Utilizing κ-carrageenan Hydrogels in Teaching, Outreach and Chemical Gardens

κ-carrageenan is a biopolymer that can be extracted from red seaweeds and has found great use for example in the food industry due to its excellent gelling, thickening and stabilizing abilities. The work done in this thesis investigates the utilization of κ-carrageenan in the growth of calcium phosphate chemical gardens with an educational touch. The term chemical gardens is used to describe the plant-like, seemingly growing, structures that result from the precipitation that occurs when placing crystal seeds of transition metal salts in silicate solution. Stemming from these classical chemical gardens a new multidisciplinary field termed chemobrionics has emerged. This is a field in which the research is focused on learning more about these structures, the processes behind them and how they can be controlled. While chemical gardens and chemobrionic structures in general continue to fascinate scientists, science itself does not seem to fascinate nor interest enough of young students today. Therefore, a future lack of STEM (science, technology, engineering, mathematics) professionals in all settings is a growing concern, nationally and globally.

The two main aims of the thesis were to investigate the use of κ-carrageenan hydrogels 1) as a base for developing teaching and outreach materials to offer teachers more available lab work, as these are usually appreciated by the students, and 2) as a replacement for the metal salt crystals usually used in chemical gardens in hope that hydrogels containing metal ions would give more control of the growth process. To make both the teaching material as well as the developed chemical gardens accessible for teachers to recreate at schools, a further, no less important, aim was to develop all processes so that they could be done utilizing food grade κ-carrageenan.

A writing board consisting of a κ-carrageenan hydrogel containing red cabbage juice was developed for the teaching of electrochemistry and for being used in outreach activities at the Faculty of Science and Engineering at Åbo Akademi University. The working principle of the writing board is based in the pH changes associated with the electrolysis of water. Additionally, two chemobrionic systems utilizing κ-carrageenan hydrogels for growing chemical gardens were created using teaching friendly materials and processes. Structures in the form of calcium phosphate tubes were grown from the interface of κ-carrageenan hydrogels containing either calcium ions (Ca-gel system) or phosphate ions (Pgel system). In both cases, the hydrogels were layered with a counterion solution (phosphate or calcium respectively). The effect of the time spent in the counterion solution (maturation time) on the created tubes was investigated in both systems. The effect of the amount of κ-carrageenan used in the hydrogels on the created tubes were investigated in the P-gel system.

The writing board concept as an outreach activity was evaluated by upper secondary school students visiting the Faculty of Science and Engineering at Åbo Akademi University, responding positively to the concept. Unfortunately, due to the COVID-19 pandemic, only a trial version could be evaluated. A version aimed at classroom teaching was also developed but could not be evaluated for this same reason. Thus, no further development was done to either of the version.

The two chemical garden systems gave rise to tubes with observable differences in their macrostructure. Tubes grown in the Ca-gel system were straight and long while the tubes grown in the P-gel system were shorter and more kinked. The difference was partly explained by a larger pH difference between the hydrogel and the counterion solution observed in the Ca-gel system. For the P-gel system, increased amounts of κ-carrageenan in the hydrogel resulted in even shorter and thinner tubes. Analysis revealed that both systems resulted in more crystalline structures with increased time spent in the counterion solution, but the Ca-gel structures remained overall more amorphous. Further, tubes from both systems contained hydroxyapatite phases. Additional calcite phases were observed for the P-gel structures while the Ca-gel structures contained measurable traces of κ-carrageenan originating from the hydrogel. The chemical garden systems were developed with teaching and outreach in mind, but no trial or evaluation could be performed in this case either.

This work shows that κ-carrageenan can be used for creating teaching materials and as the seed containing one of the precipitating ions when growing chemical gardens. The work is adding to the knowledge of chemobrionic research by extending the library of (bio)polymers that can be used for growing chemical gardens. The relative ease, with which the hydrogel writing board could be prepared, makes the writing board as well as the red cabbage hydrogel a versatile lab work with potential use in many settings.

This work also contributes to the chemobrionic knowledge by showing that 1) tubular calcium phosphate structures can be grown from the interface of a κ-carrageenan-based hydrogel, 2) by introducing a chemical garden system where the anion (phosphate) is incorporated in the hydrogel phase of the system and 3) by showing that hydrogel systems can be inversed and thereby giving more new options for creating different kinds of structures. Additionally, it shows that hydrogel chemical gardens and materials research can be conducted while having teaching and outreach in mind during the whole process.