Polymers (Greek: poly=many, meros=part) are large molecules made up of many small parts (monomers) in a repetitive way, as a term was introduced for the first time (1833) by the Swedish chemist, Jöns Jakob Berzelius. By the combination of different monomers, the resulting polymer can exhibit various properties, such as biodegradability, photosensitivity and electrical conductivity. The latter is the main characteristic of the polymers included in this thesis. Since their commercialization, in the late 20th century soft and biocompatible conductive polymers have been substituting stiff and bio-tolerable metals in numerous cases, especially in the medical field for in vivo applications. Polymers can also be found in nature, as a product of the life cycles of animals, plants and microorganisms. The variety of natural polymers is vast, and they are categorized mainly into the groups of polysaccharides, polypeptides and polynucleotides. In these categories belong some of the most well known and investigated materials, for instance, DNA, proteins, silk and cellulose. 

The combination of synthetic materials with natural materials has intrigued the scientific community for many decades, as a way to form functional materials with hybrid properties. In this thesis, synthetic polymers, particularly conjugated polymers were combined with cellulose, the most abundant biopolymer on earth to form 2D and 3D conducting composites that can find application in plant science and stimuli-responsive systems. 

In the first part of this thesis, the widely used conjugated polymer PEDOT:PSS was combined with cellulose nanofibers to form 3D porous conducting scaffolds. The scaffolds were developed by freeze-drying method and their electrochemical, mechanical and structural properties were characterized. We investigated the effect of the freezing method on the scaffold properties and found a correlation between the mechanical properties and the pore wall thickness. Furthermore, with micro-CT, we could characterize in detail the bulk structure of the scaffolds and investigate how the incorporation of carbon fibers as addressing electrodes influences the porosity (paper 1). 

Next, we applied the conducting scaffolds for stimulating plant growth. The plant of our choice was barley, a very important crop, which was grown within the scaffold and the roots were integrated within the scaffold’s pores. We demonstrated that plants grow in the scaffolds under sterile conditions, as well as in agar which is the standard medium used in plant sterile culture. Taking a  step ahead, we developed a non-sterile hydroponics setup, where the plants could grow without any contamination. Furthermore, we applied different protocols of electric stimulation to the scaffolds for various time periods and polarizations, achieving at the end a 40% increase in the plant biomass for the stimulated plants. We investigated the growth of the plants and concluded that the enhancement of growth was taking place after the stimulation period with growth enhancement both to roots and shoots (paper 2). 

In the second part of the thesis, we harnessed the unique electroswelling capabilities of the polythiophene-based polymer p(g3T2), with two different approaches. Initially, we demonstrated the ability of the p(g3T2) material to expand reversibly on a 2D mesh when electrochemically addressed. We optimized the coating on the metallic mesh with fixed pore size and developed an electroactive filter with tunable porosity that could modulate the flow of a system on demand (paper 3). 

Although p(g3T2) has great potential for various applications, it is processed from hazardous organic solvents, such as chloroform. Therefore, we addressed this issue and developed a protocol where p(g3T2) is solubilized in ethanol, which enables the coating of a plethora of substrates that chloroform would dissolve. From a biodegradable 3D printed mesh of cellulose and polylactide to everyday labware we demonstrated that p(g3T2) can change the substrate properties when electrochemically addressed directly on the non-conducting substrate without the need for an underlying supporting electrode. Forming a biocompatible substrate able to facilitate tissue engineering studies(paper 4). 

Overall, in this thesis, we demonstrated how synthetic materials can be combined with natural materials to form functional composites with hybrid properties. Firstly, by combining the mechanical characteristics of cellulose and the mixed ionic electronic conductivity of PEDOT:PSS we can obtain a 3D phytocompatible aerogel that can have desired pore size, undergo mechanical compression and act as an active hydroponic substrate for stimulating plant growth. Then we demonstrated how polymers with controllable volume change, such as the polythiophene-based conjugated polymer p(g3T2), can be combined with everyday materials paving the way for stimuli responsive systems such as electroactive filters, and when used with a green solvent can modify everyday labware used for in vitro experiments. 

Plant vasculature transports molecules that play a crucial role in plant signaling including systemic responses and acclimation to diverse environmental conditions. Targeted controlled delivery of molecules to the vascular tissue can be a biomimetic way to induce long distance responses, providing a new tool for the fundamental studies and engineering of stress-tolerant plants. Here, a flexible organic electronic ion pump, an electrophoretic delivery device, for controlled delivery of phytohormones directly in plant vascular tissue is developed. The c-OEIP is based on polyimide-coated glass capillaries that significantly enhance the mechanical robustness of these microscale devices while being minimally disruptive for the plant. The polyelectrolyte channel is based on low-cost and commercially available precursors that can be photocured with blue light, establishing much cheaper and safer system than the state-of-the-art. To trigger OEIP-induced plant response, the phytohormone abscisic acid (ABA) in the petiole of intact Arabidopsis plants is delivered. ABA is one of the main phytohormones involved in plant stress responses and induces stomata closure under drought conditions to reduce water loss and prevent wilting. The OEIP-mediated ABA delivery triggered fast and long-lasting stomata closure far away from the delivery point demonstrating systemic vascular transport of the delivered ABA, verified delivering deuterium-labeled ABA.

The porosity of filters is typically fixed; thus, complex purification processes require application of multiple specialized filters. In contrast, smart filters with controllable and tunable properties enable dynamic separation in a single setup. Herein, an electroactive filter with controllable pore size is demonstrated. The electroactive filter is based on a metal mesh coated with a polythiophene polymer with ethylene glycol sidechains (p(g3T2)) that exhibit unprecedented voltage-driven volume changes. By optimizing the polymer coating on the mesh, controllable porosity during electrochemical addressing is achieved. The pores reversibly open and close, with a dynamic range of more than 95%, corresponding to over 30 mu m change of pores widths. Furthermore, the pores widths could be defined by applied potential with a 10 mu m resolution. From among hundreds of pores from different samples, about 90% of the pores could be closed completely, while only less than 1% are inactive. Finally, the electroactive filter is used to control the flow of a dye, highlighting the potential for flow control and smart filtration applications.