Untitled Document




BME Homepage

How Material Selection involves biomedical engineering

As a Biomedical Engineer, or any researcher aiming to develop a device, one of the most important aspects in development is the selection of device materials. In comparison to other fields related to microscale devices, such as the semiconductor industry, the use of conductive materials for developing circuit electrodes are not necessarily applicable for developing implantable electrodes within the body. This is observable in all aspects of micro and nano-electromechnical devices that are designed for biomedical purposes such as implantable epiretinal and subretinal prostheses. For example, certain materials such as copper work very effectively in conducing charge; however they are virtually ineffective as implantable electrodes when applied to biological tissue. This makes the task of the biomedical engineer to choose the correct material all that much more difficult.

Selection of an Anode Membrane Material

Electrochemical microreservoir system:

            The composition and type of membrane material chosen for use in an implantable and controllable release microchip is critical for reliable use in vivo. The membrane material must remain stable within a solution in absence of an applied electric potential to prevent premature release of the drug or chemical from the microchip. Another membrane material requirement is that it can dissolve rapidly and selectively when a specific electric potential is applied through the membrane. Biomedical applications of the device make selection of an appropriate anode membrane material difficult due to small amounts of dissolved oxygen and chloride ions naturally found in biological fluids. Dissolve oxygen and chloride ions are capable of spontaneously corroding numerous types of metals that could otherwise be used as possible membrane materials for non-biomedical purposes. [1]
            The initial membrane chosen to model the proof-of-concept device for implantable microchips was gold [1]. Gold is a noble metal that is easily deposited and patterned, has low reactivity with other substances, resists spontaneous corrosion in most aqueous solutions over a broad pH range, and retains a clean surface without a substantial native oxide layer.
            Gold in aqueous solutions free from complexing substances is immune to corrosion within water when no electric potential is applied. When gold is placed in a solution with small amounts of chloride ions, the electric potential and pH region thermodynamically favors the formation of water soluble chlorogold complexes. Gold corrosion by the formation of water-soluble chlorogold complexes is also kinetically favored. When an electric potential is applied to gold within a soldium chloride solution, the current density begins to rise indicating the formation of tetrachloroaurate (III). Tetrachloroaurate(III) or Au(III) is the dissolved form of the gold membrane that resolves within the electrolyte. Gold membrane corrosion can occur within 10-30 seconds for a 0.1-0.3 micron thick membrane at +1.04 V vs SCE (saturated calomel electrode: def. a reference electrode which uses the reaction between mercury metal and mercury(I) chloride, to fix its potential, the aqueous phase in contact with the mercury and the mercury(I) chloride (Hg2Cl2 calomel) is a saturated solution of potassium chloride in water).
            Gold is also biocompatible and has been used in dental materials, transdermal gene delivery, and drugs for treating rheumatoid arthritis. The amount of gold, or Au(III) that is released due to electrochemical opening of the anode membrane is around 2-6 ng. The effects of Au(III) release in vivo has not been extensively studied and is dependent on the local concentration and implantation site.

Electrothermal microreservoir system:

Similar to the electrochemical device, the electrothermal based device reservoir membrane should remain stable within a solution in absence of, in this case current, to prevent premature release of the drug or chemical from the microchip. Another requirement is that the membrane should rupture almost immediately and selectively when current is applied through the membrane. Since the electrothermal based device does not require a specific surrounding medium to open the reservoir membrane, the type of material chosen is based on the efficiency of power. The electrothermal based device utilizes current sent through the membrane to increase the local resistivity, thereby melting or degrading the membrane. The material should require less current to produce activation and provide greater resistivity.