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Abstract Sensors based on the detection of small resistance variations are universally recognized as piezoresistive. Being one of the simplest, most common and most investigated classes of sensors, continuous efforts are focused on creating improved devices with higher performance that can be used in many commercial and non–commercial applications (e.g. evaluation of strain, pressure, acceleration, force etc.). Consequently, despite the fact that more than 150 years have passed since the discovery of the piezoresistive effect in some classes of metals and semiconductors, the development of such sensors remains interesting and topical. Moreover, with the advent of second–generation robotics, research on piezoresistive sensors has undergone a massive increase. This paper aims to be a short, self–consistent vademecum which would be useful to researchers and engineers, since it focuses on the fundamentals of theory, materials, and readout–circuit design pertinent to the most recent developments in the field of piezoresistive sensors. [ABSTRACT FROM AUTHOR]

• Transducer fabricated by a thin PVDF film wound along its longitudinal axis in a logarithmic spiral shape. • Contiguous curved films that vibrate at specific frequencies, depending on the radius of the related portion of the spiral. • Wide band ultrasonic transducer in the range 30–95 kHz. • Omni–directionality on both the horizontal and vertical planes as ultrasonic receiver in the range between 30–95 kHz. This article proposes an ultrasonic sensor inspired by the shape of a bat's cochlea, which is part of one of the most sophisticated echolocation systems to be found among mammals. In this case, polyvinylidene fluoride (PVDF) was used for the fabrication of the sensor, which is manufactured by folding a sheet of the piezopolymer according to a logarithmic spiral geometry. It is characterized by a very broad frequency band, in agreement with numerical simulations and impedance analyses, and has a uniform radiation pattern both on the horizontal and vertical planes. The sensitivity of the sensor was evaluated in the range between 30–95 kHz, a well–suited frequency band for most bio–systems. [ABSTRACT FROM AUTHOR]