Introduction
The growing demand for cell-based assays to determine the interaction occurring between cells and chemical and biological materials or compounds has pushed the development of sensing systems and methodologies that are capable of providing continuous results in real time, at high throughput and at low cost. The combination of biological components, such as proteins, with electrically conductive interdigital structures, is a non-invasive methodology that has the potential to fulfill these requirements.
Sensors for biological applications with interdigital structures already exist today. In fact, various commercial variants that use electrically conductive structures composed of biocompatible metals, such as gold or platinum, are currently available and typically produced using photolithographic methods. Since cell-based sensors are used to examine viable cells, a biocompatible interface to a fluidic system is required, and can typically be accomplished through bonding the wells to the printed circuit board that comprises the electrically conductive interdigital structures. It is important to note that these structures are often expensive to produce as a result of the complex production method of the biocompatible interdigital structures.
Haydale’s Graphene-Containing Ink
Haydale has recently developed and tested the biocompatibility, cytotoxicity, conductivity and suitability of a new graphene-containing ink for gravure printing. The ink was successfully found to meet the requirements concerning printability and application in a cell-based sensor. With this novel ink, printed structures can be produced with a surface resistance of 10 Ω/sq based on a layer thickness of 25 μm. A newly developed micro-engraving machine with an ultra-short pulse laser was used to create the print cylinder that is capable of producing structures with lateral dimensions measuring less than 10 μm.
For sensor production, a compact roll-to-roll printing system for two-color printing has been set up with established printing parameters. Additionally, the system is equipped with an integrated corona unit for surface activation of the substrate and each of the two printing units are equipped with a near-infrared drying unit. The plant, graphene ink and the developed processes are all suitable for high speed printing of conductive graphene electrodes, which are designed as interdigital structures. Even very fine electrode structures with widths of 52 microns and a distance of 52 microns between the adjacent fingers can be printed with this technology.
This graphene-containing ink is nano-enhanced, electrically conductive and biocompatible as a printed layer. The biocompatibility of the ink was demonstrated in cytotoxicity tests according to ISO 10993-5 with different cell lines, which led to the modification of the ink to allow for its application through rotary gravure printing. To ensure the accuracy and definition of the printing technology at high speed, the ink needed to be formulated to have a suitable viscosity and rapid drying.
