Solar Powered Mosquito Repellant

The vast majority of Challenges posted by InnoCentive Seekers are of such a confidential nature that we are never permitted to reveal even the Seeker’s identity, much less the actual solution. We are very fortunate in this instance, that the Seeker, SunNight Solar, and the Solver, Tom Kruer, have been willing to talk about the winning solution.  Below is an overview of the solution, provided by Solver Tom Kruer.

The simple, passive solar device illustrated to the left is is designed to attract and kill malaria-carrying mosquitos. This solution was chosen as the winner of the InnoCentive Challenge entitled "Reducing the Risk of Malaria with a Solar Powered Device".

Not surprisingly, coming up with a viable solution to this Challenge involved the typical phases of product development project. Research uncovered the relatively few critical features that the product absolutely needed to have in order to be successful. The design phase generated and evaluated a number of alternative approaches which incorporated these critical features.  Two of these designs broke all the rules (including the ultimate solution) which the Seeker agreed to at least look at. A couple of crude functional prototypes were constructed to prove that the concept was feasible.  After that, it just was time to crunch some cost estimates and write up the submission for InnoCentive.

The Research

My son, Nathan, and I started this project by doing extensive reading and analysis of the published research regarding the behavior of Malaria vector mosquitoes performed by leading universities and institutions.  It helped that Nathan is a student with access to a great university library and had an interest in winning some money (yes, I shared the prize with Nathan).

This literature review led us to two important conclusions that helped guide the rest of the design effort.  The first conclusion was that using insecticides for eradication was probably not the best idea as these toxic chemicals kill off as many or more beneficial insects as the target pests and mosquitoes eventually develop a tolerance to whatever insecticide is employed.

The second conclusion was that mosquitoes are attracted to their human blood hosts by the combination of the following factors when inside a human habitat (2):

  1. The temperature of human extremities during rest (~ 29 +/-3 degrees C); and,
  2. Moisture emanating from the skin; and,
  3. The complex, multi-component scent of human sweat & resident microflora.

Once these conclusions were verified through more focused research, a device was designed to effectively attract, trap, and kill the female Anopheles mosquito by using a complete combination of the above three factors.

(1) There has been a great deal of scientific research into how carbon dioxide along with visual clues act as mosquito attractants.  However, when the research is evaluated in detail, it is apparent that CO2 and visual indicators are used by the mosquito only when coming from a distance.  These attractants have little, or no effect within an enclosure.

The Design

The proposal which won the challenge is quite unique in that it uses a nontoxic phase change material (PCM) to retain passive solar heat during the day (see operation).  This stored heat is then released when the unit is brought indoors (when mosquitoes are feeding). Basically, the Eureka moment came when I realized that if all we needed was heat from the sun, we did not need expensive and complex photo-voltaic cells and batteries.  Inexpensive wax works just fine in a passive solar mode and phase change wax can be made to closely simulate human body temperatures.  Rough calculations showed that the released heat from only about 1 liter of paraffin wax, in combination with a natural convection chimney (analogous to a power plant cooling tower) distributes a water-based mosquito attractant throughout the enclosure for about 10 hours.

The black blow molded bottle which holds the phase change wax was initially configured to allow all the material to be approximately the same distance from an exterior surface.  This requirement led to a tall cylindrical shape with a large cavity in the center. The shape was further tweaked to provide the shape needed to achieve natural convection air flow.  A natural convection chimney was selected to achieve air flow instead of a motor-powered fan for what should be obvious reasons of reduced cost and complexity.

Phase Change Model

The third major aspect of the design was settling on which attractant to use.  The attractant suggested for use in developing third world countries is simply a small amount of water combined with human sweat (collected in a sweatband during the day).  It is noted that using sweat is a very simplistic approach, but it makes sense in so many ways.  First of all, sweat has been scientifically and experientially proven to work.

Attractant CupTherefore, this simple suggestion can be implemented immediately without any further research.  Secondly, there will always be a FREE supply of human sweat in the household... or from a more pungent "supplier" in the region (how about that for a job!).  Thirdly, research has found more volatile compounds in adult sweat that attract mosquitoes than found in other human fluids or artificially derived compounds.  And, again, its free.

The sweat attractant is located in a injection molded cup at the bottom of the unit with simple cotton wicks to expose the liquid to the rising air column. The cotton wicks could be cut from old clothing.  The attractant cup snaps into a hollow in the bottom of the phase change bottle.

The design also needed to offer a wide range of mountings since not all mosquito species fly at the same height from the ground.  Thus, the trap was design to be placed either on the ground (with the attractant cup providing an insulating thermal break for the heating element), on a tabletop, hung by a wire hook, or a pole up through the center of the attractant cup and the cavity in the heat element bottle.

Heat, moisture and scent are all present, thus meeting the critical requirements found in the research phase.  The mosquitoes are trapped in a simple tortuous path container placed on top and eventually die by dehydration.

Total manufactured cost was estimated to be roughly US $5.00 of material with assembly labor of an additional US $1.25.

An alternative design was also considered for this product.  This version positioned the phase change material in a plastic pouch which wrapped around a liquid trap. The trap mimicked nature's pitcher plant which my son had seen in his travels to the rainforests of Peru .  Although equally unique as the final design, this design was deemed not as effective at distributing the scent nor trapping the attracted mosquitoes.

Operation

At the start of each day, the heating element/clear chimney assembly is removed from the attractant cup and placed on the ground, roof, or table outside the home where it can be exposed to the full sun.  At the same time, the sweatband is put around the ankle of the adult resident who is most frequently bitten by mosquitoes(2).

The phase change material within the black bottle melts during the day by absorbing heat from the sun.  The phase change assembly can be stood upright or laid on its side.  If stood upright, the flat surface of the chimney resting on the surface cuts off air flow through the hyperbolic gap and diminishes any cooling effect of wind or breezes.  Nothing will be damaged if it rains or tips over!

As the PCM heats up in the sun, the sweatband (or, alternatively, a cotton sock) will collect some sweat and foot odor from the "donor".

At the end of the day, prior to dusk, the sweat band is washed with a few CC's of water and wrung into the attractant cup.  The warmed heating element assembly would be placed back on top of the attractant cup with the conical surfaces ensuring proper alignment.  The trap would be placed on top of the heating element, if it had been removed.  Note that leaving the trap in place while the unit is heated by the sun may hasten the death of any entrapped mosquitoes.

At this point, it is recommended that the heated unit be placed in a dark corner downwind of beds or people in the habitat.  The reason for this is that the traps should be located in inconspicuous locations where they will not be disturbed or vandalized.  Furthermore, the research shows that mosquitoes search for hosts by traveling upwind, guided by scent.

Once inside the habitat, the heating element will begin to release its stored heat.  Heated air, laced with the moisture and scent from the cotton wicks, flows up through the hyperbolic gap between the bottle and chimney.  The air exits through the openings in the top of the trap and is distributed downwind to start attracting local mosquitoes.  Very intentionally, there is no CO2 present in the air so as to avoid attracting more distant mosquitoes to the habitat.

The mosquitoes track the scent upwind to the trap rather than to the humans.  They find their way to the wicks only to find no host. The mosquito then takes off vertically and follows and/or are forced by the natural convective air flow up through the gap between the bottle and the chimney.  They pass through the trap holes into the trap assembly.  Once inside the trap, they cannot escape due to the tortuous path.

It is important to continually monitor the number of mosquitoes in the trap day to day.  If after several days the trap does not appear productive, it should be moved to an alternate location, or height.  Remember the primary goal is to trap blood-feeding mosquitoes. It is less of a priority to maintain consistent sites that may have poor yields.

(2) Note that the ankle is the preferred site for the cotton/elastic sweatband as a number of researchers have proven the effectiveness of foot odor as an attractant over other human scents.

A prototype of this solution will be built and tested by SunNight Solar.  If it is effective, SunNight Solar expects to begin production as early as this Spring.  For more on SunNight Solar and the malaria Challenge, click here.