Electrical conductivity of synthetic DOPA-melanin polymer for different hydration states and temperatures

On the origin of electrical conductivity in the bio-electronic material melanin

“…The water contents of the samples were determined using adsorption isotherms published previously. 23 Our measurement system was designed to alleviate two basic issues with the previous published experiments: (1) the problem of contamination arising from the use of salts to achieve set levels of environmental humidity 16 and (2) the improvement in signal-to-noise (S/N) afforded by using the aluminium chamber as a Faraday cage. 17 The results of the sandwich geometry measurements are shown in Fig.…”

“…The skin pigment eumelanin (hereafter referred to as melanin 7 ) is one system that has recently emerged as a strong candidate for bioelectronics because of the advances in producing device-quality thin films [8][9][10] and the prevailing model that it is perhaps the only biological amorphous semiconductor. 7,[11][12][13][14][15][16] A full model of charge transport in melanin has remained elusive for a number of reasons including (1) its electrical properties are very sensitive to hydration 9,[16][17][18][19][20] and (2) there are difficulties associated with forming good ohmic contacts for accurate and reproducible measurements (due to surface roughness). These issues have led to observations of apparent n and/or p-type transport dependent upon experimental conditions, 19,21 anomalous Arrhenius behaviour, 9,13,16,18,22 dominant capacitative effects reminiscent of ionic materials, 17,18,20 and a general lack of agreement as to how electrically conductive melanin actually is.…”

“…7,[11][12][13][14][15][16] A full model of charge transport in melanin has remained elusive for a number of reasons including (1) its electrical properties are very sensitive to hydration 9,[16][17][18][19][20] and (2) there are difficulties associated with forming good ohmic contacts for accurate and reproducible measurements (due to surface roughness). These issues have led to observations of apparent n and/or p-type transport dependent upon experimental conditions, 19,21 anomalous Arrhenius behaviour, 9,13,16,18,22 dominant capacitative effects reminiscent of ionic materials, 17,18,20 and a general lack of agreement as to how electrically conductive melanin actually is. 7 In this letter, we report the first systematic and controlled study of the conductivity of melanin as a function of hydration (both in the dark and under illumination) with different electrical contact geometries.…”

“…16,17 Although the methodologies for controlling hydration in these two studies were quite different, the experiments shared one feature-the use of the sandwich contact geometry (inset of Fig. 1(a)).…”

“…This led us to question the validity of the sandwich geometry given the dramatically reduced surface area available for water absorption and the relatively short timescales allowed for equilibration in the previous studies. 16,17 Based upon this insight, we redesigned the traditional sandwich geometry experiments under more carefully controlled conditions and also implemented "an open contact" geometry to ensure equilibrium hydration conditions via a surface van der Pauw (vdP) electrode architecture (inset of Fig. 1(b)).…”