A wireless induction heating system in a microfluidic device was devised for cell lysis to extract DNA and RNA from Escherichia coli. The thermal responses of nickel, iron and copper heating units were studied by applying an alternating magnetic field as a function of geometry of unit, strength of magnetic field, and kind of metal. Heating units were prepared by cutting metal film using a fiber laser, and the units were integrated into a microchannel system using a soft lithographic process. Variation and distribution of temperature on the surface of the heating units was observed using a thermographic camera and temperature labels. The amount of protein released from E. coli by thermal lysis was determined by protein concentration measurement. Hemoglobin released from red blood cells was observed using colorimetric intensity measurement. Extracted DNA was quantified by real-time polymerase chain reaction, and the profile was compared with that of a positive control of ultrasonically disrupted E. coli. The stability of RNA extracted by induction heating was quantified by the measurement of 23S/16S rRNA ratio and comparison with that by normal RNA extraction kit as a gold standard. A solid-shaped nickel structure was selected as the induction heating element in the microfluidic device because of the relatively small influence of geometries and faster thermal response.
The amount of protein extracted from E. coli and hemoglobin released from red blood cells by induction heating of the nickel unit in the microfluidic device was proportional to the strength of the applied magnetic field. The lysis of E. coli by induction heating was as effective as lysis of DNA by the ultrasonication method because the threshold cycle values of the sample were compatible with those of the positive control as measured by ultrasonication. Thermal lysis of E. coli by induction heating represents a reasonable alternative to a commercial RNA extraction method as shown by the comparative ratio of 23S/16S obtained. In this study, wireless induction heating in a microfluidic device achieved convenience, portability, and economic efficiency by wireless operation, easy fabrication, disposability, and treatment of a large number of cells.