Motivation

Current electrochemistry setup can cost over £1000 as they include a counter and working electrodes made of materials such as gold as well as AgCl reference electrodes. While this kind of set-up has its own advantages such as great biocompatibility, outstanding conductivity and low reactivity with organic molecules, pure gold wires are too expensive to make it affordable and cost-effective. Instead, we opted for carbon electrodes which are up to 7 times cheaper than gold wire when comparing price per cm² - the main geometric feature of interest in electrochemistry set-ups and 250 times cheaper when looking at the three electrodes required to stimulate gene expression.

Characteristics & Performance

To characterise our system a series of square wave voltammetry and cyclic voltammetry experiments were performed. For these experiments, a 90% content carbon working electrode of 2mm diameter was used, a 60% carbon counter electrode of 5mm diameter and a custom-made Ag/AgCl reference electrode was used (protocolHYPERLINK). To drive the specific redox potential into our system amperometry experiments were performed. For these, the setup only differs in the use of 60% carbon working electrodes of 5mm diameter as opposed to 90% content carbon working electrode of 2mm diameter. A table for the technical specifications of these electrodes is found here:

This cheap 3-electrode setup was capable enough of stimulating gene expression as shown in figure XX -(THOMAS PICTURE OF GFP DOT) and figure XX (FIGURE OF VOLTAMMETRY) (FIGURE OF PAPER VOLTAMMETRY)

Price comparison

Comparison of price per setup

TABLE HERE

Comparison of price per cm²

TABLE HERE

Motivation

Once we experimentally demonstrated how of gene expression could be locally induced with our affordable 3-electrode setup, the next goal was set to build a higher-end electrode array able to induce genetic expression locally at different regions in a lawn of cells.

Concept Design

The electrode array is printed onto one side of a Printed Circuit Board (PCB) in the form of 100 (10x10) circular working electrode pads, 4 counter rectangular pads, and 1 reference pad with the shape of a cross as it can be seen in figure XX ~(figure of pad configuration) . All the electronic components that allow controlling the logic of the array are placed on the other side of the PCB. We want each individual pad to select one of two working electrode potentials. The circuit can be broken down into several modules:

• Electrode cell: This corresponds to a memory storage unit, a set of two transistors and the electrode pad itself connected together. The storage unit is operated by a Data Flip-Flop and stores 1-bit (0 or 1), based on the data flip-flop output the transistors select one of the available Working potentials and outputs it into the pad.
• Power module: logic chips are powered through a voltage difference between a constant voltage supply (Vcc) and a ground(GND) supply. The logic HIGH (1) of components corresponds to a voltage of Vcc while the logic LOW (0) corresponds to a voltage equal to GND. We want logic LOW to correspond to 2.5V below the reference voltage of the electrochemical array, and logic HIGH to correspond to 2.5V above the reference voltage. this is to ensure that the Data Flip-Flops can force the FETs into either an ON or OFF state. Thus, we cannot use the real ground as logic GND. Instead, we run the logic (Arduino and PCB chips) from a non-grounded battery, in order to be able to shift logic levels up or down. This shifting is achieved by an op-amp, with its non-inverting input connected to the reference electrode. The op-amp is connected in negative feedback to a 2.5V voltage regulator. This means that it sources and sinks current as required to maintain its output (2.5V relative to logic GND) at the reference voltage, effectively centring the logic HIGH and LOW around the reference voltage. This ensures that the logic circuits can reliably switch the FETs.
• ROW-Selecting module: in order to program our electrode pads we need to sort through every single row and feed individual data into each pad. A demultiplexer selects electrode rows one at a time, telling each row in turn to read the data supplied to it.
• Data-feeder: The data, which is either a 1 or a 0 (HIGH or LOW) is fed into the individual cells of the selected row. This operation is accomplished by a serial-input-parallel-output (SIPO) shift register which outputs 10 bits at a time (one per column of the electrode array).

Components Description

• Memory Unit - Data Flip-Flop: data flip-flops are simple devices that can be built with logic gates. DFF store one bit of information: HIGH or LOW. In our system DFFs are used to store the fate of the array cell and will give orders to the FETs on which potential to choose from.

• Selection Unit - MOSFETs: metal-oxide-semiconductor-field-effect transistor or MOSFET are in practice switched independently from any human mechanical input. MOSFETs behaviour is characterised by their terminals: Gate, Source and Drain and whatever electronic events are happening at these terminals. These minuscule devices respond to a certain voltage operating at the Gate terminal and will allow for the flow of electrons(i.e current) when certain conditions are met. whenever the voltage at the gate is greater than a certain value, electrons will flow, otherwise, they won't. In our system, MOSFETs are used to select for either of the potential coming from the potentiostat, either oxidising or reducing which will determine if the array cell will be an oxidising or a reducing cell.

• Row Selection Unit - Demultiplexer:demultiplexers are logic devices that allow selecting for one output line out of many available according to the logic input in the so-called select lines. In our circuit, one demultiplexer is used to select for a row of array cells which data is going to be fed into (from the shift register)

• Data feeder - Shift Register: shift registers are technically a row of DFF all connected together, upon a clock “order” a data flip-flop will share it’s “memory” with next data flip-flop hence whatever data is stored in the SR ( 1110) will be shifted to the right (0111). In our circuit, a serial to parallel SR is used to determine the value of the bits that are going to be fed into the DFF in the preselected row by the demultiplexer.

• Power control - opamp: operational amplifiers or op-amps are extremely useful devices that can be used for many applications. In our circuit, we use an op-amp for one of it’s most basic purposes: a voltage follower. A voltage follower “copies” the voltage from one terminal without drawing any current from it.

• Power control - voltage regulator: a voltage regulator transforms whatever voltage it has at its input to a specific voltage determined by its internal configuration. In our circuit, the voltage follower is an elegant and easy way to have the Arduino and the PCB running from one single battery. The Arduino runs on a 9V battery and the same voltage is then sent to the voltage regulator which then power the logic devices in the PCB.
• Capacitors: capacitors can be seen as tiny batteries, they are able to hold a charge and then release it. they have many applications of course but in our circuit, we use most of them as decoupling capacitors which damp the noisy oscillations coming from high-frequency noise sources.