MSE 03 Creating Conductive Ink Circuits
- Jinwook Chang
- Nov 4, 2022
- 9 min read
Updated: Oct 3
Discover how simple graphite-based conductive inks can turn paper into flexible, low-cost circuits that light up LEDs and power creativity.
Introduction
Conductive inks are a specialized type of material that are designed to enable conductive pathways on a wide range of different materials. They consist of a dispersion of conductive particles within a binder matrix, carried in a liquid medium, which is typically a solvent. When being applied, which can be done through numerous means such as printing, painting, and drawing, the solvent evaporates and the binder solidifies, which leaves a continuous network of conductive material that can carry electrical current. Over the past few decades, conductive inks have moved from being more of a niche technology used primarily in specialist electronics manufacturing to a mainstream material for flexible electronics, wearable devices, printed sensors, and interactive packaging. Its advantage lies in the fact that they can be applied in custom patterns on non-traditional substrates, offering higher levels of flexibility, lightness, and design freedom that conventional circuits or etched printed circuit boards cannot match.
The origins of conductive ink technology can be traced to early conductive paints and metallic pastes, which were used for circuit repair, electromagnetic interference shielding, and specific applications in the aerospace and defense industries. As research in materials science advanced, more sophisticated formulations were invented, including inks containing nanoscale silver particles, conductive polymers, and advanced carbon nanomaterials such as graphene. This investigation focuses on the preparation of a low-cost conductive ink using graphite powder as the conductive phase, clear nail polish or polyvinyl acetate (PVA) glue as the binder, and isopropyl alcohol or distilled water as the solvent. The ink will be applied to flexible materials to produce fully functioning circuits capable of powering small devices such as LEDS.
Aim
This experiment aims to create a graphite-based conductive ink using readily accessible materials and to evaluate its ability to produce flexible, low-cost electrical circuits. Furthermore, this investigation aims to highlight the graphite-based conductive ink within the broader context of modern electronics manufacturing. By comparing its performance, cost, and environmental impact to that of commercially available metal-based inks, this study will highlight both the potential uses and the limitations of such a material in real-world scenarios and applications.
Context
Traditional printed circuit boards, called PCBs, are manufactured through processes that require high capital investment, specialized equipment, and hazardous chemicals. Processes such as etching, drilling, and plating all contribute to a production chain that is efficient at large scales but is unsuitable for rapid prototyping or scenarios where flexibility is required. Furthermore, rigid PCBs cannot be readily integrated into unconventional shapes or small volumes, such as wearable electronics or surfaces that typically undergo mechanical bending. Conductive inks address many of these limitations. They can be applied directly to a substrate using low-cost, accessible methods such as screen or inkjet printing, or even manual application with a brush or pen. This leads to the reduction of waste and shortens the time it takes to get from a design to a prototype
In the commercial sector, silver nanoparticle inks dominate applications where high conductivity and high resolutions are paramount, such as in photovoltaic cells and radio-frequency identification (RFID) antennas. Copper-based inks offer a lower-cost alternative but can often suffer from oxidation, which degrades conductivity unless protective coatings are applied. Graphite-based inks, while less conductive, provide advantages in terms of cost, chemical stability, and safety. Their simplicity and safety make them an ideal introduction to conductive inks for students, which allows students to draw functional circuits without the need for soldering or interaction with potentially dangerous materials.
The electrical conductivity of a material depends on the presence of free charge carriers and their mobility within the material. In metals, such as copper or silver, there exists a sea of delocalized electrons, which allows for high levels of conductivity. In graphite, conductivity comes from its layered structure. Each carbon atom is bonded to three others in a planar hexagonal arrangement, which leaves one electron delocalized, for each central carbon atom, and free to move within the layer. This electron mobility is what allows a charge to pass through this material and ultimately conduct electricity. In a conductive ink, graphite particles form a network through which electrons can travel.
The binder in the ink serves multiple functions. Mechanically, it locks the conductive particles in place and ensures adhesion to whatever material it has been applied to. However, because the binder is electrically insulating, too much binder content can disrupt particle-to-particle contact and increase resistance. Therefore, the formulation and ratio of binder to graphene must be kept in check.
The solvent affects ink viscosity, application process, and drying time. Rapid solvent evaporation can cause the solvent beneath to become trapped, which can negatively affect the microstructure. Slow evaporation may improve particle settling and contact, but increases the drying time.
Materials
The materials required for the preparation and testing of a simple graphite-based conductive ink are inexpensive, widely available, and safe to handle. These are the materials required for this experiment:
Graphite powder (or finely ground pencil lead): This serves as the conductive phase. The layered crystal structure of graphite enables electron movement, making it suitable for low-cost conductive inks.
Binder: Either clear nail or polyvinyl acetate (PVA) glue. The binder ensures adhesion of the graphite particles to each other.
Solvent: Isopropyl alcohol (≥70%) for rapid evaporation and smooth application, or distilled water as an odorless alternative. The solvent impacts the viscosity.
Mixing container: A small disposable cup or jar to hold and combine the ingredients.
Stirring tool: A wooden stick or spoon for mixing the components uniformly.
Applicator: A paintbrush or cotton swab, for applying the ink to the substrate in controlled patterns.
Substrate: Paper. This choice of substrate provides the physical support for the conductive pattern and is flexible.
Multimeter: Used for measuring electrical resistance and verifying
Electronic components: small devices such as LEDs, resistors, or buzzers to put into the circuit.
Power source: A coin cell battery or 9V battery with connecting leads provides power for the circuit.
Procedure
Step 1: Prepare the Conductive Ink
Mix Graphite Powder:
Add about 1 tablespoon of graphite powder into the mixing container.
Add Binder:
Pour 1 teaspoon of clear nail polish (or wood glue) into the container. The binder will help hold the graphite particles together when applied.
Add Solvent:
Add a small amount of isopropyl alcohol (around 1/2 teaspoon) to thin the mixture for easy application. If using water, add it sparingly to avoid overly diluting the ink.
Mix Thoroughly:
Stir the mixture until it has a smooth, paint-like consistency. Adjust the amount of graphite or solvent to achieve the desired thickness.
Step 2: Test and Adjust Conductivity
Apply a Small Sample:
Using a brush or applicator, draw a small line on your substrate (paper, plastic, or film).
Dry Completely:
Let the ink dry for 10–15 minutes (time may vary depending on the binder used).
Measure Conductivity:
Use a multimeter to check the resistance of the dried line. Place the probes at both ends of the line.
Adjust the graphite-to-binder ratio if the resistance is too high.
Step 3: Design and Apply Circuits
Sketch the Circuit:
Draw the layout of your circuit on the substrate using a pencil or pen as a guide.
Apply Conductive Ink:
Trace over the circuit design with the conductive ink using a fine-tip applicator or brush.
Ensure smooth, continuous lines to avoid breaks in conductivity.
Dry Completely:
Allow the ink to dry thoroughly before testing the circuit.
Step 4: Test the Circuit
Attach Components:
Connect an LED, resistor, or small electronic component to your circuit using tape or solder.
Power the Circuit:
Attach a battery to the circuit using wires or alligator clips.
Observe:
Check if the component (e.g., LED) lights up or operates correctly. If not, inspect the circuit for gaps or high-resistance areas.
Analysis
Conductivity:
Measure the resistance of various lines to determine how the ink performs under different conditions (e.g., longer lines or thinner applications).
Flexibility:
Test the circuit by bending the substrate to evaluate how well the ink adheres and maintains conductivity.
Optimization
Better Conductivity:
Add more graphite powder to the mixture for improved conductivity.
Flexibility:
Use a flexible binder, like silicone-based adhesives, for circuits on stretchy materials.
Applications
Flexible Circuits:
Use the conductive ink to create simple sensors, touchpads, or artistic electronic designs.
Repairs:
Fix broken connections on PCBs or small devices with conductive ink.
Observations
Initial trials with nail polish as the binder showed that it dried rapidly and exhibited moderate conductivity. However, repeated bending of the substrate led to the formation of small hairline cracks in the conductive layer, resulting in increased resistance and, after continuous damage, loss of conductivity. The hard, brittle film formed by nail polish was found to be less suitable for applications requiring significant flexibility. Switching to PVA glue as the binder meant that samples dried more slowly but retained conductivity even after being bent multiple times. The more elastic nature of the PVA film means that it is able to accommodate more mechanical deformation without breaking the circuit.
Increasing the graphite content lowered resistance but made the ink thicker and more difficult to apply evenly. A balance had to be made between maximizing conductivity whilst maintaining ease of application. Measurements showed that shorter traces consistently exhibited lower resistance, as expected. Although longer circuits could still exhibit conductivity but with lower efficiencies.
Scientific Discussion
Graphite is an allotrope of carbon characterized by its layered structure. Each layer, known as graphene, consists of carbon atoms arranged in a two-dimensional hexagonal lattice. Strong covalent bonds within the layers provide structural stability, while weak Van der Waals forces between the layers allow them to slide over each other. This makes graphite soft and easy to grind into fine powders suitable for ink formulations. Between the layers, the conductivity is much lower. When graphite is processed into powder for inks, particle orientation becomes random, and conductivity depends on the degree of particle contact achieved in the dried film. The binder in the ink encapsulates the graphite particles, holding them in place and adhering them to the substrate. A continuous network of particle-to-particle contacts is essential for good conductivity, making the mixing process important. The solvent plays a temporary role, allowing the binder and particles to be applied evenly before evaporating.
Qualities of Conductive Inks
Graphite-based conductive ink circuits possess several notable qualities:
Low cost – Materials are inexpensive and widely available.
Customizability – Circuit designs can be created or altered rapidly without complex tools.
Light weight – The thin, painted traces add negligible mass to the substrate.
Flexibility – Circuits can be applied to bendable surfaces, although flexibility depends on binder choice.
These characteristics make graphite-based conductive inks particularly useful for rapid prototyping, educational projects, and low-power applications where really low resistance is not a requirement.
Applications
Graphite-based conductive inks have a wide range of potential applications, including:
Education tools: Allowing students to draw functioning circuits as part of STEM learning.
Prototype development: Quickly testing ideas before committing to a manufactured PCB.
Wearable electronics: Integrating simple sensors or LED arrays into clothing or accessories.
Interactive displays: Adding touch-sensitive areas to posters or books.
Repairs: Restoring broken connections in low-power devices.
Environmental monitoring: Deploying disposable sensors for temperature or humidity.
While their lower conductivity limits use in high-current applications, for many low-voltage, low-power uses, they offer a cost-effective and versatile solution.
Economic and Environmental factors
From an economic perspective, graphite-based inks are significantly cheaper to produce than their silver- or copper-based counterparts. The low material cost makes them ideal for educational use, where budgets may be constrained, and for applications where the circuits are disposable or experimental.
The environmental profile of graphite is favorable compared to that of metals requiring energy-intensive mining and refining. Graphite is an abundant, non-toxic, and chemically stable material. The primary environmental concern comes from the choice of binder and solvent. Solvents such as isopropyl alcohol are volatile organic compounds (VOCs) and should be used in well-ventilated areas to minimize inhalation risks. PVA glue, being water-based, avoids VOC emissions but requires longer drying times. Waste generation in this process is minimal, especially when compared to traditional PCB manufacturing, which produces significant chemical effluent. By using graphite inks with biodegradable substrates such as certain types of paper or compostable plastics, the overall environmental footprint of the circuit can be reduced.
Conclusion
This investigation demonstrated that a simple conductive ink can be produced from graphite powder, a binder, and a solvent, and that such an ink is capable of forming functioning circuits on flexible substrates. The performance of the ink depends strongly on the choice of binder, the ratio of graphite to binder, and the application method.
Nail polish-based inks offered rapid drying and acceptable conductivity but are prone to cracking under mechanical stress. PVA glue-based inks dried more slowly but maintained functionality after repeated bending.
While graphite-based inks cannot match the conductivity of metal-based formulations, they provide a safe, low-cost, and environmentally friendly option for applications where high conductivity is not essential. Their accessibility and versatility make them well-suited to education, rapid prototyping, and low-power flexible electronics.
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