Based on this year's project, we made a device which called "East-Wind" that can detect the concentration of toxic ions in water. In order to describe this device clearly, we will introduce it in three aspects. Just as our project describes, iGEMers from all around the world can make their own East-Wind by reading introductions as follow. You can cancel, add, or change some components of the device to get your own target. It is a modularized device and it can make anything you want.-----* Introduction *-----
Our team members are always wondering that how we can apply our project to practical problems. As we all know, synthetic biology is an engineering subject and the meaning of engineering is solving problems. With the development of genetic circuits, we naturally think of electronic engineering which can make our ideas into reality. It is awesome to combine genetic circuits and electronic circuits. That is why we persist in moving into an unfamiliar territory and making East-Wind.
East-Wind can be divided into three main parts: microfluidic chip driven by centrifugal force; three electrode system, constant voltage circuit, current-voltage conversion circuit and multi-position amplifier circuit; 3D printing shell.
We choose poly(methyl 2-methylpropenoate) (PMMA) as the material of our microfluidic chip. Emerging biotechnology and Biomedical research uses PMMA to create microfluidic lab-on-a-chip devices, which require 100 micrometre-wide geometries for routing liquids. These small geometries are amenable to using PMMA in a biochip fabrication process and offer moderate biocompatibility.
The picture shows the microfluidic chip's sandwich-like structure. There are five functional parts in the third figure from left to the right. From top to bottom, they are the hole, pipeline-1, hatching house, pipeline-2 with capillary valve and testing room. The injection hole which is covered by silver paper is where you put the sample in and its size is suitable for pipette tips. The pipeline-1 is wide enough to make sure that water sample can easily enter the hatching house. The hatching house is where the engineering bacteria grow and start biochemical reactions. On a separate note, the freeze-dried engineering bacteria have been placed in the hatching house. The pipeline-2's width is 100 micron which can be considered as a capillary valve so that it can prevent liquid from entering the testing room when the chip is not spinning. For two reasons the room's top is covered by silver paper. on the one hand, we can prick a tiny hole on silver paper to balance pressure when the liquid enters the room; on the other hand we can easily make the screen-printed electrode pass through the silver paper to gain data.-----* Circuit *-----
I. Three-electrode system
Conducting our project experiment requires at least two electrodes. The working electrode, which makes contact with the analyte, must apply the desired potential in a controlled way and facilitate the transfer of charge to and from the analyte. The second electrode acts as the other half of the cell. This second electrode must have a known potential with which to gauge the potential of the working electrode, furthermore it must balance the charge added or removed by the working electrode. While this is a viable setup, it has a number of shortcomings. Most significantly, it is extremely difficult for an electrode to maintain a constant potential while passing current to counter redox events at the working electrode.
To solve this problem, the roles of supplying electrons and providing a reference potential are divided between two separate electrodes. The reference electrode is a half cell with a known reduction potential. Its only role is to act as reference in measuring and controlling the working electrode's potential and at no point does it pass any current. The auxiliary electrode passes all the current needed to balance the current observed at the working electrode. To achieve this current, the auxiliary will often swing to extreme potentials at the edges of the solvent window, where it oxidizes or reduces the solvent or supporting electrolyte. These electrodes, the working, reference, and auxiliary make up the modern three-electrode system.
With a view to the device's portability, we choose screen-printed electrode to carry our point. The following table shows the screen-printed electrode's materials.
II. Constant voltage circuit
In order to maintain the potential between reference electrode and counter electrode, we design a constant voltage circuit to replace a potentiostat. A potentiostat is a controling and measuring device. It comprises an electric circuit which controls the potential across the cell by sensing changes in its resistance, varying accordingly the current supplied to the system: a higher resistance will result in a decreased current, while a lower resistance will result in an increased current.
III. Current-voltage conversion circuit & multi-position amplifier circuit
In order to be compatible to different sizes of current, we design a multi-position amplifier circuit. The circuit is controlled by analog switches which are driven by Arduino PWM. We use syntax 'analogWrite(pin, value)' to define the high or low potential of each pin of analog switches so that different resistors will be put in the circuit.
Theoretical formula as follows:
The following table shows a viable solution.
We put the three-electrode constant voltage circuit, current-voltage conversion circuit and multi-position amplifier circuit on a single PCB. It's better to work in a metal box due to electromagnetic shielding characteristic.
IV. Other parts
The motor driver can send water samples to the testing room from the hatching house by centrifugal force. In order to achieve this goal, we choose gear motor to increase stability in the premise of ensuring the performance. The gear is controlled by Arduino, you can use PWM output to change rotation direction.
In order to achieve the optimum growth temperature of engineering bacteria, we use the heating wire and the temperature control switch. When the temperature is below 35℃, the switch is closed and the heating wire starts heating. When the temperature is above 35℃，the switch is open and the heating wire stops heating.
We design and print a shell for East-Wind. It's very hard for us to use the way to manufacture this small and light shell with many hatches. 3D printing is a suitable, low price and environmentally-friendly choice. We choose acrylonitrile butadiene styrene (ABS) as our 3D printing material.
To design a set of hardware using in situ, our work focuses on portability and stability, combining modularized device, microfluidics and freeze-drying. We made a miniature device called East-Wind with microfluidic chips and freeze-dried bacteria for steady transportation of our engineered bacteria. East-Wind, which can detect the concentration of toxic ions, is divided into three main parts: microfluidic chips, electrode system and 3D-printed shell. East-Wind is a modularized device that its parts can be added, canceled or reformed. To detect the survival rate and the quantity of our freeze-dried bacteria, an accessory and a series of microfluidic chips using with East-Wind together are designed. This accessory can detect bacteria of different OD-value by optical signals. The chips we designed in the way of continuous integration contain the simplest chip in device, the multichannel chip, and modularization chip. Worldwide iGEMers can make their own East-Wind by reading our wiki.
 Yao Yu-sheng, Xie Yong-ping, Wen Tao, Design of Potentiostat for Three-electrode Electrochemical Sensors, Instrument of Technique and Sensor, 2009, 9, 23-25.
 HU Geng, JIN Yang, YANG Shi-yuan, WANG Hong, CAI Hao-yuan, JIANG Jun-feng, Weak-current Measurement in a Micro-electrochemistry System, MICROELECTRONICS & COMPUTER, 2009, 6, 1-4.
 WANG Zhao-yu, WU Xiao-ming, LIU Zhong-ming, Design of Three -electrode Electrochemical Detection System Based on C8051F020 Microcontroller, Chinese Journal of Medical Physics, 2013, 1, 3909-3912.
Xiamen University, Fujian, China
No. 422, Siming South Road, Xiamen, Fujian, P. R. China 361005