Biosensors can be used to detect toxic substances in a highly specific and sensitive manner. Furthermore, they can be cheaper and easier to handle than conventional detection methods (Kaur et al. 2015). For this reason, they are a field of active research and a popular topic in the iGEM competition. Most of these biosensors make use of living microorganisms, which raises a number of problems. Among them are legal issues and safety concerns, as it must be prevented that genetically modified organisms are released into the environment. Moreover, these sensors are often not very user-friendly, as they have a limited shelf life and their application outside the laboratory is complicated. In order to solve these problems and thus make biosensors applicable in everyday life, we developed cell-free biosensors that can be used as paper-based test strips.

Our principal approach towards this end relies on cell-free protein synthesis (CFPS). This system is suitable for classical repressor- or activator-based biosensors, but by carrying out the expression of the reporter protein in a cell extract, biosafety problems are prevented. Furthermore, the cell extract remains functional when it is applied onto paper, and after lyophilization, such a test strip can be stored for a long time (Pardee et al. 2014).