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Overview Integrated Design Prototype Implementation: Test & Iteration Resources and Cost Back to top ⬆



Motivated by the desire to apply Plink to more testing scenarios, we designed the Plinker and E-Pili Cartridge as hardware prototypes of Plink that can be replicated and can be derivatively designed for different needs. Since our IHP is primarily aimed at stakeholders in the community healthcare system, the needs of different stakeholders were considered and responded to in the design of the hardware. Our hardware prototype in the project is designed for community family doctors and is derived as [Plinker for Family Doctor].

出于希望将Plink应用到更多检测场景的愿望,我们设计了Plinker和E-Pili Cartridge,将其作为可复制和可为不同需求衍生设计的Plink的硬件原型。由于我们的IHP主要面向社区卫生系统中的利益相关者,在硬件的设计中考虑和回应了不同利益相关者的需求。项目中我们的硬件原型为社区家庭医生而设计,衍生为[Plinker for Family Doctor]。

Integrated Design

From Requirements to Design Choices

In the IHP section we interacted with stakeholders to gather their opinions and needs on the topic. We visualized their needs in fig.1 below and translated them into design points for the hardware as a response. In the design choices section we listed the ideas and solution options corresponding to the different design points based on the needs to describe the possible means to fulfill the needs. We have labeled the final adopted solution in blue and explained why, as shown in fig.1 below.


Figure 2

Fig.1 Requirements -- Design Choices

Frame of 3 Modules

In our hardware design, we follow the technical principle of the biosensor structure: E-Pili acts as a sensitive element and responds to changes in the concentration of the substance being measured by means of an immune response, changing its own conductivity and the magnitude of the resistance value. Therefore, the environmental signal (substance concentration) can be converted into an electrical signal output.


Module 1 -- E-Pili Cartridge

In Module 1, we determine the form of the combination of E-Pili and hardware.


In our design, since the system needs to detect samples with possible pathogenic hazards. In order to maximize the protection of the operator of this hardware, we have broken down our hardware into two segregated parts, a disposable test consumable and a multi-use measurement and data processing unit. Our E-Pili will be incorporated into the disposables and the cost of our disposables is quite manageable due to their feature of being mass-produced by engineered microorganisms. We have named the disposable E-Pili Cartridge.

在我们的设计中,由于我们的系统需要检测具有可能的病原危害性的样本。为了最大程度的保护这台硬件的操作者,我们将我们的硬件分解成一次性检测耗材和多次使用的测量和数据处理装置这两个之间相互隔离的部分。我们的导电菌毛将结合在耗材上,由于其具有可以被工程微生物大规模生产的特点,我们的耗材的成本是相当可控的。我们将耗材命名为E-Pili Cartridge。

The E-Pili Cartridge needs to provide a platform that allows the modified E-Pili and the disease marker to be tested to fully bind. After several iterations we finally settled on using the most commonly used structure for rapid screening kits, lateral flow immunochromatography, as the substrate for our disposable. We immobilize the purified conductive hairs in the region to be tested on the NC membrane, and the sample solution to be tested added on the other side will flow across the NC membrane under the moisture gradient provided by the absorbent pad, and eventually bind to the hairs in the region to be tested.

E-Pili Cartridge需要提供一个让修饰后的菌毛和待测疾病标志物充分结合的平台。经过了多次的迭代后我们最终确定了使用快速筛查试剂盒最常用的结构---侧流免疫层析作为我们的耗材的基底。我们将纯化后的导电菌毛固定在NC膜上的待测区域中,在另一侧加入的待测样品溶液会在吸水垫提供的水分梯度的作用下在NC膜上流动,最终在待测区域和菌毛结合。

Figure 2

Fig.2 Lateral Flow Immunochromatographic Test Strip Structure

Module2 -- Test Module

In Module 2, we implement an accurate measurement of the conductivity of the area to be measured.


In our system, the accuracy of quantification is directly proportional to the precision of the conductivity measuring instrument. In order to obtain better quantitative results, we design a PCB for detecting the resistance value and analyzing it.


Based on the STC15 development board, we designed a high-precision conductivity measurement system, which uses the STC15W4K56S4 development board as the core chip. To ensure accurate conductivity values, the system employs a reference resistor switch comparison method, enhancing the credibility of the measurement results.


For complex biological signals, the system adopts multiple sampling and signal processing techniques to perform pre-processing operations such as filtering and denoising on the measurement data to improve the accuracy and stability of the data. By transforming the processed data into an execution scheme, the system is able to carry out further data analysis and calculation, which provides a strong support for the study of bioelectrical conductivity.


Eventually, the measurement results are displayed intuitively on the screen in the form of data results, providing a visualized data presentation effect.



Additionally, with the WiFi module, the system can reliably transmit real-time measurement data to a mobile phone, allowing users to monitor and analyze data in real-time for decision-making purposes.

Furthermore, the conductivity of E-Pili may be affected by the ambient temperature and humidity. As a system with high requirements for quantitative accuracy, our hardware must be able to realize the quality control of the generated data and automatic correction within a certain error range. For this reason, we also designed the Blank program and the quality control steps in this module.

Quality Control System

In classic qualitative kits for antigen detection, a simple idea is to use the inclusion of a quality control line (C-line) as a criterion for determining whether the system is working properly in order to avoid false-negative results. In our system, since our hardware is able to read the conductivity change of the material over a period of time, an easy way to think of it is to use the curve of the change in the conductivity of E-Pili by a standard buffer of known pH as a reference for the quality control of the system and correction for environmental factors.


In our design, we need to add a total of three solutions to the Cartridge. The first addition is a buffer of known pH, and the last two additions are the sample solution to be tested and the elution solution, respectively. The first addition of the buffer and the measurement of the change in the conductivity of the bacterial hairs is treated as a Blank step and is incorporated into the design of the process and the Plinker program for systematic quality control and correction of environmental factors.


The first addition of buffer serves three purposes:


1. Since the Cartridge containing E-Pili should be stored in a low-temperature, dry environment, the buffer can wet the dried E-Pili to bring them into working condition.

1. 由于含有菌毛的检测耗材应该存贮在低温干燥的环境中,缓冲液可以润湿干燥的菌毛使其进入工作状态。

2. This step serves as a blank control group (Blank) for subsequent measurements and is used directly to correct subsequent assay results.

2. 该步骤作为后续测量的空白对照组(Blank),直接用于修正后续的检测结果。

3. The conductivity curve generated by this step can be compared with the model stored in the system. If the deviation is within a certain range, it is considered to be an effect of the environment (the correction process can also be modeled in relation to the temperature and humidity of the environment to improve accuracy), and this part of the effect is eliminated by the algorithm during the subsequent measurements. If the deviation exceeds a certain range, the system is considered faulty and we need to check if there is a problem with the experiment or change the Cartridge to avoid possible false positives or false negatives.

3. 该步骤产生的电导率随时间变化曲线可以和系统内存储的模型进行对比,如果偏差值在一定范围内,则被视为环境因素的影响(该修正过程还可以结合环境的温度和湿度建立模型来提高精确度),后续的测量过程中该部分的影响会被算法消除。如果偏差值超过一定的范围,则认为该检测系统出现问题,我们需要检查实验的操作是否存在问题,或者更换Cartridge以规避可能出现的假阳性或者假阴性。

Module3 -- Support Module

In Module 3, we have designed our device to be more user-friendly.


In order to lower the threshold of quantitative testing, our support module includes a wifi module for data transmission, a display screen and buttons for user interaction. The test module and support module are integrated into Plinker, which will likely be used multiple times by healthcare professionals in a variety of scenarios, so the product is styled with ergonomic considerations to make it easy for users to insert the Cartridge into the exact area they are intended to be inserted.

为了降低定量检测的操作门槛,我们的Support Module包含进行数据传输的wifi模块、以及与用户交互的显示屏幕与按键。其中test module和support module集成在Plinker中。Plinker将可能被医护人员在多种场景下多次使用,因此产品的造型有人机工学上的考量,方便用户将耗材准确的插入指定的区域。

Figure 3

Fig.3 Three Modules of Plinker

How to use

Figure 4

Fig.4 Usage Diagram

In our research, the community healthcare system does not usually conduct immunization testing, and healthcare workers are not involved in the use of the same functionality as Plink in their daily work, nor have they been trained to operate the device. Therefore, equipping Plink in the community requires low learning costs and user friendliness, and we hope that healthcare workers in the community health system can complete accurate tests efficiently and at a low cost.


Figure 5

Fig.5 Plinker Interaction Diagram

1.Start Plinker and select the detection item.

1. 启动Plinker,选择检测项目。

2. Add buffer dropwise to the E-Pili Cartridge, insert into the Plinker, and press Start.

2. 向E-Pili Cartridge中滴加缓冲液,插入Plinker,按下Start

3. Drop the sample to be tested into the sampling hole.

3. 用户受到提示后往耗材上样孔中滴加待测样品

4. Add eluent to the cartridge for elution

4. 用户受到提示后往耗材上样孔中加入洗脱液洗脱

5. The user receives a detection end prompt and receives detection data.

5. 用户收到检测结束提示,并得到检测数据。


Module1 -- E-Pili Cartridge

Our Cartridgeare referenced in the form of antigen kits that use lateral flow immunochromatography to construct a binding platform for E-Pili and the molecule to be tested.


We purchased an absorbent pad, a sample pad, and an NC membrane, combined the three with reference to the existing structure, and fixed the E-Pili on the NC membrane near the end of the absorbent pad. Based on this result, we added a interdigital electrode to the upper layer of E-Pili to detect the conductivity change of it, which was connected to the test module through a wire extending to the outside.


In order to minimize the cost of manufacturing the hardware, off-the-shelf materials were used to fabricate the detection electrode of the E-Pili Cartridge as readily available as possible.

出于降低硬件的制造成本的考量,我们选用尽可能易获取的现成材料来制作E-Pili Cartridge的电极。

The Cartridge electrode consists of two parts: a polyimide (PI)-based copper interdigital electrode for contacting E-Pili, and pogopin for connecting the interdigital electrode to the Plinker electrode. Since our test consumables need to be inserted into the hardware, we chose the commonly used pogopin (an electrode with a spring-loaded contact at the tip, which allows for a certain amount of spatial tolerance in the contact between the different parts but keeps the circuit on) in order to allow for a stable connection to the test module. Since the fork-finger electrode needs to contact the hairs downward and the pogopins upward, we bent it so that the fork-finger part can make full contact with the hairs while the two ends of the electrode can make good contact with the pogopins.


In terms of case design, we made an opening at the end of the sample pad as a loading hole, and two round holes at the end of the absorbent pad as contact points for the Cartridge and device electrodes. Due to the small size of our Cartridge prototype and fabricated by FDM printing, the two parts of the casing are not connected using snap fasteners (which may be damaged due to lack of strength), but rather 2° beveled cutouts.


Figure 6

Fig.6 Structure of E-Pili Cartridge

After 3D printing the Cartridge, we found that the prints could not be assembled due to insufficient tolerance, the 2° bevel cutout was too large and the pogopins could not be accurately seated in the holes due to the tolerance. In the new version of the Cartridge, we used light-curing resin SLA 3D printing and changed the 2° cutout fixation method to four round holes. At the same time, we widened the notch where the test strip is placed.

我们在3D打印出Cartridge后,发现由于打印件由于公差过大而无法组装,2°的斜切口过大且Pogopin也因公差无法准确卡在孔位中。我们在新一版Cartridge使用光固化树脂SLA 3D打印,并将2°的切口固定方式改为四个圆孔位。同时我们还加宽了放置试纸条的凹槽。

After testing, the new version of the Cartridge can be assembled very smoothly. The STL file of the casing can be downloaded from the link at the bottom.


Figure 7

Fig.7 E-Pili Cartridge Prototype_V2

Module2-3 -- Test & Support (Plinker)

PCB Design

For the design of the measurement circuit, we used multiplexing technology and resistor dividers to switch between different voltage ranges. We utilized switch circuits to switch between different resistor ranges, allowing us to achieve the resistance range switching. To achieve more accurate resistance measurements, we used suitable ADC modules to display the measured resistance values on a digital display screen. We tested the circuit using a breadboard, dupont wires, and other electronic components to ensure its proper functioning. Afterward, we added filtering circuits and signal processors to optimize the measurement results, making them more accurate and reliable, while taking the actual situation of the tested objects into consideration.


To design the circuit, we initially designed the required circuit diagram based on the circuit design requirements and determined the relevant components. We then carried out theoretical calculations to determine the specific parameters of the components. Next, we simulated the circuit in simulation software and fitted and debugged the related parameters with existing components. Finally, we soldered and tested the components. After the testing results were positive, we used CUBEMX to configure and generate the code, and implemented the main function and related functions in KEIL. Through the above process, we successfully designed a signal amplifier processing circuit with high reliability and accuracy.


Figure 8

Fig.8 Circuit Diagram

For the PCB layout, we tested the feasibility of the circuit on a breadboard and designed the circuit using KICAD and JiaLiChuang EDA software based on the test results. By drawing the schematic and circuit diagrams, we transformed the prototype circuit diagram into a PCB diagram that is easier to assemble and lay out. During the design process, we considered various factors such as line width, voltage, and component spacing to ensure the reliability and stability of the PCB board. After completing the design, we sent the created PCB layout to a PCB manufacturer to produce the actual PCB board. During the PCB board manufacturing process, chemical and mechanical processing, circuit pin and hot melt processing, electroplating, and chemical treatment processes were used to print the PCB's position according to CAD diagrams and generate the PCB's circuit. Finally, we soldered the electronic components onto the PCB board, allowing the PCB board to fully implement the circuit function.


Figure 9

Fig.9 Feasibility Test of the Circuit on A Breadboard

Finally, we solder the electronic components into the PCB according to the PCB diagram, so that the PCB board can realize the circuit function completely. For the PCB design, we considered the problem of combining hardware with the actual measurement object and designed the reference resistor as an adjustable resistor. Initially, we tested with resistances of 10^n, using resistors ranging from 10Ω to 100kΩ, which were connected in descending order between the 16-way switches c0 to c5. After testing, we measured the approximate transformation range of the tested object using precision instruments and replaced the resistors with more precise ones that were closer to the resistance range and had higher accuracy, while updating the programming algorithm to complete the calibration of the resistance. This was more conducive to data processing and algorithm optimization of the tested object."


Figure 10

Fig.10 Finished Prototype of PCB

Connection & Casing Design

Various options have been considered to determine the connection between the detection module and the Cartridge.


Figure 11

Fig.11 Process Solutions for Connection Methods

For example, in order to minimize material waste and reduce the manufacturing cost of Cartridges, we had hoped to place the electrodes at the end of the device, with a mechanical structure designed to allow the electrodes to move and contact E-Pili in the Cartridge. However, we subsequently considered that this approach would introduce additional errors into our system, and the part of the electrode in contact with E-Pili would need to be re-cleaned after each measurement, which would not allow for easy and efficient operation by healthcare workers. Therefore, we chose to prevent the test module from being contaminated by the sample to be measured by building an electrode into the Cartridge as a firewall while simplifying the user's operation.


In addition, we considered the human-machine relationship for desktop use. Since the Cartridge need to be inserted into the Plinker device and then go through the desktop operations of blank, filling and eluting steps, it is important to make sure that it is easy and comfortable for the user to drop liquids into the Cartridge once it is inserted into a Plinker. We tested a variety of angles and found that it was more comfortable to fill the sample when the Cartridge was at a slight angle of 10° above the horizontal.


Figure 12

Fig.12 Comparison of Two Connection Methods

In order to make a prototype ready for user testing, we designed a casing for the Plinker that could be raised 10° on the Cartridge side and 20° on the screen side. The interior is large enough to accommodate the screen, PCB, and cables. For easy viewing and debugging, we laser cut 3mm clear acrylic panels and assembled them to create the Plinker's casing. For the insertion of the E-Pili Cartridge, we SLA 3D printed a slot using light-curing resin that fits seamlessly into the Plinker casing to accommodate the Pogopin electrodes on the Plinker side (which is used to connect to the Pogopin electrodes on the Cartridge side), as well as the Cartridge itself. The STL format model of the slot and the laser cut DXF file of the housing can be downloaded at the link at the bottom.

为了做出一个可以进行用户测试的原型,我们为Plinker设计了一个可以将Cartridge侧抬高10°,屏幕侧抬高20°的外壳。内部空间足以容纳屏幕、主板以及杜邦线。为了方便观察和调试,我们激光切割3mm透明亚克力板,并将其拼接出Plinker的外壳。在E-Pili Cartridge插入的部分我们用树脂光固化3D打印了一个可以和外壳无缝契合的插槽,插槽用于容纳Plinker端的Pogopin电极(用于与Cartridge端的Pogopin电极连通)以及Cartridge。插槽的STL格式模型和外壳的激光切割DXF文件可以在底部的链接下载。

Figure 13

Fig.13 Side View of the Plinker

To make it easier to test the Plinker later, we used copper alligator clips to connect the pogopin electrodes extending from the Plinker end, and the Test Module at the other end. we also placed a plastic sheet between the two pogopins to prevent short circuiting.

为了后续能够更方便的测试Plinker,我们用铜鳄鱼夹连通延伸出来的Plinker端的Pogopin电极,在另一端连接Test Module。我们还在两个pogopin之间垫了一个塑料片以防短路。

Figure 14

Fig.14 Copper Alligator Clips to Connect the Pogopin Electrodes Extending from the Plinker End

Figure 15

Fig.15 The Plinker assembled

Implementation: Test & Iteration

Test of WiFi Module

To validate the WiFi functionality of the hardware device for measuring conductivity, a series of tests is required to ensure that the module works properly and operates in conjunction with the mobile application. Firstly, it is necessary to check if the module is functioning properly, set the WiFi connection parameters, and verify the connection status. Additionally, attention should be paid to the stability of the network environment and correct connection settings. This involves conducting connection tests and tests for sending and receiving messages to verify the stability of data transmission and reliability of communication. When writing the code, it is important to use the serial communication function for calling to ensure the proper functioning of the module. Finally, download a TCP client software on the mobile device and input the IP address and port number of the ESP8266 module for connection testing. This is done to ensure that communication is working correctly. This process verifies the stability of data transmission and reliability of communication for the WiFi module.


Real-time monitoring of temperature changes through temperature sensors, and transmission of monitoring data to the mobile phone through the WiFi module. If the data received by the mobile phone is consistent with the temperature data monitored in real time by the computer, the feasibility of the WiFi module can be proved.


Plinker and E-Pili Cartridge Connection

We finished assembling the E-Pili Cartridge and inserted it into the slot, dropping buffer into the Cartridge. After a few seconds, the resistance value on the screen changed, meaning that the connection between Plinker and E-Pili Cartridge was successful.

我们将E-Pili Cartridge组装完成后插入插槽中,在Cartridge中滴入缓冲液,几秒后屏幕上的电阻值发生变化,意味着Plinker和E-Pili Cartridge之间的连接成功了。

Figure 16

Fig.16 Before and after Inserting E-Pili Cartridge into Plinker

Simulation Experiment

Since we still do not have high purity E-Pili available for testing our hardware, a simulation experiment was set up in order to test our hardware.


This experiment utilizes the difference in conductivity of saline solutions with different ionic concentrations to simulate the change in conductivity of E-Pili under different conditions of concentration of the substance to be tested.


We added Tris-HCl buffer to the sample hole and again added Tris-HCl buffer with an additional 5% KCL at the sample hole when the conductivity indication stabilized. The conductivities of these two solutions were measured individually (by adding these two solutions dropwise directly to the interdigital electrode) as 3.134 mS and 3.845 mS, respectively. Finally, we can get the following results:


Figure 17

Fig.17 Simulation Experiment Results

We can observe that after the addition of Tris-HCl buffer, the buffer flowed laterally across the NC membrane, and eventually the number stabilized at 2.712 mS (after about 1 minute). After the number stabilized, we added Tris-HCl buffer with an additional 5 percent KCl, and the number started to change after about 15 seconds, and finally the number stabilized at 3.533 mS (about 1 minute).


Although there is a certain error (0.5mS) between the conductivity value measured after chromatography and that measured by directly adding the solution dropwise to the interdigital electrode, the difference between the conductivities of different ion concentrations measured by the two methods is relatively close (0.711mS vs. 0.821mS). This result can be a good proof that our system is relatively accurate.

虽然经过层析后测量得到的电导率数值和直接将溶液滴加到电极片上所测量得到的数值存在一定的误差(0.5mS)但两种测量方法所测得的不同离子浓度的电导率的差值是比较接近的(0.711mS VS 0.821mS)。这一结果可以较好的证明我们的系统是相对精确的。

Resources and Cost

An E-Pili Cartridge costs 3.05 CNY, which translates to $0.418, and a Plinker costs about 78 CNY, which translates to $10.689.

一个E-Pili Cartridge成本为3.05人民币,折合为0.418美元;一个Plinker成本约为78人民币,折合为10.689美元。

E-Pili Cartridge

Item Count Price (RMB)
E-Pili Cartridge
NC Membrane 1 0.3
Sample Pad & Absorbant Pad 1 0.1
HIPS Casing 1 0.3
PVC Sheet 1 0.06
Interdigital Electrode 1 2.0
Pogopin 2 0.15
Total 3.05


Item Count Price (RMB)
LCD Screen121.0
ESP8266 Module18.0
2.54mm Pin header and nut11.0
Resistor Element Kits12.0
16-channel analog multiswitch14.0
ABS Injection Molded Case110.0
Total 78