**1. Introduction**

Ionic actuators are an interesting class of materials which can change their shape over electrochemical stimuli [1]. "Ionic" means that these materials rely on ions (from electrolyte) to carry out their shape change. These materials are also called "artificial muscles" [2] due to relative similarity to the behavior of natural muscles. Applications can be found in soft robotics [3,4], smart textiles [5], energy harvesters [6,7], and biomedical applications [8]. From the variety of materials belonging to this class, conducting polymers [9] and multiwall carbon nanotube/carbide-derived carbon (MWCNT-CDC) fibers [10] were chosen, which di ffer in their actuation mechanism and formation. Conducting polymers, such as polypyrrole, are a class of materials that follow redox-active processes (Faradaic actuators) [11]. During oxidation a positive charge is formed on the PPy network, in order to maintain the electroneutrality, counterions (anions with solvent) move into the PPy film [12]. Consequently, the polymer film swells (shape change), and during reduction those anions leave the film

(anion-driven actuator). The whole process is reversible. Alternatively, if the anions are too large or too strongly interacting, like dodecylbenzene sulfonate (DBS), they become immobile inside the PPy film, while during reduction the charge of DBS- anions is compensated by the ingress of cations into the film (cation-driven actuators, like PPy/DBS [13]). MWCNT-CDC fibers are formed by dielectrophoresis [14], where the fiber is drawn from a droplet of mixed MWCNT and CDC solution over a sharp needle under AC voltage. The MWCNT-CDC actuation mechanism follows the electrical double layer (EDL) processes (non-Faradaic actuators [15]), where the charging of the double layer results in C-C bond length change [16], but the actuation is mostly explained by ion migration [17,18].

For the development and characterization of actuator materials, an accurate measurement setup is an important tool. There have been several hardware constructions using different sensors, for example isometric transducers to measure force, and isotonic transducers to measure strain. For each sensor system, an individual experimental setup is required [19,20]. Recently, a unique hardware setup based on linear variable distance transducers [21] was reported for the measurement of the length change (strain) of conducting polymer devices. The setup consisted of a beam balance with constant weight on one side and the PPy helix tube on the other side. The disadvantage of this setup is the absence of any isometric (force) measurements. Some other constructions have been proposed based on optical signals (lasers) [22], whereas on the end of the hung film a thin wire suspends a reflecting plate through a pinhole of the cell and the up and down movement can be measured by laser displacement. In general, the combination of an isometric transducer with a linear actuation stage (LAS) has been shown to be capable of strain and stress measurements without changing the setup [23–25]. The main problem with all these hardware setups is the neglect of synchronization between the driving potentiostat and the linear actuation device.

Our main interest, therefore, was to focus on a suitable software solution to perform synchronized isotonic and isometric measurements at the same time scale. Here, we want to demonstrate the suitability of the software solution on two different materials: one having high strain and stress—the conducting polymer (PPy/DBS), the other pushing the limitations of the software solution with low strain—the MWCNT-CDC fiber. Numerous manufacturers of stress and strain measurement equipment have advertised various software solutions, which provide isotonic or isometric measurement functionalities with driving signal generation possibilities [26–28]. The primary drawback of these systems, which in general are composed of two different devices (a potentiostat, a linear muscle analyzer with force sensor), is that the devices start and run independently from each other, [29] leading to disconnected time scales/time frames of the measurements— meaning poor synchronizability. Aurora Scientific [27] does provide a complete set of hardware (isotonic/isometric sensor, signal generator, data acquisition device) with a LabVIEW-based software (named dynamic measurement control (DMC)) that controls the hardware resources, executes experiments, and analyzes measured data [27,30,31]. Despite the apparent suitability of the DMC software and other similar products, these solutions are rarely available as open source. Even if they were, the isotonic/isometric measurement software solutions are directly designed for a specific hardware setup [28,32]. Another drawback is the amount of data, collected in binary format and in MB sizes, as the lowest sampling rates available are in the order of 1000 points/s, which make the data analysis rather complicated. Therefore, our interest was to obtain a flexible isotonic and isometric measurement solution that can be composed of devices from different manufacturers and handle data in a manageable text file format in kB scale. The most resource-efficient solution was to develop a custom software for running the desired experiments meeting the requirements.

Here, we present the resulting measurement system and demonstrate its performance on two model ionic actuator systems—a conducting polymer film and a carbon composite fiber.

#### **2. Material and Methods**
