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This paper reports a novel generation of CMOS stress mapping chips comprising 32 square field effect transistors (FET) with four source/drain contacts (piezoFETs) exploiting the shear piezoresistive effect in n-type (NMOS) or p-type (PMOS) inversion layers. The sensor chips with a total die area of 2.5 x 2 mm2 are integrated with analog circuitry and digital logic. When exposed to homogenous shear or normal stress, all 32 integrated stress sensors show a linear response in excellent agreement with theoretical predictions and exhibit identical stress sensitivities. Piezo-FETs fabricated as separate devices are characterized with respect to stress sensitivity, intrinsic offset, and noise behavior. Stress sensitivities are enhanced by incorporating a central hole into the piezo-FETs. Sensitivities of -448 iV/(V MPa) and 477 iV/(V MPa) were measured for NMOS and PMOS devices, respectively.

Functional electrical stimulation (FES) is a technique that stimulates nerves by electrical charge, but carries the risk of charge accumulation, voltage pile-up, electrode corrosion and finally tissue destruction. Using biphasic stimulus current pulses, the main transferred charge is compensated by reversing the current direction. However, due to PVT variations in integrated circuits mismatch in the biphasic waveform always occurs. Charge balancing (CB) has thus become an integral part of FES to ensure safe chronic stimulation [1]. This paper presents a CMOS integrated, 22V compliant active charge balancer accomplishing "cause and consequence" based compensation, ensuring both instantaneous and long-term balanced condition. By inherently incorporating an idle window, the consequence-based compensation Inter-Pulse Charge Control (IPCC) offers a self-regulated and reference-free removal of charges without interfering with naturally evoked action potentials. Compared to other approaches, its simple architecture at 1.4μA current consumption and its ability of compensating a stimuli mismatch of up to ±100% with respect to the idle window, make it suitable as independent balancer offering autonomous CB for arbitrary and even monophasic neural stimulators. The main feature of the cause-based Offset Compensation (OC) is up to ±36% biphasic stimuli mismatch correction via an adjustable PI-controller providing an overall G m of 1.5nS at only 860nA consumption. Figure 22.6.1 illustrates a neural stimulation setup with a biphasic stimulation current source, the presented active CB circuits, and the equivalent circuit model of an electrode-tissue interface [1]. An exemplary curve of the electrode voltage V E around common mode (CM) during stimulation is depicted, in which an unbalanced stimulus consecutively raises the remaining DC potential of V E . To prevent irreversible electrochemical reactions a voltage safety window is defined that must not be exceeded [1]. The illustration of electrode current I E shows that the consequence-based CB approaches aim to reduce the electrode potential after each stimulus, whereas the cause-based methods try to symmetrize the anodic and cathodic charge of the stimulus. The latter typically utilizes long-term integration of the electrode voltage [1,2], but might show a settling process with overshoots during startup. For reliability reasons, most of today's certified medical devices are equipped with passive CB, mostly DC-blocking capacitors, despite their disadvantage in size, settling time, and uncontrolled balancing. Active consequence-focused compensation based on the injection of a fixed amount of charge by short current pulses is proposed in [1,3,4]. However, the ensuing possibility of "an unwanted neural stimulation has not been proven yet. [...] the maximum amount of mismatch charge, which can be compensated, depends on the adjusted charge per pulse and the number of pulses allowed over time" [1], demanding a precise configuration of the system during long-term trials.The main advan...