Introduction
As advancements in technology have enhanced our quality of life, the role of indoor air purifiers, integral to public health, has seen a surge in daily utilization. Conventional air purifiers predominantly rely on activated carbon as an adsorbent to sequester airborne contaminants. However, activated carbon presents limitations, such as a constrained adsorption capacity leading to increased adsorbent consumption, and diminished efficacy in adsorbing mixed gases. Moreover, there's a noted desorption phenomenon, attributed to the disparity between the molecular diameter of the adsorbed species and the pore dimensions of the activated carbon. Furthermore, activated carbon, being non-renewable, demands energy-intensive activation processes - physical activation under elevated temperature and pressure or chemical activation using agents like phosphoric acid and potassium hydroxide[1]. Such processes not only entail significant energy consumption but also contribute to environmental pollutants.
In light of the above challenges, and to champion the ethos of sustainable development and enhanced human health protection, our team undertook the design and fabrication, of a microbial-based air purification system. For this innovative approach, we opted for sodium alginate as the matrix to encapsulate our engineered bacteria, while employing M9 culture medium to ensure bacterial viability. Our design incorporates a gas-liquid circulation system, optimizing the capture efficiency of airborne contaminants. We've integrated photosensitive sensors to monitor the efficacy and lifespan of the culture solution, coupled with ultraviolet irradiation mechanisms at the discharge point to preclude potential escape of engineered bacteria.
Our microbial-based air purifier not only excels in its primary function of air purification but also boasts an quiet operational modality, an intuitive user interface, and capabilities for temperature and humidity sensing, all seamlessly controllable remotely. This holistic design seeks to offer individuals an enhanced and environmentally-responsible domestic ambiance.
Structural Overview
As a complete device, our air purifier has mechanical structure and can be divided into serval systems.
Mechanical Structure
The mechanical structure is made using 3D printing technology and CNC machining technology, providing a lightweight, highly durable, and transparent integrated design. The shell part uses 3D printed parts as the frame structure, and uses high transparency acrylic plates as filling, which can easily show the internal structure while maintaining stable operation inside the machine.
Initially, the entire structure should be meticulously crafted in SolidWorks, according to the previous draft. Subsequently, based on the distinctions in machining techniques, the mechanical components can be categorized into those suitable for 3D printing and those apt for CNC machining.
For 3D printing parts, their 3D models must be sliced into pieces first with support structure. Next, the sliced file was sent to a 3D printing machine to be printed. Finally, the 3D printed parts should be washed using ethanol and cured using UV light.
As for CNC machining parts, according to the 3D model, the tool path of our milling machine should be routed and simulated. After that, the G code was generated and sent to the milling machine, and the parts were machined from a large acrylic board using a 2mm diameter milling cutter. Finally, the 3D printed parts should be washed using ethanol and cured using UV light.
Circuit
In the circuit section, we have implemented the underlying functions of the control system, power supply and drive system, air and liquid circulation system, sensor system, human-machine interaction system, and UV sterilization system. We designed a PCB board to help us build the circuit part using EDA software. Compared to bread boards or general-purpose PCB boards, PCBs have the characteristics of high degrees of freedom, simple wiring, small size, and high reliability, making them cheap and easy to produce on a large scale.
To design a PCB board, the schematic diagram and the PCB diagram should be designed first. Then the design files were sent to PCB original equipment manufacturer to be manufactured.
System Composition
Control System
The control system features an ESP Devkit V1, based on the ESP32 microcontroller[2]. This board, known for its wireless capabilities, connects to the Internet via Wi-Fi for remote operations. It is paired with a Bottom PCB, which serves as a conduit, interfacing with other systems and managing power supply. In essence, commands from the Internet are channeled through the ESP32 Devkit to the Bottom PCB, which then directs these to the appropriate subsystems, ensuring streamlined remote control and monitoring.
Power Supply and Driver System
Within the power supply and driver subsystem, we implemented an L298N driver board to actuate the water pump and facilitate a voltage step-down from 12V to 5V[3]. An S8050 transistor was harnessed to control a substantial current, thereby driving an ultraviolet LED dedicated for disinfection processes. Additionally, the relay was strategically employed to govern the operations of the air pump. The system predominantly operates on a 220V 50Hz AC power source. This is subsequently transformed to a 12V DC output via a DC-DC voltage converter. The L298N further steps this down to a 5V DC. Specifically, the 220V AC is allocated for the air pump[4], the 12V DC powers the water pump, while the 5V DC is designated for the control system and ancillary components.
Air and Liquid Circulation System
In this system, we used four PVC pipes to hold the culture fluid. Two PVC pipes are used as solute exchange pipes for air and liquids. Here, a large amount of air pumped out by the air pump will pass through the pipeline from bottom to top, exchanging solutes dissolved in the air during the upward floating process. The other two PVC pipes are used to purify harmful substances in the culture fluid purification pipeline. These pipelines are filled with sodium alginate microspheres containing microorganisms, and the culture medium will slowly pass through these microspheres from bottom to top and be purified. Afterwards, the culture fluid will return to the solute exchange pipes again. Two water pumps are used to move the liquid inside these two pipelines. This device can be easily replicated (adding pipes) to increase the rate of air purification while keeping the other systems unchanged.
Sensor System
The sensor system mainly includes temperature and humidity sensors and culture fluid lifespan sensors. The culture medium lifespan sensor is based on the TCS34725 color sensor to achieve color detection[5]. By detecting the color of the culture medium, the remaining lifespan of the culture medium is determined, and the I2C protocol is used to communicate with the control system. This is because microorganisms produce indigo during the metabolic process to change the color of the culture medium. By detecting the color of the culture medium, the remaining lifespan of the culture medium can be determined. The temperature and humidity sensor are based on the DHT22 chip to achieve temperature and humidity detection and communicates with the control system through an IO port[6].
Human Machine Interaction System
This system includes a USART HMI screen and a potentiometer, where the potentiometer is used to adjust the air pump speed. The touch screen display is used to display current data and accept user instructions and communicate with the control system through the UART protocol.
UV Sterilization System
The UV sterilization system includes a UV LED array. The entire array is powered by 5V DC and requires a large amount of current consumption, so a transistor drive circuit is required. The periphery of this system is wrapped in opaque tape to prevent accidental adverse effects of ultraviolet radiation on microorganisms in the circulatory system. This system is located at the extreme end of the air passage and can effectively prevent the escape of microorganisms inside the equipment, ensuring biosafety.
Programming
USART HMI Screen Design
USART HMI Screen means one monitor used for human mechine interface (HMI), communicating with other devices, such as MCU, by using USART protocol. We used TJC USART HMI Editor to desigh the GUI, compiled and downloaded the GUI program to the Screen. Our USART HMI Screen can get current temperature, humidity and culture medium lifetime from the MCU of the control system and desplay them on the screen. At the same time, it can also get ON/OFF instructions from the user and send them to the MCU.
ESP32 MCU Program Design
We use the Arduino IDE to program ESP32 in the control system.
The program is divided into driver layer, controller layer and application layer. The driver layer allows the ESP32 to communicate with all peripherals. The controller layer helps the ESP32 encode the sent data and decode the received data. The application layer is able to exchange data between different devices and switch all systems on and off through finite state machines. The ESP32 main program is initialized after startup, and then takes turns operating the finite state machine and data sending and receiving in a polling structure.
Prototype Presentation
The culmination of rigorous design, testing, and iterations has resulted in the final prototype of the air purifier, meticulously illustrated in the succeeding figures. These visuals break down the multiple facets of the air purifier to provide a comprehensive understanding of its integrated systems and components.
Figure 13. (A) showcases the overall exterior and design of the air purifier, encapsulating its sleek and functional build. This holistic view provides a glimpse into the device's ergonomic design and its aesthetic appeal.
Subsequently, we delve deeper into the heart of the machine in Figure 13. (B): the Control System, along with the Power Supply and Driver System. This figure illuminates the intricate circuitry and power management mechanisms vital to the seamless operation of the purifier.
Figure 13. (C) illustrates the Human-Machine Interaction System. This interface is crucial for ensuring user-friendly interactions, providing real-time feedback, and enabling customizable settings to cater to individual preferences.
The Water Pumps, essential components of the air and liquid circulation system, are elucidated in Figure 13. (D). These pumps are pivotal in ensuring the efficient movement of air and liquid throughout the system, guaranteeing optimal purification.
Figure 13. (E) shines a spotlight on the UV Sterilization System. Capitalizing on ultraviolet irradiation, this subsystem plays a quintessential role in neutralizing harmful pathogens, ensuring that the air circulated is not only pure but also hygienic.
Lastly, Figure 13. (F) portrays the Air Pump, another integral part of the air and liquid circulation system. Its role is indispensable in maintaining the desired air pressure and flow rate, ensuring that the purifier operates at its peak efficiency.
Future Design
This device can effectively demonstrate the principles of our work. However, based on user feedback, the device still has the following areas for improvement:
- Increase circuit integration. The water pump and air pump drive system are integrated into the PCB to further improve the integration level of the system, to reduce the volume of equipment, reduce costs, and facilitate large-scale production. Later, multiple models can be derived according to the different needs of users for air purification capacity, corresponding to different air purification capacity.
- System modular design. The drive system, circulation system, sensor system, etc. are packaged into an air purification unit, so that an air purifier can install different numbers of air purification units according to the different needs of users for air purification capacity and realize the modularization and personalized of the entire equipment.
- From photosensitive resin material to rPET. The 3D printing materials we currently use are photosensitive resins, which are not environmentally friendly and are not easy to mass produce. To avoid this, the material should be replaced with recycled PET (rPET) and mass produced using an injection molding process.
- Purify a variety of harmful substances. In this program, we mainly study how to use microorganisms to purify nicotine from the air. In fact, many other harmful substances in the air, such as formaldehyde, can be purified by changing the types of microorganisms.As long as different sodium alginate beads are replaced, the function conversion of the air purifier can be completed.
