In Vivo Evaluation of a Subcutaneously Injectable Implant with a Low-Power Photoplethysmography ASIC for Animal Monitoring

Figures

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  Figure 1. Block diagram of the subcutaneous PPG system where the blocks in solid boxes have been integrated using the ON Semiconductor 0.5-μm CMOS process and the blocks in dashed boxes have been implemented with smaller COTS components.

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  Figure 2. Micrographs of the two ASICs (before being wire bonded to the PCB) and a photo of the overall system. (a) The delimited die area measures approximately 1.3 × 0.7 mm2 and represents the transimpedance amplifier, current digital-to-analog converter, and switched integrator. (b) The delimited die area measures approximately 0.7 × 0.9 mm2 and represents the FSK transmitter. (c) Picture of the injectable optical probe with an OSRAM BioMon sensor in the glass encapsulation (top) and the two sides of the fully assembled injectable PPG system.

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  Figure 3. (a) Schematic of the PPG analog front end. The inset shows the programmable options for choosing among the built-in Rf-Cf value pairs. (b) Transistor-level schematic of the OTAs used in the AFE.

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  Figure 4. Simulation results of the PPG readout circuit. (Blue) Output of the transimpedance amplifier. (Orange) Output of the switched integrator. (Yellow) Control signal for switch S1, which is also used for the LED driver.

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  Figure 5. (a) The control signals generated by the microcontroller repeating every 5.3 ms. The inset shows SI_RST (blue) and SI_EN (orange) used for operating switches S2 and S1, respectively. (b) Measured output of the switched integrator showing the instant when switch S2 resets the feedback capacitor and switch S1 enables the integration of the photocurrent immediately after that.

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  Figure 6. Screenshot of the GRC interface showing the different blocks that comprise the software-defined FSK receiver.

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  Figure 7. Distribution of power consumption drawn from a 2.5 V source by the PPG readout system.

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  Figure 8. Noise at the output of the switched integrator for (a) a photocurrent of 3 μA, and (b) a photocurrent of 10 μA, showing the effect of the PD shot noise.

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  Figure 9. (a) Relative power spectral density at the front end of the SDR receiver. (b) Two superposed plots of power spectral density measurements recorded with a real-time spectrum analyzer to obtain the absolute levels of the received signal.

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  Figure 10. (a) Histograms of received binary signal levels showing the actual distribution for the symbols ‘0’ and ‘1’. (b) Gaussian fit for probability density functions (P0 and P1) of the received signal levels for the two symbols.

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  Figure 11. Visual description of the two experimental setups. (a) Wireless transmission was tested with the handheld capsule transmitting PPG data to the laptop through the dipole antenna and the Airspy R2 receiver. (b) In vivo performance was tested on an anesthetized rat with the optical probe injected in the thoracic area. The probe was connected to the ASIC test board and the AFE4490 evaluation board alternatively and the data was displayed on an oscilloscope.

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  Figure 12. (a) Subcutaneous injection procedure demonstrated with a cat cadaver. (The picture has been republished from [13] with authors’ permission ©2014 IEEE.) (b) An anesthetized rat with the subcutaneously injected optical probe temporarily wire-connected to the commercial AFE4490 system for benchmarking and validation.