The microgravity conditions of space travel create unique physiological demands on the astronauts’ skeletal structures, resulting in a reduction in bone strength from restructuring of the micro-architecture and loss of key minerals. An ultrasound system was developed that is capable of quantitatively correlating a series of measurement parameters to the physiology. The Nautilus transducer is a spiral-wrapped ultrasound transducer fabricated from piezoelectric Mn:PIN-PMN-PT single crystal that uses micromachining to take advantage of unique resonance modes within the crystal. The combination of integration of a single crystal and use of multiple resonances provided a bandwidth superior to commercial devices with the capacity for high sensitivity.
The focus of this work was to build on the Nautilus technology, to show the additional capability of the transducer over conventional technologies, and to develop the library of knowledge correlating ultrasound to bone physiology and the pathology of osteoporosis. Thedirect application of the technology is for a bone characterization tool to lead towards countermeasures. The transducer could also be used for possible bone loss therapy, or for nondestructive testing (NDT) of space structures to evaluate micro-crack progression over longduration missions.
The device consists of a pattern of multi-resonance arrays with common electrodes that is wrapped around a central hexagonal mandrel such that the six radiating faces of the resonators are coplanar and can be simultaneously applied to the sample to be measured. The fabrication process begins with a Mn:PIN-PMN-PT piezocrystal wafer. Micromachining is used to etch the resonator pattern into the crystal. The resonators consist of rectangular bars of varying length and width, but common thickness. After machining, the pattern is filled with epoxy to create a composite structure, and the faces are lapped to remove the crystal substrate and excess epoxy. Electrode material is applied to the faces. Silver wires are placed in contact with the electrode surfaces and held in place with adhesive tape. The composite arrays spotting to the six faces are then secured in a housing with urethane potting material, and the silver electrode wires are bonded to a standard connector on the external surface of the housing. The central mandrel where the hexagonal shape is wrapped may be inert material, or it may be an ultrasonic composite that adds additional bandwidth to the device.
The device operates as transmitter and receiver of a source of acoustic energy. It is operated by connection to an electronic system capable of both providing an excitation signal to the transducer and amplifying the signal received from the transducer. The excitation signal may be either a wide-bandwidth signal to excite the transducer across its entire operational spectrum, or a narrow-bandwidth signal optimized for a particular measurement technique. The transducer face is applied to the skin covering the bone to be characterized, and may be operated in through-transmission mode using two transducers (for example, in measuring the signal transmitted through the calcaneus), or in pulse-echo mode (for example, measuring the signal reflected from vertebrae).
The transducer is based on Mn:PINPMN-PT piezoelectric single crystal. As compared to the commonly used piezoceramics, this piezocrystal has superior piezoelectric and elastic properties, which results in devices with superior bandwidth, source level, and power requirements. The advantages of piezocrystal have been demonstrated in devices ranging from electromechanical actuators to sonar transducers and is becoming the widespread material of choice for medical ultrasound imaging.