The role that SARS-CoV-2 has played in causing the COVID-19 pandemic needs no introduction. Richer nations have tried to mitigate the number of casualties by going into lockdown, however the damage this causes to individual livelihoods cannot be tolerated in developing countries, where COVID-19 linked starvation is as great a risk as the disease itself.
Therefore, it is of critical importance that other solutions are found to prevent the spread of COVID-19. One way to reduce transmission is through frequent cleaning of face coverings and shared tools, as SARS-CoV-2 can survive on surfaces for days. The developed world has access to washing machines, disinfectants and sterilisers. Meanwhile, in many developing countries, the majority of the population does not have access to electricity, while 896 million people use health care facilities with no water service and 785 million people lack even a basic drinking-water service.
Therefore, a device is required for sterilising materials and tools that
- Does not use water.
- Does not require mains electricity.
- Does not become a source of infection if multiple people use it.
It has recently become known that inactivation of SARS-CoV-2 occurs when it is held at temperatures of 56 degrees C for 30 minutes or 70 degrees C for 5 minutes (Chin et al. is the most well-known study that shows this, although a comprehensive summary of work by others can be found here).
We have developed a dry-heat steriliser that is based on a Telkes solar cooker and uses sunlight to heat potentially-contaminated objects to temperatures in excess of 70 degrees C for 30 minutes.
Our solar steriliser is designed to be loaded and unloaded using the elbows or a walking stick, minimising the contact between people's hands and surfaces that could be harbouring SARS-CoV-2.
The temperature sensor is the only element that requires electricity. It is designed to operate from 3x AAA batteries, which should last for well over two and a half weeks if used continuously, or more than a month if switched off at night. We also suggest some small solar panels that could be used instead, although this part has not been prototyped. No hardware changes are required to switch between solar panels and batteries, as the PCB has been designed with this in mind.
Some of the sections that make up the body will need to be manufactured in a workshop, but only basic metalworking and woodworking tools are required. The device can be assembled much like a piece of flat-packed furniture - there is nothing particularly complicated about the process, although it will take significantly longer than putting together a chair!
Our steriliser has been designed around an international standard size "200-litre" drum (200 litres is only the nominal size and the one in our parts list is, like most "200-litre" drums, slightly larger). However, it can easily be modified to suit whatever size drum you have available. In addition, the height of the steriliser means that able-bodied operators will need to sit or kneel when loading and unloading, and this may be a feature that you wish to change. We are releasing the SARS-CoV-2 Solar Steriliser under a CC0 Creative Commons license to enable all of this and more. By waiving all of our rights to the fullest extent allowed by law, we aim to facilitate rapid deployment of the SARS-CoV-2 Solar Steriliser and promote the creation of derivative works.Assembly Instructions: Steriliser
Technical drawings for all non-standard parts can be found under Schematics.
Begin with the wooden base. Screw the caster wheels into place and turn on the brakes. Slide the rotation rod into the hole that passes all the way through the side.
Requires: 1x drawing no. 13, 1x drawing no. 31, 7x caster wheel, 28x M8x30 wood screw.
Glue the wooden sidepanels and rear underblocks onto the base.
Requires: 2x drawing no. 8, 1x drawing no. 24, 1x drawing no. 30, 12x 1” dowel (cut to 3” length).
Glue the wooden frontpanel to the sidepanels and rear underblocks.
Requires: 1x drawing no. 17, 10x 1” dowel (cut to 3” length).
Glue the wooden front underblock to the wooden frontpanel.
Requires: 1x drawing no. 21, 6x 1” dowel (cut to 57mm length).
Screw the standoff screws into the wooden sidepanels and attach the metal sidepanels to the standoff screws, using nuts to secure them in place.
Requires: 2x drawing no. 32, 18x M6x80 standoff screws, 18x M6 nut, 18x M6x60 machine screws.
Glue the insulation wall into the base and fill the gap between the insulation wall and the wooden frontpanel with mineral wool. Try to build it up nearly to the top of the wooden front panel, so that it forms a slope down to the top of the insulation wall.
Requires: 1x drawing no. 23, 4x 1” dowel (cut to 3” length), mineral wool.
Using 60 degree brackets, screw the metal frontpanel to the wooden frontpanel, covering the mineral wool and ensuring there are no gaps between the metal frontpanel and the metal sidepanels or the insulation wall.
Requires: 1x drawing no. 16, 3x 60 degree bracket, 3x M8 wood screw, 3x M8 machine screw, 6x M8 nut.
Lay the barrel on its side, so that the opening faces upwards. Fill the bottom half of the barrel (as viewed from above) with mineral wool and place the metal barrel plate on top. Line the barrel up between the two metal sidepanels and slide the barrel mount through the entire assembly.
Requires: 1x drawing no. 9, 1x drawing no. 11, 1x drawing no. 26, mineral wool.
Slide the metal barrel fasteners through the holes in the barrel mount and rotate them until they line up with the bolt holes in the side of the barrel. The metal barrel plate will move around during this process. Then bolt them in place.
Requires: 2x drawing no. 10, 4x M20x60 bolt.
[Non-Optional] Secure the metal barrel plate in place by running stainless steel cable ties through the holes and around the metal barrel fasteners/barrel mount. Coat all exposed surfaces inside the barrel with black paint.
[Optional] If the steriliser will be used for treating tools, and not just fabrics, secure the wire rack in place in a similar fashion.
Requires: 1x drawing no. 27, 6x stainless steel cable ties (12x if the wire rack is installed too), black paint.
Glue the wooden back supports in place.
Requires: 1x drawing no. 3, 1x drawing no. 4, 10x 1” dowel (cut to 3” length).
Using 45 degree brackets, screw the metal backpanel to the wooden backpanel. Then mount the wooden backpanel on the rear of the steriliser. Fill the space between the metal and wooden backpanels and the space between the metal and wooden sidepanels with mineral wool.
Requires: 1x drawing no. 1, 1x drawing no. 2, 3x 45 degree bracket, 3x M8x30 machine screws, 3x M8x30 wood screws, 6x M8 nuts, 6x 1” dowel (cut to 3” length), mineral wool.
Slide the large wooden wheel struts through the two holes in the barrel support closest to the body of the steriliser. Slide the small wooden wheel struts through the perpendicular holes in the barrel support and glue them to the large wooden wheel struts using wooden dowels.
Requires: 4x drawing no. 42, 2x drawing no. 43, 2x ½” dowel (cut to 57 mm length).
Slot each wheel half onto the large wooden wheel struts, gluing them in place and applying wood glue where they intersect the small wooden wheel struts. When the barrel is in the sterilising position, the halves with the loading and sterilising notches should be orientated as in the images below.
Requires: 2x drawing no. 39, 1x drawing no. 40, 1x drawing no. 41, 4x 1” dowel (cut to 3” length).
Place a wooden rotating plate on each side of every wheel strut and fasten them in place.
Requires: 8x drawing no. 12, 16x M6x60 machine screw, 16x M6 nut.
[Optional] Use the M8 machine thread to wood thread screw to attach the wooden animal bolt handle to the modified animal bolt. Secure it in place with a wooden dowel. If you are comfortable drawing back the bolt without using your hands and without the wooden animal bolt handle attached, there is no need to carry out this step.
[Non-Optional] Screw the animal bolt to the wooden side panel (right), with the wooden animal bolt mount in between the two.
Requires: 1x drawing no. 14, 1x drawing no. 37, 1x drawing no. 38, 1x M8x75 machine thread to wood thread screw, 1x ¼” dowel (cut to 100 mm length), 4x M6x45 wood screw.
Slot the PCB mount onto the end of the barrel mount and secure it in place using a wooden dowel. Once this is done, see Assembly Instructions: Electronic Temperature Sensor for guidelines on how to mount the PCB, temperature sensor and reset button. If you are using solar panels to power the temperature sensor, mount them on the four faces of the PCB mount that are perpendicular to the PCB and would otherwise have nothing else attached.
Requires: 1x drawing no. 25, 1x 1” dowel (cut to 181mm length).
For mounting the PCB, temperature sensor and reset button, requires: 1x reset button, 1x PCB, 1x drawing no. 15, 4x M4x10 wood screw, 4x M6x30 wood screw, 4x No. 10 x 1” machine screw.
Glue the bottom sections of the wooden window frame onto the angled edges of the wooden panels at the front, side and rear. Ensure the frame touches the metal panels at the side and rear, as these provide extra support.
Requires: 1x drawing no. 6, 1x drawing no. 19, 2x drawing no. 34, 22x 1” dowel (cut to 3” length), 8 x ½” dowel (cut to 3” length).
Glue the middle sections of the wooden window frame in place.
Requires: 1x drawing no. 5, 1x drawing no. 18, 2x drawing no. 33.
Carefully place the glass window in the frame.
Requires: 1x drawing no. 22.
Glue the top sections of the wooden window frame in place, securing the glass window so that it can no longer be removed.
Requires: 1x drawing no. 7, 1x drawing no. 20, 1x drawing no. 35, 1x drawing no. 36.
Mount the reflectors using the 60 degree brackets.
Requires: 2x drawing no. 28, 2x drawing no. 29, 18x 60 degree bracket, 18x M8x30 machine screw, 18x M8x30 wood screw, 36x M8 nut.
The design of the temperature sensor and the PCB design is documented in the attached GitLab project, where a full README is available. The main board for the temperature sensor is shown below.
To construct the PCB:
- Fabricate the PCB using the Gerber files. Assemble the board using the schematic diagram for reference. Use an IC socket for the microcontroller so that it is easy to transfer the PIC in and out.
- Download and install the latest MPLAB X IDE and the XC8 compiler for programming 8-bit Microchip PIC devices. Open the main project in MPLABX and connect a hardware tool for programming. (A simple option is to purchase this microchip development board, the same one we used for development, which targets the correct range of PIC devices.) Program the PIC and the plug it into the main board
- Prepare a 50cm length of two-core cable to screw into the sensor terminal block on the board. Solder the two active pins of the temperature sensor to the other end of the wire. (To reduce the chance of a mistake, clip off the unused pin 3 in advance. See the datasheet.) Make a note of which wire is data and which is ground. Feed the temperature sensor through the hole in the PCB mount that leads into the pipe, so that the sensor is in the drum (protruding through the hole in the image below) and the other end is poking out of the PCB mount.
- Prepare a 10cm length of two-core wire. Solder one end to the push button, ensuring that you use the normally open (NO) connections. Mount the push button onthe wooden push button plate (drawing no. 15) and mount that on the wooden PCB mount (drawing no. 25).
- Mount the PCB to the wooden PCB mount (drawing no. 25) using the four M4 screws at the edges of the board. Connect the sensor wire to the terminal block marked SENSOR on the main board. Ground should be connected to the rightmost side of the terminal block (near solar) in the picture above. Connect the push button wires to the START terminal block (the polarity is not important).
The device should now be ready to operate. To test, insert 3xAAA batteries into the holder, ensure SW1 is set to the BATT. position. Switch on the device using SW2. The green ready LED should start to blink once every 3 seconds.Calibration
If the inside of the steriliser gets too hot, there may be detrimental effects on the items you wish to sterilise, and the temperature sensor that we have prototyped this with will burn out. Equally, if the inside of the steriliser does not get hot enough, the sterilising process will never finish (the "In Use" light will remain on permanently). Therefore, some minor calibration is required. This need only be performed very occasionally in places where there is very little variation in the weather. However, in more unpredictable climates, calibration may need to performed daily.
The maximum temperature of a Telkes solar cooker can be in excess of 200 degrees C. Therefore, in many locations, the SARS-CoV-2 Solar Steriliser should not be pointing directly at the sun. The PCB is designed to assist with identifying which direction it should be pointing in if you do not have a suitable thermometer available.
Follow the operating instructions to carry out a test run with items that are representative of those that you wish to sterilise. The steriliser will work at a range of different angles relative to the sun, so do not worry about getting it perfect the first time you try this.
If the "Too Hot" LED comes on, you need to rotate the steriliser so that it is facing further away from the sun. If, after an hour, the "In Use" LED has not gone off, you need to rotate the steriliser so that it is facing more towards the sun. So long as the internal temperature reaches between 70 and 90 degrees C for half an hour, the "In Use" LED will go off and the "Ready" LED will come back on. This indicates the steriliser is pointing in the right direction.If you find the steriliser needs to be pointing in different directions depending on the time of day, then you must calibrate it at each time of day. In this case, we recommend marking the direction on the ground around the base, along with the time at which it must be pointing in that direction (much like the face of a sundial).Operating Instructions
Check that the "Ready" LED is illuminated and that all the others are off.
Ensure the barrel is in the loading position and that the animal bolt is holding it in place. Load the barrel with the items you wish to sterilise.
Retract the spring-loaded animal bolt and rotate the wheel anti-clockwise until the bolt clicks into the hole corresponding to the sterilising position. Both of these actions may be performed with the use of your elbows or a pair of walking sticks that only you handle.
Press the large, red button with your elbow or a walking stick that only you handle. The "Ready" LED will switch off and the "In Use" LED will turn on.
Once the steriliser has reached 70 degrees C and stayed at that temperature or above for 30 minutes, the "In Use" LED will switch off and the "Ready" LED will come back on. If this has not happened after an hour, the steriliser may need to be recalibrated.
Place a container that is used only for clean items in the open section underneath the barrel. Retract the spring-loaded animal bolt as you did before and rotate the wheel anti-clockwise. As the barrel turns upside down, it will deposit its contents into the container. Continue rotating the wheel until the bolt clicks into the hole corresponding to the loading position. The steriliser is now ready to be used again.Notes on the Project Difficulty and Time-to-Build
The project difficulty and time-to-build assume that the end user (or someone acting on their behalf) fully constructs and assembles the SARS-CoV-2 Solar Steriliser as it is presented here. There are a number of factors which could make this easier or quicker for the end user.
- The SARS-CoV-2 Solar Steriliser is designed so that the custom parts can be manufactured in one location, shipped in kit form and assembled in a second location. This means that businesses or central/local governments could take on the most difficult and time-consuming parts of the process, leaving the end-user to simply screw everything together.
- The SARS-CoV-2 Solar Steriliser is designed for hands-free loading and unloading. However, this is an unnecessary feature if it is being deployed in an area where only one person will be touching the steriliser. In such a scenario, we would recommend reducing the complexity of the system by using a conventional Telkes solar cooker design, augmented with the temperature sensor that we present here. For this variation to be operated safely, it is important to ensure that the user has access to oven gloves (one pair for loading and one pair for unloading) as traditional solar cookers do not have gravity-assisted unloading like the SARS-CoV-2 Solar Steriliser.
When designing the wooden panels, we chose to use balsa wood, because it can easily be cut to shape without needing expensive equipment. This is partly why balsa is so popular with model makers but, because it is widely used in this way, there is now a popular misconception that balsa is a very flimsy wood. However, if there are no flaws in the underlying design, items made out of balsa can be incredibly strong (see here and here for some general data, as well as here and here for examples of how much weight a balsa wood bridge can hold, and here for some strength-test photos from a balsa wood furniture maker).
We have based the thickness of our panels on the worst-case assumption that only low-density balsa is available. If medium- or high-density balsa can be obtained, the panel thickness can (and should be) reduced.
If woodworking facilities are available, we would recommend using plywood instead (ideally Baltic Birch, because it is sold in sheets of up to 1.5m x 1.5 m). In this case, the panel thickness should once again be reduced, as otherwise the steriliser will become too heavy for it to be used effectively.
Finally, if the sheets of wood available to you are not big enough for the size of steriliser that you want to build (likely in the case of balsa, less likely in the case of plywood), it is easy to construct bigger sheets by laminating them (sticking the smaller sheets together). When thickness laminating, we recommend ensuring that the grain in layer x is at 90 degrees to the grains in layers x-1 and x+1, as this will increase the strength of the finished product.
Holes that are metric (e.g. 8 mm diameter) and do not go all the way through the panel are intended for wood screws and so may be created using the screw itself during the assembly process. Holes that are imperial (e.g. 25.4 mm = 1 inch diameter) and do not go all the way through are intended for wooden dowels and so should be cut during the construction of the panel. Holes that go all the way through should be cut during the construction of the panel, regardless of whether they are metric or imperial.FAQs
Why have you not included a UV-C light source for ultraviolet germicidal irradiation?
UV-C has been used as a disinfectant for well over 100 hundred years. It is regularly used in commercial sterilisers to decontaminate surgical equipment, as well as in other applications, such as water treatment. UV-C light sources can be divided into two categories: those that emit far UV-C (around 222 nm) and those that emit longer wavelengths (usually between 250 nm and 280 nm).
There is strong evidence that far UV-C light is safe for humans to be exposed to because "it cannot even penetrate through the dead-cell layer on the surface of our skin or the tear layer on the surface of our eyes". However, there are two reasons why we have not incorporated far UV-C in our steriliser:
- The far UV-C light sources that are currently available are expensive and require mains electricity.
- If far-UVC cannot penetrate the dead-cell layer on the surface of our skin, it will not penetrate any fabrics that are placed within our steriliser, so is useful for sterilising surfaces only.
We have not incorporated "standard" UV-C LEDs in our steriliser because:
- "Standard" UV-C is not safe for humans to be exposed to - it causes skin cancer and damages eyesight. Although the glass window in the SARS-CoV-2 Solar Steriliser should not be a cause of exposure, any cracks that form, while not always sufficient to impair the operation of a solar oven, would cause UV-C to leak out if the device continued to be used. There would also need to be a mechanism by which the UV-C LEDs turn on only when the barrel is in the sterilising position. All-in-all, the risk of injury is far too high to warrant using "standard" UV-C.
- "Standard" UV-C breaks down chemical bonds that leads to rapid ageing of certain materials. Although there is conflicting evidence as to whether or not items such as N95 masks are affected (e.g. Liao et al., Zhao et al.), it could damage elements of the steriliser itself. The parts that may experience degradation include the insulation (if there are gaps that cause UV-C to leak into this area) and the temperature sensor.
- It is unclear that "standard" UV-C will sufficiently penetrate fabrics either.
There is one more reason why we have not used UV-C in general and that is simply that it is not required. Although it is certainly a nice-to-have, and recent developments in far UV-C devices are exciting, the downsides are not worth tolerating given there is strong evidence that thermal sterilisation is sufficient.
What if I can’t get hold of a particular component?
Worry not, none of this project relies on proprietary hardware, so you can just substitute in something similar. However, be aware that you will probably need to modify the design to accommodate the new component, whether that's because it has different dimensions, the screw holes are in different places or because it doesn't work in quite the same way as the component we recommended. To facilitate this process, we have provided CAD files for everything and schematics for all the major parts.
Can this be used as a general steriliser to target a wider range of pathogens?
A wide range of pathogens can be inactivated at similar temperatures to SARS-CoV-2, including other coronaviruses (see here for a review) and Ebola. As solar ovens can exceed 200 degrees C, there is also scope for using the SARS-CoV-2 Solar Steriliser to inactive pathogens that are targeted by commercial dry-heat sterilisers (see here for the most common time-temperature relationships). However, the temperature sensor used in this project can only tolerate temperatures below 100 degrees C, so an alternative must be found if you want to exceed this limit. The code will also need to be modified.Further Development
You can extend and further develop the microcontroller code if required (for example, if you wish to incorporate additional temperature sensors, which can be achieved without needing to modify the PCB). The easiest way to prototype using the PIC is to purchase a Microchip Curiosity Low Pin Count development board and connect it to a breadboard as shown in the picture below.
The code for the project is included in the GitLab project (attached). The code is located in the sensor.X folder (which is the MPLABX project folder), and is documented here.Disclaimer
This project, including, but not limited to, all instructions, suggestions for potential modifications, CAD files, Gerber files and schematics, is provided as-is and with no warranties. No express or implied warranties of any type, including for example implied warranties of merchantability or fitness for a particular purpose, are made with respect to this project, or any use of this project. The project’s authors make no representations and extend no warranties of any type as to the accuracy or completeness of the information or content presented in this project.
The project’s authors and contributors specifically disclaim liability for incidental or consequential damages and assume no responsibility or liability for any loss or damage suffered as a result of the use or misuse of any of the information or content presented in this project.
Any claims made by this project with regard to the ability of the SARS-CoV-2 Solar Steriliser to kill pathogens is based on the best-available scientific literature at the time of publication. The SARS-CoV-2 Solar Steriliser has not been tested on samples that are known to have been contaminated with pathogens of any kind, including, but not limited to, SARS-CoV-2. Those who use any of the information or content presented in this project for any purpose, including, but not limited to building, operating, modifying or designing derivatives of the SARS-CoV-2 Solar Steriliser, accept that they do so at their own risk.