Critical ecological developments, like exploitation of finite resources, pollution and climate change lead to the urgent demand for new approaches concerning efficient energy production, especially in the building industry, which is defined by huge resource and energy consumption. Functional behavior, i.e. intrinsic material reactions onto various external triggers, like humidity, can be used as autonomous and therefore efficient concepts to increase efficiency in technical applications. In this context, with the movement and actuation principles of plants and trees, nature serves as a source of bioinspiration for the development of cellulose-based technical biopolymers, which are hygroscopic and show a pronounced stimuli-responsiveness onto changing relative humidity in ambient air. They can be implemented, e.g., as energy-efficient climate-adaptive building envelopes.

In this paper, with Cottonid, a traditional cellulose-based technical biopolymer, produced over chemical modification of native cellulose in form of paper layers, is characterized concerning its functional performance. A manufacturing parameter study over instrumented actuation tests in combination with chemical and optical analytics enabled the identification of  structurally optimized Cottonid variants with increased hygroscopic properties and to derive structure-function-relationships.

Concerning the choice of cellulose source, actuation tests revealed, that paper based on wood pulp seems to produce a more hygroscopic Cottonid variant than paper based on cotton linters, since, due to its higher impurity, it inherits more hydroxyl sites for the adsorption of water molecules. Further, choice of chemical catalyst during parchmentizing results in different process windows, i.e. possible combinations of duration of the treatment and process temperature, which can be exploited to increase time and energy efficiency of the manufacturing process. Analysis of sorption hysteresis enabled an evaluation of porosity, i.e. degree of homogenization, of the Cottonid variants, revealing a relationship between amount of defects in initial condition and hygroscopicity. The findings pave the way for future manufacturing of tailor-made Cottonid elements with adjusted functional properties for adaptive issues.