Development and Characterization of Micro Needles Based Electrode for Biopotential Measurement

Abstract

Bio potential signals of human brain activity are used to monitor the health of an individual. The electroencephalogram (EEG) is a record of the electric signals of the brain activity and is useful in diagnosis, treatment of various diseases, and monitoring the fatigue and alertness of a personnel. The current electrodes are not suitable for portable and long-term recording applications. Also, the conventional biopotential electrodes demand for additional skin preparation and need conductive gel. However, conductive gel may cause allergic reactions or skin irritation. Microneedle electrodes are dry electrodes and do not require any skin preparation and conductive gel. Microneedles pierce into the skin and keep direct contact with the conductive epidermal layer of the skin and are promising for long-term recording applications. The research work is carried out to identify suitable materials for microneedle fabrication for biopotential measurement. Microneedle design is carried out by taking skin anatomy and their properties into consideration. Finite Element Analysis (FEA) is carried out to optimize pitch and base diameter for maximum penetration of the skin and minimum strain of the microneedles. Research is carried out initially using micro-stereolithography (MSL) system to realise microneedles. The research work investigated fabrication of microneedle using Hexanediol diacrylate (HDDA) by UV photo polymerisation. Novel method of step less fabrication of HDDA microneedle by MSL is presented. Investigation carried out to co-polymerise HDDA and PMMA using MSL system is presented. Samples are characterized using FTIR. FTIR spectrum of samples shows limitations in curing PMMA.Replication technique carried out for micro mould fabrication is presented. The Teflon negative mould is used for making positive polydimethylsiloxane (PDMS) micro mould, which is again used for making PDMS negative mould. The PDMS microneedle fabricated by this work is used as master for subsequent microneedle fabrication using PMMA. It was observed that the PMMA sticks to PDMS mould. Further, the work is carried out to fabricate microneedles using micromachining centre. Layer-by-layer machining strategy is adopted with varying machining parameters at different heights of microneedles. Microneedle array consisting of 10×10 microneedles having base and tip diameters 120 μm and 5 μm, height 200 μm and pitch 400 μm are micro milled. Fabricated needles are tested for compression, shear and penetration into chicken skin. Characterisation results of microneedles and chicken skin penetration test results are presented. The results of tests show the microneedles have capability for skin penetration. To make PMMA microneedles electrically conductive, electroless silver plating has been carried out. Electrical testing of needles conducted shows that the needles offer less electrical resistance. The results show that PMMA microneedles with conductive layer are found to be suitable for biopotential measurement.