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.
The BIOGRAPHY Project
The BIOGRAPHY research project has developed novel production techniques utilizing Haydale functionalized graphene ink for a rapid transfer of research results from materials science to industrial applications. The initial tests of this project regarding the applicability of the printed biosensors in the targeted fields of application are currently being conducted.
Cost-Effective production of the sensors is made possible through a roll-to-roll gravure printing technology, in which printed foils are bonded to bottomless well plates to set up sensors in well plate format similar to that which is shown in Figure 1.
Figure 1. 24 printed interdigital electrodes.
These sensors are produced through the sequential printing of two substances, in which the interdigital structures are printed using conductive ink that is comprised of graphene platelets. During the second step, an additional protein layer is printed onto the electrode structures to improve the adhesion of cells to the sensor, for example as a confluent monolayer. Register control, which is otherwise understood to be the synchronization of the printing units to each other, is used for ensuring the precise alignment of the layers to each other, which is particularly crucial for the quality and function of the sensors.
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.
Composition of the Graphene-Containing Ink
The main composition of the ink includes main component types of a resin (or binder), solvents and additives, which can include pigments or other functional additives. The combination of targeted properties was made possible through a careful selection of the constituents, specifically the addition of Haydale’s Graphene Nanoplatelet (GNP) material as a key active ingredient. GNPs are high-aspect ratio carbon nanomaterials that are processed and surface-functionalized through the use of Haydale’s patented HDPlas® plasma-based technology.
The low energy HDPlas® process adds a functional chemistry in a uniquely benign manner that reduces the amount of damage caused to the nanomaterial structure, thereby maximizing the ability to impart the nanomaterial properties to the final ink. This process of chemical functionalization promotes the efficiency of the dispersion into the carrier resin used in the ink, as this process allows the ink to more readily exhibit the desired characteristics described above. The ink was made by using a variety of mixing and dispersion methods, in which the GNPs were mixed into the resin vehicle and then subjected to a series of compounding procedures to ensure adequate uniformity through the ink.
The resultant biocompatible ink exhibits a paste-like viscosity that can be diluted through the utilization of an appropriate solvent. The selection of the solvent is directly influenced by numerous factors, some of which include chemical compatibility, printing speed and drying factors, such as temperature and time. At a suitable solvent concentration, the viscosity of the ink was 0.1 Pa*s, whereas the surface tension was between 31 and 36 mN/m and the square resistance was determined for prints made with a two-color printing machine and the new Haydale graphene ink. The test cylinder had a well depth of 60 μm. The measured square resistance of the approximately 5.5 μm thick printed layer was around 100 Ω/sq. Normalized to a layer thickness of 25 μm, this results in a square resistance of approximately 20 Ω/sq.
Electrically conductive interdigital structures of various geometries were successfully printed as shown in Figure 2. Note that even the smallest electrode structures with widths of 50 µm and a distance of 50 µm between adjacent electrodes could be produced by gravure printing.
Figure 2. Printed interdigital electrodes. Electrode width and gap: 50 µm.
The sensors developed within the BIOGRAPHY project can be used both for toxicity studies, as well as for the validation of the effectiveness of anti-infective agents, such as antiviral substances. For these purposes, the indicator cell lines are used to form a confluent cell monolayer on the interdigital electrode-based sensor and electrically insulate it. When the added substances destroy the electrically insulating, the confluent cell layer, this will therefore indicate, for example, the cytotoxicity of these substances. To recognize this cellular response, the reduction of the electrical resistance of the interdigital structure can be visualized.
For the second potential application, viruses, instead of toxic substances, are added to the electrically insulating cell layer, thereby leading to a morphological change of the cells or to a detachment of the cells from the cell monolayer, which is otherwise referred to as the cytopathic effect (CPE). The addition of antiviral substances inhibits the infection of the cells or the viral replication in the cells and thus the CPE. This circumstance can be useful in deducing the changes in the measured impedance values from the achieved inhibition of the CPE and thus the efficacy of the antiviral substances.
The BIOGRAPHY project is funded by Innovate UK, the UK’s innovation agency, which primarily works with people, companies and partner organizations to discover and drive the scientific and technological innovations that will promote the growth of the UK economy. The project is also funded by the German Federal Ministry of Education and Research (BMBF) and managed by the Project Management Agency Karlsruhe (PTKA).
This information has been sourced, reviewed and adapted from materials provided by Haydale Limited.
For more information on this source, please visit Haydale Limited.