*2.1. Materials*

Solvents: propylene carbonate (PC, 99%) and ethylene glycol (EG, 99.8%) were purchased from Fluka (Bucharest, Romania). Pyrrole (Py, ≥ 98%, Sigma Aldrich, Taufkirchen, Germany) was vacuum-distilled prior to use and stored at a low temperature in the dark. Sodium dodecylbenzene-sulfonate (NaDBS, technical grade) and bis(trifluoromethane) sulfonamide lithium salt (LiTFSI, 99%), polyvinyl pyrrolidone (PVP, average mol. wt. 40,000) were acquired from Sigma Aldrich (Taufkirchen, Germany). MilliQ+ water was used for aqueous solutions. Amorphous titanium carbide (TiC)-derived carbon (CDC TiC-800) was purchased from Skeleton Technologies Ltd (Großröhrsdorf, Germany). Multiwall carbon nanotubes (MWCNT) were purchased from Sigma Aldrich(Taufkirchen, Germany) (O.D. × I.D. × L = 7–15 nm × 3–6 nm × 0.5–200 μm).

#### *2.2. Formation of Ionic Electroactive Materials*

#### 2.2.1. Multiwall Carbon Nanotube/Carbide-Derived Carbon (MWCNT-CDC) Fiber

The MWCNT and CDC material were dispersed in deionized water containing PVP as the surfactant and sonicated with a UPS200S (Hielscher, Teltow, Germany) ultrasonic processor for 30 min at 50% amplitude. PVP, CDC, MWCNT, and water were mixed at a ratio of 1:2 and: 4:1500 (wt %), respectively. The surfactant was added to stabilize the dispersion for extended periods. Fibers were prepared according to the method described recently [14]. Briefly, the tip of a chemically etched (diameter 0.1 μm) tungsten needle was inserted into a droplet of MWCNT-CDC suspended in deionized water deposited onto a stainless-steel plate. Then, an AC voltage was applied between the tungsten tip and the metal plate (0–350 Vpp, 2 MHz), and the tungsten tip was withdrawn until a MWCNT-CDC fiber of the desired length was formed. The initial spacing between the needle and the plate electrode (i.e., the height of the droplet) was 3 mm. The retraction of the needle was performed with an M-413.3PD precision stage (Physik Instrumente, Karlsruhe, Germany). MWCNT-CDC fibers with embedded CDC particles in the range of 25% were obtained (with an average diameter of 150 μm) and dried in an oven at 100 ◦C.

#### 2.2.2. Polypyrrole Doped with Dodecylbenzene-Sulfonate (PPy/DBS)

PPy/DBS was polymerized galvanostatically at 0.1 mA cm<sup>−</sup><sup>2</sup> (40,000s) in a 2-electrode cell with a stainless-steel mesh counter electrode and a stainless-steel sheet working electrode (18 cm2) in a 0.1 M NaDBS, 0.1 M Py, in EG/Milli-Q (1:1) mixture. The temperature of the polymerization was −40 ◦C. The obtained PPy/DBS films were washed in ethanol to remove excess of pyrrole with additional washing steps in MilliQ+ water to remove excess of NaDBS, and dried in an oven. The film, in thickness of 18.5 μm, were then stored in LiTFSI propylene carbonate (0.2 M) electrolyte.

#### *2.3. Linear Muscle Analyzer Set up*

To measure strain and stress without changing the test setup, a Linear Actuation Staging (LAS) with an isometric transducer (force sensor) were combined. The setup is shown in Figure 1a and the connection hierarchy in Figure 1b.

**Figure 1. a**: Stress and strain measurement (linear muscle analyzer) setup [10]. (1) Linear Actuation Staging (LAS), (2) beaker with electrolyte, (3)(4)(5) electrodes of the potentiostat, (6) ionic electroactive material sample, (7) force sensor. **b**: Connection hierarchy of the devices used for measurement setup.

Figure 1a shows that the sample (6) is attached between two clamps. The lower clamp is static and is fixed to the chassis (1) of the LAS (Linear actuation stage). The upper clamp is attached to a force sensor (7) which is mounted on a plate of the LAS (Physik instrumente M-414.3PD, min step size 0.5 μm, Karlsruhe, Germany) that enables it to perform linear movements. Figure 1b shows the hierarchy of the devices. The signal of the force sensor (Panlab TRI202PAD, Barcelona, Spain) is read by a voltmeter (Keithley 2182A, Beaverton, United States) which exchanges data with the computer over a General-Purpose Interface Bus (GPIB) to Universal Serial Bus (USB) converter (Prologix Rev 6.4.1, Asheville, North Carolina). The LAS (Physik Instrumente M-414.3PD, Karlsruhe, Germany) is controlled by a controller (Physik Instrumente C-863, Karlsruhe, Germany), which accepts commands from the computer via a serial port. Lastly, the potentiostat (Biologic PG581, Göttingen, Germany) is connected to the computer using a USB interface.

To examine the samples, PPy/DBS films and MWCNT-CDC fibers were cut in strips of 1.0 cm \* 0.1 cm and fixed between the force sensor and on the fixed arm with gold contacts that served as a working electrode in the linear muscle analyzer setup (Figure 1). The initial length of the films between the clamps was 1 mm. The strain, ε, was calculated from the formula ε = Δ*l*/*l,* where Δ*l* refers to *l –l1* with *l* as the original length of the film (1 mm) and *l1* the change of length obtained from isotonic measurements. The load applied on PPy/DBS films was 6 g (58.8 mN, 3.2 MPa) and the load on MWCNT-CDC films was 60 mg (0.6 mN, 33.2 kPa). For isometric (force) measurements, the stress, σ*,* was calculated using the formula σ = *F*/*A,* where *F* is the force in N acting on an object (*F* = *m\*g* with *m* the mass and *g* means acceleration due to gravity as a constant 9.8 m s<sup>−</sup>2) and *A* represents the cross-sectional area of the object (width \* thickness). A platinum sheet was used as the counter electrode in the measurements cell and Ag/AgCl (3M KCl) as the reference electrode. For PPy/DBS films and for MWCNT-CDC fibers, 0.2 M LiTFSI propylene carbonate solution was applied. For isotonic and isometric measurements, cyclic voltammetry (scan rate 5 mV s<sup>−</sup>1) was used for driving in the potential range of 0.65 to −0.6 V.

#### Characterization of the Materials

PPy/DBS films and MWCNT-CDC fibers were examined by scanning electron microscopy (Helios NanoLab 600, FEI, Hillsboro, OR, USA). The electrical conductivity of the fibers was measured by a simple two-point probe method.

#### *2.4. Linear Muscle Analyzer Software Description*

The software of the measurement system was developed [33] in the LabVIEW 2015 environment, and it provides di fferent control modes (Figure S1), which allows it to calibrate the force sensor, test sample elasticity, run electrochemical analysis with the potentiostat (real time measurements), and measure weight change (stress) and length change (strain). For real time measurement requirements, the shortest data acquisition period must be 1s, and after the user has activated an experiment, the devices must be initialized in 10s.

#### 2.4.1. Calibration of the Force Sensor

Calibration of the force sensor is necessary for converting the force sensor's output voltage into values, which represent milligrams. To operate the software, the measurement called "bias" will calibrate the force sensor by measuring the baseline voltage arising from no load on the force sensor. In the next step, the signal arising from di fferent known weights is measured. Equation (1) gives the voltage-to-milligram transition coe fficient C:

$$\mathbf{C} = \begin{array}{c} m\_w \\ \hline \mathcal{U}\_w - \mathcal{U}\_B \end{array} \tag{1}$$

where, *mw* is the weight of the calibration load in milligrams, *Uw* is the voltage of the force sensor's output which is measured by voltmeter when the calibration load is applied, and *UB* is the bias voltage. The coe fficient is stored in a .txt file. In order to calculate the force from raw voltage measurements, Equation (2) is presented:

$$F = \mathbb{C}(\mathcal{U} - \mathcal{U}\_B) \tag{2}$$

where, *F* is the calculated force, and *U* is the potential measured from the force sensor's output.
