EnderScope: A Low-Cost 3D Printer Based Microscope for Microplastic Detection

Session

Sustainability and Carbon Net Zero

Authors

Ms Niamh Burke (2), Ms Gesine Müller (1), Dr Vittorio Saggiomo (3), Dr Emmanuel Reynaud (2), Dr Mark Pickering (2)

Affiliations

1. Georg August University
2. University College Dublin
3. Wageningen University and Research

Keywords

Open-source Microscopy, Microplastics, Fluorescence Microscopy

Abstract text

Here we describe a low-cost microscope for automated scanning and detection of microplastics in filtered seawater samples - the EnderScope. This microscope is based on the mechanics of a low-cost 3D printer (Creality Ender 3). Using the reliable and well-calibrated motion system of the 3D printer, it is possible to automate the scanning of a large filtered sample, as large as the print bed (>20x20cm). We aimed to simplify and reduce the cost of this tool where possible, prioritising accessibility and useability over image quality. In essence, this tool only needed to be capable enough to detect microplastics.

Microplastics are defined as small plastic particles <5mm in size (Andrady, 2011). Due to the way in which microplastics are transported in the environment, many microplastics will end up in the sea as marine microplastics (Horton and Dixon, 2018). While marine microplastic pollution is a global problem, the extent of the problem remains unknown. Under target 14.1 of the UN Sustainable Development Goals we have a responsibility to prevent and reduce all marine pollution by 2025 (General Assembly resolution 71/313, 2017). Microscopy can be a key tool in allowing us to understand of the extent of the microplastic pollution problem, and will also allow us to assess the effectiveness of any mitigation strategies. In order to obtain a baseline measurement of the microplastic content of the ocean we need scalable and reproducible technologies that allow people, including those outside the traditional research laboratory environment, to measure microplastics in their local environment. 

A simple way to measure marine microplastics involves taking a sample of seawater, digesting the organic material and filtering the remaining inorganic material through a filter paper. The filter paper is then stained with a fluorescent dye, Nile Red. When coupled with traditional fluorescence microscopy, Nile Red allows for these plastic particles to be manually counted and detected. However, this is a resource and labour intensive task. In addition, a commercial fluorescence microscope is still a relatively expensive instrument (costing thousands of euros) which is a barrier to a truly accessible and scalable solution. An automated, low-cost and easy to build fluorescence microscope would not only make this Nile Red staining method more efficient, but would allow this technique to be accessible to a wider range of people. 

The EnderScope is based on the mechanics of a low-cost 3D printer (Creality Ender 3 Pro, €230). The hotend of the printer is replaced with an optics module. However, instead of sacrificing the 3D printer to create a new tool (as done in similar systems (Merces et al., 2021)) the microscope is designed such that the 3D printer retains its printing capabilities. This is achieved through a quick-change mechanism, allowing the user to easily swap between the hotend of the original 3D printer and the optics module. 

The EnderScope is capable of both reflected light and fluorescence imaging. In both configurations we aimed to make the design as simple and as cost effective as possible, for example, by using low-cost LEDs for illumination (5mm round head LED, 2 pin, €0.08) and photographic gels (Neewer 35PCS Universal Photography Speedlite 47x77mm Square Full Color Balance Gel Filter Kit, €8.50) as emission filters. Many of the optomechanical parts are 3D printed to reduce costs. Images are acquired using the low-cost Raspberry Pi High Quality Camera (€50) and a low-cost finite conjugate objective lens (4x, NA = 0.1, €8). The system is operated from a Raspberry Pi single board computer (Raspberry Pi 4 Model B, €50).

The approach we used to assess the function of the system was twofold. First, we characterised the overall performance of the imaging system by determining the modulation transfer function (MTF) using the slant edge method. Specifically, we determined this in both reflected light and fluorescence configurations which allowed us to measure the impact of the simple emission filters used in the design. This was done by taking an image of a slanted edge of a black square located on a USAF 1951 resolution test target and applying the following ImageJ/Fiji plugin: SE_MTF_2xNyquist.jar. We also confirmed that the system was suitable for the purpose for which it was designed (i.e. detecting microplastics) by taking sample images of Nile Red stained plastic particles. 

Figure 1: Slanted Edge MTF Resolution Test

The MTF profiles for images acquired with the EnderScope in both reflected light and fluorescence modes are shown below in figure 1B and 1D respectively. The MTF plots predict the performance of the system (in terms of modulation) as resolution is increased. Typically, MTF plots show a sloped line, starting with a modulation factor close to 1 and then sloping downwards as resolution is increased. We see a slight difference in the slope of the MTF curves from the image taken in reflected light (fig 1B) and fluorescence mode (fig 1D). In the image taken in fluorescence mode, the modulation drops off at a lower resolution compared to reflected light mode. This can be explained by the addition of the photographic gel emission filter into the optical path. 

Our system is capable of detecting Nile Red fluorescence and resolving microplastics, as shown in figure 2. In this image the plastic particles are 0.2-5 mm in size, typical of marine microplastics. 

Figure 2: Image of Nile Red stained microplastics acquired using the EnderScope

Costing <€350, this scanning microscope, capable of detecting microplastics in filtered samples, is a fraction of the cost of commercial alternatives. This was achieved by sacrificing image quality for the sake of usability and accessibility, while also ensuring that our device is suitable for its intended purpose. While the device may be used for other imaging purposes, we believe it is the approach taken when designing the EnderScope that will open up new possibilities for accessible, low-cost imaging tools. 

References

Bibliography

Andrady, A.L. (2011) ‘Microplastics in the marine environment’, Marine Pollution Bulletin, 62(8), pp. 1596–1605. 

General Assembly resolution 71/313 (2017) ‘Work of the Statistical Commission pertaining to the 2030 Agenda for Sustainable Development’. A/RES/71/313.

Horton, A.A. and Dixon, S.J. (2018) ‘Microplastics: An introduction to environmental transport processes’, WIREs Water, 5(2), p. e1268. 

Merces, G.O.T. et al. (2021) ‘The incubot: A 3D printer-based microscope for long-term live cell imaging within a tissue culture incubator’, HardwareX, 9, p. e00189.