Diseases like polio have been eradicated in many countries through vaccination; however, they are still prominent in parts of the developing world. One of the greatest challenges in reaching children for vaccination is the sensitive nature of vaccines themselves, which spoil if not kept at precise temperatures from manufacture to use.
The IV Lab team, in partnership with IV’s Global Good program, set out to address this by developing an insulated container to strengthen and extend vaccination services in developing countries. Our Passive Vaccine Storage Device is designed to keep vaccines at the appropriate temperatures for a month or more with repeat vaccine retrievals and no need for electricity.
Our Approach — Minimize Heat Leak
Super insulation techniques have existed for decades to help store cryogenic fluids and protect spacecraft from the extreme temperatures of outer space. Using similar principles, we developed an insulated container optimized for vaccine storage.
Three different heat transfer mechanisms affect the long hold capability of a device:conduction, convection, and radiation. Our goal was to manage heat transfer through different technological methods, ranging from vacuum management techniques to radiation shells. The heat that does get into the device is absorbed by ice. When it melts, it changes from a solid at 0o C to a liquid at 0o C, but absorbs 334 kJ per kg of ice. That means that per Watt of heat getting into the device, one kilogram of ice will maintain cold temperatures for just about 4 days! While ice is amazing this way, its weight and volume add up; the size and weight of the device is somewhat proportional to the time we want to maintain cold vaccines divided by how big our heat leak is. So, as it turns out, the best way to keep the vaccines cold is to never let much heat into the device in the first place!
Early research on how to do this began through an investigation of the inner workings of the cryogenic dewar (effectively a highly efficient Thermos). Originally, the idea was that we would simply put vaccines and ice into an off-the-shelf liquid nitrogen dewar, and all our troubles would be gone. However, when we tried this, the devices which hold liquid nitrogen cold ( (-196oC) for months only held ice cold (at 0oC) for days!
Nonetheless, the basic idea inspired us, but we realized that we needed to become experts in vacuum dewar construction. We read literature, we talked to experts from around the world, and contracted with a few to accelerate what we knew about the problem.
We took advantage of techniques like wrapping our inner shell with multi-layer insulation or MLI (very reflective thin sheets of foil and separators commonly used on spacecraft) to reduce radiative heat transfer. We knew that with a high vacuum (<10-5 Torr), convective heat transfer is minimized. However, in order to replace the vacuum generated through cryo-pumping (as seen in the cryogenic dewar), we needed to design a new way to create and hold a high vacuum inside the walls of our device. This meant different materials choices, and different fabrication and assembly techniques.
Our next step was to construct prototypes to validate our concepts and refine the technology for use in the field. Our Passive Vaccine Storage Device went through six different prototype versions with performance measurement and design iterations on each. We spent time using Comsol’s multiphysics modeling tools to predict performance, and then used empirical measurements to refine our model for use in the next round of design. Using these results, we built performance prediction tools which enabled us to trade design choices based on feedback we received from a number of points.
As we progressed through the prototyping process, we went from creating devices designed specifically to test technical concepts to those that explored functionality features like how health workers accessed vaccines. Below is an outline of the focus for each prototype.
P1: Proof of Concept
- Demonstrate technical feasibility
P2: Advanced Concepts
- Demonstrate the potential functionality of low heat leak devices
- Reduced weight & increase internal volume
- Include conceptual vaccine dispensing mechanism
P3: Research & Development Test Bed
- Refine the theoretical model
- Improve the device’s thermal performance
P4: Improve Practicality of Design
- Weight reduction
- Redesigned vial dispensing mechanism
P5: Integrate and Optimize for Field
- Integrate structural drop tolerance systems
- Integrate the vial storage container, water block system, and monitoring/communication system with the device
P6: Field Readiness
- Physical robustness to drop and vibration
- Easier payload service and access
- Electronics reporting via cell network
As with many inventions, the journey from idea to the Passive Vaccine Storage Device faced many challenges. One challenge of great significance was our project’s need to achieve and maintain a high level of vacuum passively between the walls of our device.
High vacuum is one of the key components that drives our super-insulating performance. In order to achieve and maintain a high level of vacuum passively, the device’s construction materials were chosen carefully for their outgassing properties. Everything outgasses, regardless of how hard, smooth, dense or flexible a material may be, there will always be gas released. This is a common phenomenon, which is negligible in every which way unless you are trying to maintain a vacuum, in which case outgassing becomes your number one concern. We’ll dive deeper into outgassing and vacuum system design in future blog posts.
Another complexity along our journey included a balancing act between two competing objectives – heat loss and transportation robustness. It was important to accurately test our devices’ ability to keep vaccines cold in extreme climates. In order to test the hold time for each prototype, the device was placed in an environmental chamber to simulate a Sub-Saharan African climate. This allowed us to collect valuable data to make informed decisions as we refined each prototype. Recently, we built a new environmental chamber for testing of the P6 device.
Not only does our device need to keep vaccines cold for long periods of time, but it must also withstand “everyday” use and transportation. The payoff on this may be huge, as some level of durability may enable different models of transport to better reach children in remote areas. But what is everyday use? Is it rolling the device down a hill? Dropping it off the back of a truck? Transporting it on a motorcycle with vibrations equivalent to a space shuttle launch? The short answer is: it’s complicated. However, what if we could make a device that could withstand all types of conditions? How would that affect our holding time? Could the structure withstand the forces? Could these design objectives be met or would we have to compromise? Through extensive durability testing, we managed to find a reasonable compromise.
In addition to hold time and durability testing experiments, we also invested in a variety of field use case studies and field-testing. This took us around the globe to Southeast Asia, Uganda, Gambia, and Senegal. These field studies helped us refine the vaccine dispenser mechanisms and storage stacks, the temperature monitor, user interface, communications interface, and GPS location monitor. In February 2013, seven Passive Vaccine Storage Devices (P6) were deployed in Senegal for a field study to demonstrate to local health officials the length of time the devices can passively keep vaccines cold, and to demonstrate field device communications and electronic monitoring. On a daily basis the P6 units in Africa were able to send us live updates, including the units’ GPS positions viewable on interactive maps here in Bellevue, the number of days since fresh ice had been added, temperature, and more.
What began as a challenge to solve a problem of great importance became a 4-year journey of invention and a significant milestone in the growth of IV Lab. The Passive Vaccine Storage Device was brought to life by our multidisciplinary team, who each contributed their ingenuity and passion to the project over the years.