Study of low-cost, flexible graphite-based sensor for human finger movement

Paper-based sensor devices have grown prominently in recent decades owing to their flexibility, light weight, biodegradability, and low cost. Pencil traces on the paper are the protuberant component to sense the physical parameter variation, and the geometry on the paper substrate plays a vital role in sensing applications. This study used the commercially available pencil as an electrode on a paper substrate that senses the mechanical strain on human finger movements. Two different geometries of the paper substrate were considered, and the optimized geometry was identified. Two different grades (HB and 8B) pencils were used as the drawing instrument to draw the graphite on the paper. The repeated experimental results show a better version of the proposed sensor for measuring human finger movements. The grade HB pencil provides excellent results compared to the other grade pencils.


Introduction
The production of flexible and wearable sensing devices has grown exponentially in recent decades due to material and structural science advancements.The nanostructure formation of the materials plays a vital role in fabrication of flexible health monitoring devices, microfluidic devices, touchpads, transistors, displays, and energy-storing devices.Different substrates have been utilized to develop flexible sensors.The reported research outcomes show that polydimethylsiloxane (PDMS) is a substrate for flexible force sensors, textile material is used to detect human motion, and plants and plant-derived biomaterials may also use to develop the adjustable sensor [1][2][3][4].The fabrication of flexible sensor is required the clean room facility, high cost material and also skilled person.However, the recent literature shows that the sensors can be made using the official material in the home.Hence, paper and pencil graphite is used to fabricate the sensor to measure the strain as human finger movement.Paper is a low-cost thin material prepared from cellulose and pulp fibers and a versatile material.There are different types of papers available to fabricate the paper-based device.The type of paper has been selected based on the sensing application [5].The more excellent performance of wicking ability in Whatman brand chromatography paper is used to fabricate flexible electrochemical sensors.Due to the high protein-binding capabilities of the Nitrocellulose membrane, it has been used to develop the sensor based on gold nanoparticle based assays.Office printing paper is also used to fabricate electronic, electrochemical, and wearable devices [6][7][8].The paper has various properties, such as lightweight, biodegradability, biocompatibility, transport liquid, and flexibility.With these advantages, enormous paper-based devices have been developed such as paper-based pregnancy testers, paper-based glucose test kits, paper-based gas sensors, paper-based touch sensors, paper-based sound source devices, paper-based power sources, etc. Paper generally acts as a substrate in a paper-based device [9][10][11].Micro or Nano forms of a chemical component may deposit on the paper substrate, and these devices produce output signals under specific mechanical, physical, or chemical parameter changes [12].Pencil graphite is highly reactive, sensitive, low cost, and easy to handle compared to other carbonbased materials.Pencil lead is graphite with clay particles and sometimes contains a little wax.The pencils have been categorized into grades (H and B) based on the graphite-to-clay ratio.The 'H' grade denotes the hardness due to the higher clay content.The other grade, 'B' is for blackness due to higher graphite content [13].Pencil-trace drawn on printing papers is the simplest and easiest way of constructing graphite-based devices [14].Pencil on Paper (PoP) based sensor fabrication technique is getting significant attention from researchers because of its simplicity, portability, environmental friendliness, and low cost.The PoP sensor can be fabricated minimally by supporting simple office tools (paper, pencil, and scissors) [15].The fabrication process doesn't require a cleanroom facility and complicated process steps like the MEMS-based sensor fabrication techniques [16].This paper is organized as follows.The graphite on the paper and its impacts are discussed in section 2.The proposed methodology for sensing the human finger movement and its output are presented in section 3. The signal conditioning circuit utilized in this work is discussed in section 4. The results & discussion is discussed in section 5 then followed by a conclusion in section 6.

Graphite on paper
The prime objective of this work is to develop a graphite-based sensor on paper to sense human finger movement.Pencil leads (HB, and 8B, Graded Pencil Usl-967) were used for the electrode, and plain documents (A4, 80 GSM, India) were used as substrate materials.Two different rectangular geometries (50 X 5) mm and (30 X 5) mm have been considered to trace the pencil graphite on paper.Two different grade pencils such as HB (Hardness) and 8B (Blackness) are employed to achieve the additional amount of graphite on paper.MECO makes 45CF -4 ½ Digit 20000 Count LCD Digital Multimeter were used to measure the resistance value.Figure 1.shows the 79.03 Kilo Ohm resistance output for the (50 X 5) mm rectangular geometry of twenty numbers free hand traced HB pencil.Figure 2. shows the 49.97 Kilo Ohm resistance output for the (30 X 5) mm rectangular geometry of twenty numbers free hand traced HB pencil.) mm and (30 X 5) mm rectangular geometry of twenty numbers free hand traced using an 8B grade pencil.It is observed that the geometry of the graphite region is minimized the resistance value is reduced.The hardness of the pencil grade also plays a vital role in resistance change, the higher hardness grade has a higher resistance value, and the higher blackness grade has a lower resistance value.The number of traces drawn on the paper also changes the resistance value.Table 1. and table 2. shows the resistance value for 10 and 20 trace drawn on the paper using HB and 8 B-grade pencils.The 10number trace drawn on the paper has a higher resistance value, while the 20 traces drawn on the paper have a lower resistance value.And also, HB grade pencils for 20 traces have more resistance variation than the 8B grade pencil.More than 20 trace drawn on the paper has no significant changes in resistance value.For further study of this work, it is considered the 20 number trace drawn on the paper by using an HB grade pencil, to sense the finger movement of the Human.Human finger for a straight and bent position of (30 X 5) mm detector.Table 3. shows the resistance value of 20 traces drawn on the paper using an HB grade pencil for two rectangular geometry of (50 X 5) mm and (30 X 5) mm.It is observed that the rectangular geometry of (50 X 5) mm has a resistance variation from 109.09Kilo Ohm to 179.88 Kilo Ohm.The Wheatstone bridge circuit has been designed for measuring the resistance variation, and the circuit's output voltage occurs in the microcontroller for further processing.

Result and Discussion
The experimental output for four different positions of the human finger movement has been observed.The repeated experiment cycles for the exact position of the finger were noted 20 times, and the average value of the change in resistance is considered for the analysis.It is observed that the sensor has produced approximately 180 Kilo Ohm resistance value for the fully bent finger position, and the corresponding amplifier output voltage is 2.66 Voltage.The resistance output for the finger's small bent part is approximately 120 Kilo Ohm, and the corresponding amplifier output voltage is 0.434 Voltage.
As the bent position of the finger increases, the change in the resistance value of the sensor also increases, and the output voltage increases.The theoretical and experimental output for the different movements of the human finger is represented in table 4. The error percentage of theoretical and practical amplifier output for the finger movement of the Human is presented in Table 5.The formula utilized for error percentage calculation is represented in equation( 5).The minimum error percentage of theoretical and practical amplifier output shows better performance of the proposed method.

Conclusion
The developed Pencil on Paper (PoP) based sensor is analyzed to sense the human finger movement.Two different sizes of the rectangular geometry of the paper substrate have been studied and observed that the (50 X 5) mm geometry produced better results.The performance of HB and 8B grade pencils has been analyzed, and it's concluded that the HB grade has higher resistivity because of the hardness.The number of traces drawn on the paper was also investigated, and it identified that the 20-number trace is optimized.The repeated experimental output shows that the better performance of the proposed graphite-based sensor is well-suited for sensing human finger movement.The minimum error percentage of theoretical and practical amplifier output agreements a better version of the proposed method.Due to the paper's low-cost, biodegradable, and environmentally friendly nature, this kind of device may be utilized to sense the Robot arm movement and its automation in the future.

Figure 3 .
Figure3. and 4. shows the 8.18 and 7.49 Kilo Ohm resistance of the (50 X5) mm and (30 X 5) mm rectangular geometry of twenty numbers free hand traced using an 8B grade pencil.It is observed that the geometry of the graphite region is minimized the resistance value is reduced.The hardness of the pencil grade also plays a vital role in resistance change, the higher hardness grade has a higher resistance value, and the higher blackness grade has a lower resistance value.

Figure 5 .
Figure 5. and Figure6.shows the placement of twenty numbers free hand traced HB pencil sensor on a Human finger for a straight and bent position of (50 X 5) mm sensor.

Figure 7 .
Figure 7. and Figure8.show the placement of twenty numbers free hand traced 8B pencil sensor on a Human finger for a straight and bent position of (30 X 5) mm detector.Table3.shows the resistance value of 20 traces drawn on the paper using an HB grade pencil for two rectangular geometry of (50 X 5) mm and (30 X 5) mm.It is observed that the rectangular geometry of (50 X 5) mm has a resistance variation from 109.09Kilo Ohm to 179.88 Kilo Ohm.The Wheatstone bridge circuit has been designed for measuring the resistance variation, and the circuit's output voltage occurs in the microcontroller for further processing.

Table 1 .
Resistance value for different traces of HB grade.

Table 2 .
Resistance value for different traces of 8B grade.

Table 3 .
Resistance value for a straight and bent finger

Table 4 .
Resistance value for straight and bent finger

Table 5 .
Error Percentage of Amplifier Output