Pulse modulation CMOS image sensor for bio-fluorescence imaging applications Jun Ohta ∗ , Takashi Tokuda ∗ , Keiichiro Kagawa ∗ , Masahiro Nunoshita ∗ , Sadao Shiosaka † ∗ Graduate School of Materials Science † Graduate School of Biological Science Nara Institute of Science and Technology (NAIST) 8916-5 Takayama, Ikoma, Nara 630–0101, JAPAN Email: ohta@ms.naist.jp Abstract— For wide dynamic range, compatibility with dig- ital circuits, and low-voltage operation, the pulse modulation technique is suitable for an implanted bioimage sensor. We demonstrate bio-fluorescence imaging of the hippocampus in a sliced mouse brain using a pulse modulation-based image sensor. The sensor architecture and system configuration are discussed. In addition, we develop an imaging device for implantation into a mouse brain in order to measure the neural activity in the hippocampus. The device is composed of a pulse modulation image sensor with 128×128 pixels and a fiber illuminator on a polyimide substrate. I. I NTRODUCTION The application of CMOS technologies to bio-technologies is an emerging field. The system-on-a-chip approach pro- vides a compact, high-sensitivity measurement tool, which enables on-site measurement. Several pioneering studies have investigated two-dimensional voltage sensing of neural activ- ity [1], [2]. Cell luminescence is another signal that has a high potential for use in bio-technology applications and has been studied extensively. Recently, the Stanford group has reported an image sensor for detecting bio-luminescence, a phenomenon in which light is emitted from biological tissues due to a chemical reaction, and thus in the absence of external excitation light [3]. Another type of luminescence that is used extensively in bio-technology is fluorescence. Fluorescence is a phenomenon in which light is emitted by dyed cells that are excited by an external UV source. Fluorescence is also widely used to inves- tigate cell activity. A fluorescence microscope is usually used to capture fluorescence images. Fluorescence microscopes are equipped with a cooled CCD camera and imaging optics. Although the fluorescence microscope is a powerful tool, it cannot be applied to in vivo cell measurement. Our goal is to develop an implantable image sensor for measuring in vivo cell activity, particularly for use in the study of the Long Term Potentiation (LTP) in the hippocampus. Although functional Magnetic Resonance Imaging (fMRI) and Positron Emission Tomography (PET) can acquire in vivo images, the resolution of these techniques is not sufficient for such applications. Other potential applications of the newly developed chip include DNA microarray measurement and protein chips. For the implanted sensor, we introduce a pulse modulation photosensor, which provides high sensitivity, a wide dynamic range, and low-voltage operation [4]. We herein describe the architecture and fundamental characteristics of the newly developed sensor and demonstrate fluorescence imaging of a dyed mouse brain slice. Finally, we describe the structure of the implantable sensor, in which a molded sensor chip and a fiber illuminator are integrated on a polyimide substrate. II. PULSE MODULATION PHOTOSENSING Target materials of small volume (e.g., neural cells or DNA spots) require a bioimaging sensor that has a high sensitivity and a wide dynamic range. In conventional CMOS image sensors, the voltage change of photodiodes (PD) in each pixel caused by photocarrier generation is measured as light intensity. In order to detect extremely low intensity light, long-time exposure or accumulation is effective. However, one disadvantage of long-time accumulation is that the detection limit in the high-intensity region decreases. We propose the application of a pulse modulation measurement scheme for bioimaging sensors. In the pulse modulation method, the light intensity is measured not as a voltage drop, but rather as the time required for a specific voltage drop, as illustrated in Figure 1. For the time measurement, a pulse width modulation (PWM) or time-to-saturation photosensor is realized [6], [5], [7], [8]. In addition, for feedback of the output to the reset transistor of the PD, a pulse frequency modulation (PFM) photosensor is realized [9], [10], [11], because there is no fundamental difference between PWM and PFM, [13], [14], [15]. A PFM photosensor has been proposed to be applied to retinal proshtesis devices [10], [12]. Fig. 1. Concept of pulse modulation The pulse modulation scheme provides the advantage of 3487 0-7803-8834-8/05/$20.00 ©2005 IEEE.