Fluorescence imaging of electrical activity in cardiac cells using an all-solid-state system

Tracking spatial and temporal determinants of cardiac arrhythmogenesis at the cellular level presents challenges to the optical mapping techniques employed. In this paper, we describe a compact system combining two nontraditional low-cost solutions for excitation light sources and emission filters in fluorescence measurements of transmembrane potentials, V_m, or intracellular calcium, [Ca²⁺]i in cardiac cell networks. This is the first reported use of high-power blue and green light emitting diodes (LEDs), to excite cell monolayers stained with V_m- (di-8-ANEPPS) or [Ca²⁺]i (Fluo-3) sensitive dyes. In addition, we use simple techniques for fabrication of suitable thin emission filters with uniform properties, no auto-fluorescence, high durability and good flexibility for imaging V_m or [Ca²⁺]i. The battery-operated LEDs and the fabricated emission filters, integrated with a fiber-optic system for contact fluorescence imaging, were used as tools to characterize conduction velocity restitution at the macro-scale. The versatility of the LEDs for illumination is further emphasized through (1) demonstration of their usage for epi-illumination recordings at the single-cell level, and (2) demonstration of their unique high-frequency light modulation ability. The LEDs showed excellent stability as excitation light sources for fluorescence measurements; acceptable signal-to-noise ratio and negligible cell photodamage and indicator dye photobleaching were observed.