The Development of an Affordable Graphite-Based Conductive Ink for Printed Electronics ^†

Abstract

Printed electronics (PEs) are rapidly attracting interest, especially in wearable sensors, smart textiles, and IoT devices. Conductive inks, essential for the fabrication of PE, must be highly conductive, cost-effective, biocompatible, easy to prepare, and less viscous. Conductive inks comprise a conducting material (metals like silver, gold, copper, or carbon-based alternatives like graphite, graphene, and carbon nanotubes), a binder, and a solvent. In this work, a water-based graphite conductive ink is developed using graphite as a conductive material, corn starch powder (non-toxic and biodegradable) as a binder, and distilled water as a solvent. Firstly, corn starch powder is added to distilled water, which is heated up to 100 °C and stirred continuously until a homogeneous gel-like mixture is formed. After cooling the mixture, graphite powder is added to it, and it is stirred for an hour at 450 rpm to obtain the ink. To check the conductivity, the ink is brush-painted on a paper substrate with a dimension of 20 mm × 10 mm and the result shows a low ohmic resistance of ~560 Ω, confirming the highly conductive nature of the ink. Additionally, thermogravimetric analysis (TGA) is performed on the prepared ink to evaluate its thermal stability, and a very strong X-ray diffraction (XRD) peak obtained at 2θ° = 26.5426° and a small peak at 2θ° = 54.6145°, along with a few other small peaks, confirms the presence of graphite with corn starch. Thus, this conductive ink can be used for PEs owing to its affordability, biocompatibility, and ease of preparation.

1. Introduction

Recent years have seen a significant advancement in printed electronics (PEs) through the investigation of a variety of materials and fabrication techniques. PEs find applications in wearable sensors for health monitoring, smart textiles, and IoT devices, among other applications. They offer advantages such as cost-effective production, durability, and being lightweight [1]. These electronics primarily employ fabrication techniques such as screen printing, flexographic printing, or inkjet printing to deposit conductive inks on flexible substrates [2]. The conductive ink plays an important role in the fabrication process as it enables the printing of electrically conductive patterns on a range of substrates like paper, plastics, and textiles [3]. High electrical conductivity, low viscosity, biocompatibility, ease of preparation, and good adhesion to a range of surfaces are essential properties of conductive inks [4].

The three main components of a typical conductive ink are the conducting material, a binder agent that firmly holds the conductive particles together, and a solvent that suspends the mixture and adjusts its viscosity [5]. The most commonly used conductive materials include metallic nanoparticles such as copper, gold, and silver, as well as carbon-based substances including graphite powder, graphene, and carbon nanotubes. In general, metallic-based conductive inks often show better electrical conductivity but are less biocompatible and more expensive than their carbon-based counterparts [6]. Polymers like polyurethane, epoxy resin, alkyd resin, etc., are used as binders [7]. The two primary types of conductive inks—solvent-based and water-based—are classified based on the type of solvent used. Although solvent-based inks are preferred for their ease of use, low viscosity, and quick drying, large-scale manufacturing is restricted by their high cost and toxicity. Water-based inks, on the other hand, are non-flammable, economical, and eco-friendly [8].

Carbon-based graphite is readily available in highly pure form, making it a widely preferred choice for preparing conductive inks [9]. Currently, researchers are focusing on developing affordable, biocompatible, and easy to prepare graphite-based conductive inks. As reported in [10], a low-cost conductive ink was developed using graphite and nail polish (50:50%) with acetone as an organic solvent, exhibiting an average ohmic resistance of 2.17 kΩ. Another study proposed a conductive ink composed of graphite and carbon black as conductive materials and nail polish as a binder (35.3:11.7:53%), dispersed in acetone as a solvent [11]. An inexpensive conductive ink based on graphite powder and commercial glue was proposed in [12]. Authors [13] developed an enzyme-based biocompatible ink using graphite, chitosan and glycerol for wearable biosensors. Another biocompatible, low-cost ink was developed using graphite and gum arabic for printed electronics [5].

The current work proposes the preparation of an affordable, biocompatible, eco-friendly, and simple to prepare water-based graphite conductive ink using graphite as the conductive material, corn starch powder as the binder, and distilled water as the solvent. The prepared ink was brush-painted onto a paper substrate. The conductivity of the ink was tested and different characterization techniques, such as XRD and TGA, were performed to analyze the conductive ink.

2. Materials and Methods

2.1. Materials

Graphite fine powder (98% extra pure), corn starch powder (extra pure), and distilled water were procured from Loba Chemie Pvt. Ltd. (Mumbai, India). All materials used were of analytical grade.

2.2. Preparation of Graphite-Based Conductive Ink

The water-based graphite conductive ink was prepared by heating 20 mL of distilled water to 100 °C. Then, 1 g of corn starch powder was added to the heated water and continuously stirred at 450 rpm until a gel-like consistency was achieved. After cooling the solution for a few minutes, 3 g of graphite powder was then added, and the mixture was stirred for an hour at 450 rpm. The end product was a black, homogeneous, and viscous fluid. The ink preparation process is shown in Figure 1.

After preparation, the ink was applied to an 80 GSM paper substrate using a paintbrush and air-dried for 20 min. The remaining ink was stored in an air-tight vial for future use. It was observed that the stored ink remained stable for 3 days, after which slight coagulation began. By the seventh day, the ink exhibited noticeable lump formation.

2.3. Characterization Techniques

A sample of the prepared conductive ink was characterized using X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The XRD plot was obtained using the Empyrean Power X-ray Diffractometer (Malvern PANalytical, Almelo, The Netherlands) with a start position [°2θ] = 5.0155, an end position [°2θ] = 99.9935, and step size [°2θ] = 0.0260. The TGA was conducted using the Thermal Analysis System TGA 2 (Mettler Toledo, Columbus, OH, USA) over a temperature range of 25 °C to 400 °C. Sigma-300 Field Emission Scanning Electron Microscope (FESEM) (Zeiss, Germany) was used to study the thickness and uniformity of the deposited conductive ink.

3. Results and Discussions

3.1. Conductivity Test

To test the conductivity of the prepared ink, the ink was brush-painted on a paper substrate with dimensions of 20 mm × 10 mm. To achieve uniformity, two layers of ink were applied. Wires were connected to the substrate using copper tape to establish electrical contacts. However, the manual brush coating method can introduce variability in layer thickness, potentially affecting conductivity measurements. To study such variation, three samples were prepared. The ohmic resistance of each sample was measured five times using a digital multimeter. The mean, variance, and standard deviation were then calculated, as illustrated in

Table 1. Statistical analysis of the conductivity measurement.

Sample	Mean (Ω)	Variance (Ω2)	Standard Deviation (Ω)
A	558.8	313.36	17.70
B	658.6	581.84	24.12
C	614.6	885.84	29.76

. The lowest mean ohmic resistance was found to be ~560 Ω, highlighting the conductive nature of the ink.

A light-emitting diode (LED) was successfully powered by the prepared conductive ink. Two paper substrates with the ink applied on them were connected to the positive and negative terminals of a battery, respectively. The highly conductive nature of the ink established a conductive pathway, enabling the LED to emit light, as illustrated in Figure 2b.

3.2. Characterization

The structural analysis of the prepared conductive ink was performed using XRD, as shown in Figure 3a. The intense peaks at 26.540° and 54.610° correspond to graphite, confirming its presence [14]. The less intense peaks at 15.14°, 17.12°, 17.98°, and 22.93°, respectively, indicate the presence of corn starch. The small peaks corresponding to corn starch are attributed to its low concentration in the prepared ink [15,16].

The thermal behavior of the conductive ink was evaluated at a heating rate of 10 °C/min in a nitrogen environment. A gradual weight loss was observed up to 100 °C, attributed to the evaporation of moisture present in the composite. Negligible weight loss occurred beyond 100 °C, with a total weight reduction of 2.2% at 275 °C. A sharp weight loss was noted after 275 °C, corresponding to the thermal decomposition onset of the corn starch binder, leaving a residual weight of 93.7% at 400 °C.

Field Emission Scanning Electron Microscope (FESEM) analysis was performed to assess the uniformity and the thickness of the ink coating on the paper substrate, as shown in Figure 4. A slight unevenness was observed in the layer thickness due to the manual brush painting technique. However, an average thickness of 105.9 µm was calculated.

A comparison between this study and the existing literature is summarized in

Table 2. Comparison of the existing literature with the current work.

Reference	Ink Composition	Solvent	Substrate	Measured Resistance	Application	Advantages
[10]	Graphite and nail polish	Acetone	Paper	2.17 KΩ	Electrochemical sensors	Easy preparation and cheap
[11]	Graphite, carbon black, and nail polish	Acetone	Polyethylene terephthalate (PET) sheets	Low	Electrochemical sensors	Easy to prepare, low cost, and adequate homogeneity
[12]	Graphite and craft glue	Acetone, ethyl acetate, and glycerin	Ecoflex™	-	Electrochemical sensors	Inexpensive and easy to prepare
[13]	Graphite, chitosan, and glycerol	Acetic acid and water	Polyethylene terephthalate (PET) sheets	Resistance to the charge transfer (RCT) = 9.6 ± 0.6 kΩ	Wearable biosensors	Enzyme-based biocompatible water-based ink
[5]	Graphite and gum arabic	Water	Paper	Range of ~kΩ	Printed electronics	Simple, low cost, and environment friendly
[This work]	Graphite and corn starch	Water	Paper	~560 Ω	Printed electronics	Easy to prepare, affordable, and biocompatible

.

4. Conclusions

This study presents the development of a conductive ink for application in printed electronics using graphite, corn starch powder, and distilled water. A simple and rapid process of preparation was followed for the synthesis of the ink. The resulting ink demonstrated a low ohmic resistance, indicating its high electrical conductivity. Analytical techniques such as X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were carried out to characterize the ink. XRD results confirmed graphite as the predominant component in the formulation, while TGA revealed that the ink maintains thermal stability up to 275 °C, beyond which thermal degradation occurs. Thus, the developed conductive ink offers potential as a cost-effective and eco-friendly alternative in printed electronic devices.