The first successful fabrication of microsupercapacitors (μ‐SCs) using fractal electrode designs is reported. Using sputtered anhydrous RuO2 thin‐film electrodes as prototypes, μ‐SCs are fabricated using Hilbert, Peano, and Moore fractal designs, and their performance is compared to conventional interdigital electrode structures. Microsupercapacitor performance, including energy density, areal and volumetric capacitances, changes with fractal electrode geometry. Specifically, the μ‐SCs based on the Moore design show a 32% enhancement in energy density compared to conventional interdigital structures, when compared at the same power density and using the same thin‐film RuO2 electrodes. The energy density of the Moore design is 23.2 mWh cm−3 at a volumetric power density of 769 mW cm−3. In contrast, the interdigital design shows an energy density of only 17.5 mWh cm−3 at the same power density. We show that active electrode surface area cannot alone explain the increase in capacitance and energy density. We propose that the increase in electrical lines of force, due to edging effects in the fractal electrodes, also contribute to the higher capacitance. This study shows that electrode fractal design is a viable strategy for improving the performance of integrated μ‐SCs that use thin‐film electrodes at no extra processing or fabrication cost.
This paper proposes a new “twisted” 3D microfluidic mixer fabricated by a laser writing/microfabrication technique. Effective and efficient mixing using the twisted micromixers can be obtained by combining two general chaotic mixing mechanisms: splitting/recombining and chaotic advection. The lamination of mixer units provides the splitting and recombination mechanism when the quadrant of circles is arranged in a two-layered serial arrangement of mixing units. The overall 3D path of the microchannel introduces the advection. An experimental investigation using chemical solutions revealed that these novel 3D passive microfluidic mixers were stable and could be operated at a wide range of flow rates. This micromixer finds application in the manipulation of tiny volumes of liquids that are crucial in diagnostics. The mixing performance was evaluated by dye visualization, and using a pH test that determined the chemical reaction of the solutions. A comparison of the tornado-mixer with this twisted micromixer was made to evaluate the efficiency of mixing. The efficiency of mixing was calculated within the channel by acquiring intensities using ImageJ software. Results suggested that efficient mixing can be obtained when more than 3 units were consecutively placed. The geometry of the device, which has a length of 30 mm, enables the device to be integrated with micro total analysis systems and other lab-on-chip devices.
We demonstrate a fast and economically viable 2D/3D maskless digital light-projection based on stereolithography compared to traditional processes. Furthermore, electrodes and sensors are easily integrated without introducing leakages to the LOC.
MANUSCRIPT TEXT IntroductionFlexible electronics has become an established field that has seen tremendous developments over the last years. [1][2][3][4][5][6][7][8][9][10] The unique capability to adjust the geometry of devices to curved surfaces or surfaces of changing shape provides vast advantages over conventional electronics on rigid substrates. Hence, flexible printed circuit boards have already become an industrial standard for medical implants or consumer electronics [11][12][13][14] , where the main requirements are: large area, extreme thinness, and conformity to curved surfaces. The recent progress of organic [6,15,16] as well as inorganic [14,17,18] electronics, which are basically performed using printing or thin film technologies, has been very beneficial for the improvement of flexing electronic devices. In combination with other methods like screen or inkjet printing, novel concepts for flexible electronics have been developed for several applications including displays, [19] organic light-emitting diodes, [20] sensors, [21][22][23][24][25] radio frequency identification tags, [26][27][28] and organic solar cells. [29] Meanwhile, there are increasing activities on flexible magnetic field sensors and magnetoelectronics, [30,31] due to ubiquitous use of such kinds of devices. [18,32] By now, high-performance magnetic sensors based on the giant magnetoresistive (GMR) effect and the tunneling magnetoresistive (TMR) effect are mainly prepared using thin film fabrication technologies. [18,[33][34][35][36][37][38][39] This method allows for fabricating extremely sensitive magnetic sensors, especially on Si substrates, and it would therefore be a desirable solution to flex the silicon substrate, where the magnetic elements are fabricated onto. Magnetic tunnel junctions (MTJs) are of particular interest, because they constitute the main component of diverse spintronic devices such as spin-transfer oscillators, magnetic sensors, hard-disk-drive read-heads or Magnetic Random Access Memories. [40] MTJs are made of a magnetic multilayer thin film material. For sensor applications, the low power consumption [41] , high sensitivity and small size make them an attractive option. MTJs with amorphous Al-oxide barriers exhibit TMR ratios below 100 %. This can be improved by using (001)-oriented MgO barriers, achieving TMR ratios over 150 % at room temperature. [42][43][44] These high values can be exploited to design very sensitive TMR sensors, [45][46][47][48][49][50] such as biomagnetic field sensors [51] . They show an important change in resistance versus the magnetic field than other sensors such as those utilizing the anisotropic magnetoresistance or giant magnetoresistance. [52][53][54] Compared to conventionally used Hall effect and AMR sensors, a TMR sensor can also have higher sensitivity, lower power consumption [41] , better temperature stability and, in particular, compared to an AMR sensor, a wider linear range can be obtained. [54] Flexible MTJ devices have been reported with alumina [37,39] and MgO t...
This paper presents a biosensor-CMOS platform for measuring the capacitive coupling of biorecognition elements. The biosensor is designed, fabricated, and tested for the detection and quantification of a protein that reveals the presence of early-stage cancer. For the first time, the spermidine/spermine N1 acetyltransferase (SSAT) enzyme has been screened and quantified on the surface of a capacitive sensor. The sensor surface is treated to immobilize antibodies, and the baseline capacitance of the biosensor is reduced by connecting an array of capacitors in series for fixed exposure area to the analyte. A large sensing area with small baseline capacitance is implemented to achieve a high sensitivity to SSAT enzyme concentrations. The sensed capacitance value is digitized by using a 12-bit highly digital successive-approximation capacitance-to-digital converter that is implemented in a 0.18 μm CMOS technology. The readout circuit operates in the near-subthreshold regime and provides power and area efficient operation. The capacitance range is 16.137 pF with a 4.5 fF absolute resolution, which adequately covers the concentrations of 10 mg/L, 5 mg/L, 2.5 mg/L, and 1.25 mg/L of the SSAT enzyme. The concentrations were selected as a pilot study, and the platform was shown to demonstrate high sensitivity for SSAT enzymes on the surface of the capacitive sensor. The tested prototype demonstrated 42.5 μS of measurement time and a total power consumption of 2.1 μW.
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