Sticky Problem Solutions

Tom McKeag, Senior Advisor at the Berkeley Center for Green Chemistry and a member of the UC Botanical Garden Faculty Advisory Committee

One of the great values of the UC Botanical Garden is the view of the wider geographical world that it affords the public through its plants. This view is also useful to innovative researchers like Mia Wesselkamper, a Berkeley bioengineering and business undergraduate who is developing a sustainable alternative to the adhesive used in PLU stickers. You know: those annoying, sticky tags you find on fruit and vegetables. The acronym stands for “Price Look-Up,” and while it may speed your time at the grocery checkout, these stickers complicate any composting you may want to do at home. The adhesives used currently are a particular problem.

Mia’s project has been a search for a sticker adhesive that will break down as naturally as the vegetable it is on. It is a unique collaboration between the Botanical Garden, the Bioproducts Research Unit at the USDA Western Research Center in Albany, California, and the Berkeley Center for Green Chemistry. 

Mia began by seeking guidance from Garden staff on which plants exude sticky substances that might serve. Forty plants were recommended, ten of which were targeted for comparison, and four set aside for sampling: wooly ornamental tobacco (Nicotiana tomentosa), gum rockrose (Cistus ladanifer), monkeyflower (Hemichaena fruticosa), and prickly pear (Opuntia ficus-indica). After establishing a baseline of current industrial adhesives and their performance, Mia designed an extraction protocol, breaking the plant matter down with mortar and pestle, dissolving this in a solvent and then filtering and drying the material to a powder.

This was then mixed with glycerol or water in a controlled solution at two different ratios. Her original experiment plan called for subjecting these formulas to a force meter to test whether they met the current standard of 8-11 Newtons, but the mixes were not thick enough to test on the machine, so she decided to work directly with the ultimate intended surface: the fruit itself.

Prototype stickers on various fruit and vegetables

The opuntia sample was the clear winner, and is now the prototype for Mia’s continued investigation of consistency, spreadability, thin film adhesion, viscosity, and harvesting. There have been surprises: she found that opuntia, set in water for merely a day, yielded a highly effective glue. Indeed, the ancient Aztecs had mixed this material into the mortar that they had used to build their pyramids. She would like to know how to optimize her formulas, perhaps combining opuntia with other natural adhesives, and expand this library of useful plant exudates.

Sticky business in the lab: Opuntia goo!

Mia’s work has already garnered attention; she was recently declared a winner in the UC Berkeley Big Ideas competition and in the German Center for Research and Innovation’s Falling Walls Lab competition in San Francisco. Her project is a great example of how a living repository of plants and the experience of the people who care for them can be brought to bear on our modern challenges.