Implantable CMOS imaging device with absorption filters for green fluorescence imaging

Abstract

Green fluorescent materials such as Green Fluorescence Protein (GFP) and fluorescein are often used for observing neural activities. Thus, it is important to observe the fluorescence in a freely moving state in order to understand neural activities corresponding to behaviors. In this work, we developed an implantable CMOS imaging device for in-vivo green fluorescence imaging with efficient excitation light rejection using a combination of absorption filters. An interference filter is usually used for a fluorescence microscope in order to achieve high fluorescence imaging sensitivity. However, in the case of the implantable device, interference filters are not suitable because their transmission spectra depend on incident angle. To solve this problem we used two kinds of absorption filters that do not have angle dependence. An absorption filter consisting of yellow dye (VARYFAST YELLOW 3150) was coated on the pixel array of an image sensor. The rejection ratio of ideal excitation light (490 nm) against green fluorescence (510 nm) was 99.66%. However, the blue LED as an excitation light source has a broad emission spectrum and its intensity at 510 nm is 2.2 x 10^-2 times the emission peak intensity. By coating LEDs with the emission absorption filters, the intensity of the unwanted component of the excitation light was reduced to 1.4 x 10^-4. Using the combination of absorption filters, we achieved excitation light transmittance of 10^-5 onto the image sensor. It is expected that high-sensitivity green fluorescence imaging of neural activities in a freely moving mouse will be possible by using this technology.