In the present work, polyacrylamide hydrogels, which contain different monomer and cross-linker concentrations, are swollen in pure water and contacted with different metals to investigate the electrical properties of hydrogel/metal contacts. During the study, two different metals are used as contact electrodes. These are; platinum and 1050 quality aluminum alloy which contains 99.5% pure aluminum. Symmetric and asymmetric contact systems are examined separately. This different behavior arises from anodization (anodic oxidation) process which increases the resistance of the aluminum/hydrogel contact. Anodization is an electrochemical process used to increase the thickness of the natural oxide layer on the surface of the metal by applying an electrical potential or current in a suitable electrolyte system. The oxide film is formed by the chemical reactions between the cations from the metal and oxygen–containing anions from the electrolyte [1-4]. We studied the anodic oxidation of aluminum via polyacrylamide hydrogel, instead of traditional electrolytes such as liquid electrolytes [1-4]. We observed that the rectification differs considerably from the ordinary systems. It reaches to 830 for 7 V for a certain composition and swelling ratio of the gel. We have shown that the rectification can be tuned by changing some parameters such as polymer composition, swelling ratio, the applied voltage, etc. Introduction Synthesis of Hydrogels: Polyacrylamide hydrogels synthesized with stoichiometric ratio (0.62 mol/l AAm + 70 mg BIS/50 ml su + 50 mg APS/50 ml) [5] in Set 1. In the Set 2 and Set 3 the cross-linker concentrations are doubled and halved, respectively, while the others kept fixed. By this way we aimed to investigate the effect of the internal structure of the gel on the oxidation of the metal, and thus on the rectification. In this work, we observed that the selection of metal electrode which used for electrical measurements is very important. In contacts with inert electrodes like platinum, no oxidation is observed. But in aluminum contacts, the formation of high–resistant oxide layer is observed. So in asymmetric contacts, the current–voltage characteristic is like a diode. The aluminum oxide layer blocks the current in one direction and passes the current is the other direction. This method can be used for developing industrial rectifiers, organic circuit element. Procedures Conclusion References Results and Discussion I – V CHARACTERISTICS OF ALUMINUM/POLYACRYLAMIDE HYDROGEL INTERFACE Sevcan Tabanlı*, Ali Gelir, Yaşar Y ılmaz Department of Physics Engineering, Istanbul Technical University, Turkey. erdogansevcan@itu.edu.tr MONOMER Acrylamide (AAm) INITIATOR Ammonium per sulfate (APS) CROSSLINKER N,N’ŵethyleŶe ďisaĐrylaŵide (BIS) FREE RADICAL POLYMERIZATION HEAT BATH 60 o C deoxygenated by bubbling nitrogen  Pt/hydrogel/Pt (symmetric contact) the current can flow in both directions with the same response, so the contact behaves like an ohmic contact.  Al/hydrogel/Pt (asymmetric contact) the current can flow in both directions with different response, so the contact behaves like a rectifiying contact. In voltage–sweep experiments; Electrical Measurements: The gels were cut into thin slices, and then they were dried in room temperature for one week. Then they were smoothed by sandpaper till they come to approximately the same width, ∼1 mm. Thus they become ready for electrical measurements. After the gels were swollen to certain extends they put between the electrodes and the current was measured via Keithley 6487 Picoammeter/Voltage Source. The experimental setup for the electrical measurements is shown in Figure 1. [1] Grimes, C., Grimes, C., and Mor, G., (2009). TiO 2 Nanotube Arrays: Synthesis, Properties, and Applications, Springer. [2] Diggle, J.W., Downie, T.C. ve Goulding, C.W., (1969). Anodic Oxide Films on Aluminum, Chemical Reviews, ACS Publications, 365–405. [3] Keller, F., Hunter, M.S., and Robinson, D.L., (1953). Structural Features of Oxide Coatings on Aluminum, Journal of The Electrochemical Society, 100(9), 411–419. [4] Eftekhari, A., (2008). Nanostructured materials in electrochemistry, Wiley-VCH. [5] Alveroglu, E., and Yilmaz, Y., (2010). Synthesis of p- and n- type Gels Doped With Ionic Charge Carriers, Nanoscale Research Letters, 5, 559–565. In Figure 2(a), a slice of hydrogel is sandwiched between the platinum electrodes and the current is measured as a function of applied voltage. During the measurements, a 2 kΩ resistance was connected in series to the circuit. In Figure 2(b), the same experiments were repeated for aluminum/platinum system where one electrode is aluminum the other is platinum. Each symbol on the figures shows a new sweep of voltage first from 0 V to 10 V and then in the reverse direction, from 0 V to -10 V. It is clearly seen from these figures that when we change one of the Pt/Pt with Al, a considerable rectification is observed. Figure 2: Comparison of current–voltage plots for hydrogel were sandwiched between the platinum electrodes (2 Molar PAAm gel of Set 1 – m/m 0 =1.525±0.002) (a) and platinum and aluminum electrodes (5 Molar PAAm gel of Set 2 – m/m 0 =2.035±0.002) (b) and the current was measured. Figure 4: The current – voltage plot (a), Rectification ratio (I + /I - ) – voltage plot (b), for a certain composition of the gel; (5 Molar PAAm of Set 2, (m/m 0 = 3.0140.002) Figure 1: Schematic representation of the experimental setup for the measurements. From Figure 3, it is clearly seen that the rectification efficiency of the system increases with the number of the sweep. This is due to the increasing density of the oxide layer on Al electrode upon increasing number of sweep. Figure 3: Rectification ratio – voltage plot for hydrogel (5 Molar PAAm gel of Set 2 (m/m 0 =2.0350.002)), were sandwiched between the aluminum/platinum electrodes and the rectification ratio (I + /I - ) was calculated. 2 4 6 8 10 0 5 10 15 Rectification Ratio (I + /I - ) Voltage (V) 1 st Sweep 5 th Sweep 10 th Sweep 20 th Sweep -10 -5 5 10 -2.0 -1.5 -1.0 -0.5 0.5 1.0 1.5 2.0 1 st Sweep 5 th Sweep 10 th Sweep 15 th Sweep Current (mA) Voltage (V) -10 -5 5 10 -3 -2 -1 1 2 3 1 st Sweep 5 th Sweep 10 th Sweep 15 th Sweep 20 th Sweep Current (mA) Voltage (V) -8 -6 -4 -2 2 4 6 8 -15 -10 -5 5 10 15 Current (mA) Voltage (V) (a) (b) (a) (b) In Figure 4, the results of the measurements in which maximum rectification is observed are given. In Figure 4(a), The current – voltage plot for a certain composition of the gel; (5 Molar PAAm of Set 2, m/m 0 =3.014±0.002). In Figure 4(b), The rectification ratio – voltage plot for a certain compostion of the gel. 0 1 2 3 4 5 6 7 0 200 400 600 800 Rectification Ratio (I + /I - ) Voltage (V)