Similarly, plaque-lined blood vessels are also stiffer than normal, healthy blood vessels. 
The examining physician may detect abnormal tissue stiffness by palpation by taking advantage of the fact that cancerous tissues are stiffer than surrounding normal tissues under compression.  Thus, palpation has been a useful tool for experienced physicians to diagnose breast and prostate cancer.  However, palpation is not quantitative and depends solely on the experience of the individual physician.  So there remains a critical need to improve cancer-screening technology to reduce the number of unnecessary biopsies.