The most prolific semi-metallic element is silicon (Si) present in the earth’s crust, correspond to 25.7% by weight. Silicon is therefore used as a preferable Semi-Conductor in Cosmetic Surgery. This shiny semiconducting element having atomic number 14 with an atomic mass of 28.085 located just below the carbon in the modern periodic table. It is the base component for semiconducting devices, glass, ceramics, and silicones.
Reasons for Choosing Silicon
Silicon used as a semiconductor material for the substrate to the human cells because by varying the level of doping type, and concentration interfacial energy can be manipulated. Additionally, the chemical bonding and interface can be tailored easily as compared to any other materials. Molecules are attached with Si in a directional and stable form giving its surface for the adsorption of the foreign particles, unlike the other metallic elements (Au & Pt) and other semiconductors which have residual unstable oxide chemistry on the surface.
Silicon chips in computers use electrons as their charge carriers, while neurons in living cells use ions from the cell fluid as the significant charge carriers. Mobility of particle in silicon is about 103 cm2/Vs, whereas ions mobility in water is around 10-3 cm2/Vs. There is a difference in the movement of order 106, which gives rise to a difference between these two data processors.
Thus, three reasons for choosing silicon as the electrically conductive substrate arises. Firstly, a thin silicon coating is an ideal substrate for nerve cell growth. Secondly, the thermally developed silicon dioxide prevents the exchange of electrons leading to the electro-chemical reaction resulting in the erosion of silicon. It would finally lead to damage of cells. Lastly, it allows the creation of a fine microscopic electronic chip working in direct contact with the living cells, which in turn protected by the inert silicon oxide layer.
Nerve Cell Fabrication on Semi-Conductor
Nerves cells grow by depositing cell bonded protein on a chip, which provides support to the cells. These proteins keep the lipid center of the membrane at a specific distance from the substrate. The side chain of polymer molecules creates it arise from the membrane. It set up as a split or cleavage between the cell and the chip, which fills with the electrolyte.
This conductive split shields the electric field and subdue direct mutual polarization of the silicon dioxide and the cell membrane. The thickness of the split, also called cleft, is much larger than the thickness of the electrical double layer. The electrical resistance between them corresponds to 1 M- ohm, while sheet resistance is the order of 10 M- ohm. It cannot be increased by decreasing the thickness of the cleft or specific resistance.
The thin coating of the silicon dioxide and the cell membrane insulate the center of the conductive split from the conducting environment of silicon and cytoplasm. The mobility of a neuron gives rise to ionic and displacement current through the membrane. The current associated along the center gives rise to a transducer extracellular potential (TEP) between the cell and the chip. The TEP induced by the neuron gives rise to an electrical field across the silicon dioxide, which is examined by a field-effect transistor (FET).
The optimization of cell-chip contacts can be done through the sorting of ions channels by selective accumulation and depletion of ions in the cell with the substrate.
Ion-Channels and Transistors
A two-way contract is created on an n-type silicon wafers chip for transferring signals of a nerve cell comprising of a stimulator with a transistor. The stimulator is a p-doped region in the n-type silicon wafer. The insulation between stimulator and transistor is provided by oxidation of silicon, so a thin coating of silicon dioxide is fabricated between them.
A total of 16 contacts protrudes from the stimulator arranged in a circle. The neuron interacts through the core area with the protruding neuritis. The leads of simulator and transistor are doped in boron implantation where leads of the transistor are joined. It is desirable for signal exchange between the chip and nerve cells.
The field-effect transistor (FET) comprising of a p-doped area of the source and with a gate channel covered by an oxide layer devoid of any metal. Then the chip is bonded in the ceramic package with the help of medical glue.
Ion-Electron Integration
Voltage-gated ion channels control the electrical signals of the nerve cells. These molecules are lying in the lipid double layer of the membrane. Either they can be in an open state or closed state.
At the point when they are in an open state, they specifically transmit ionic current through the membrane. Sodium-ion (Na+) responsible for inward current while Potassium ion (K+) for outward flow in the cell. Electric charge conduction across the membrane is responsible for the opening and closing of the channel.
The electrical signals transfer between the chip and nerve cells depend upon the electrical nature of the contact, insulating silicon oxide, the thin coating of the electrolyte, and the insulating lipid double layer. The microcircuit of nerve cells gives rise to the development of the bidirectional electrical communications in brain chips and neuro-computers.
Capacitor Stimulation
Rising slew rate and falling slew rate of the voltage are applied to the capacitor. The impaled micro-pipette records the intracellular voltage. A rising voltage leads to the positive extracellular voltage between the chip and cell through the displacement current into the chip-cell junction, hyper-polarizing the membrane. Similarly, a falling voltage leads to a negative extracellular voltage between the chip and cell depolarizing the attached membrane.
In an initial step, the ion channel is actuated. In the second step, the subsequent internal Na+ current depolarizes the entire cell membrane. As a result, the action potential is evoked. The impact vanishes for intense excitement of outward K+ current is actuated that diminished the depolarization of the cell.
Patch-Clamp Measurement System
A patch-clamp chip is designed to record electrical currents in single cells using a planar electrode replacing the glass micro-pipette. The system compensates for the actual membrane voltage and the clamping voltage difference. It is used in cell culture and in Vivo transplant and ion-channel drug screening studies. Examining depression drugs on ‘ion-exchange channels’ in the neurons can be taken as an example.
Nowadays, patch-clamp systems are based on silicon on sapphire (SOS) technique; unlike the traditional method of Complementary Metal Oxide Semiconductor (CMOS). It reduces the frequency degradation and provides a faster signal between the substrate and channels.
Image Credits
Featured Image: Silicon as a Semi-Conductor in Health & Medicine
Research Scholar
Metallurgical & Materials Engineering Dept.
Indian Institute of Technology, Patna
M.Tech, Mechanical Engineering
