*2.5. Rheological Test*

The rheological properties of the SA solution with various compositions were analyzed using an AR-G2 stress-controlled rheometer (TA Instrument, New Castle, DE, USA) with a parallel plate geometry (1 mm) at 45 ◦C. The viscosity of hybrid ink was also measured with different temperatures from 35 ◦C to 65 ◦C. The shear rate was varied from 0.01 s<sup>−</sup><sup>1</sup> to 1000 s<sup>−</sup>1. The oscillation frequency measurements were conducted at 35 ◦C and at a frequency of 1 Hz to measure storage and loss modulus of ink solution. The tan δ values was calculated as [18]:

$$\text{Tan } \delta = \frac{G''}{G'} \tag{3}$$

where *G* and *G* are storage and loss modulus, respectively.

## *2.6. Conductivity Test*

The hydrogel sample was clipped onto a universal tensile machine (AGS-X, SHIMADZU, Kyoto, Japan) at room temperature around 20 ◦C. The speed was 10 mm min−1. After the hydrogel sample was stretched at certain strain, the resistance was measured by a multi-meter. To limit water evaporation from 3D printed hydrogels, VHB were used to wrap up hydrogel sample during test. To achieve a wearable strain sensor, 3D printed hydrogels covered with VHB were connected to electric wires

and placed on a finger with the help of copper tape. To study conductivity of hydrogels at various concentration of ions and SA, calcium alginate (CA)/polyacrylamide(PAAm) hydrogels were fabricated via an injection molding method as previous work [11]. Di fferent concentration of CaCl2 (50 mM, 100 mM, 300 mM and 500 mM) were used to soak hydrogels. Three concentrations of alginate (10 mg/mL, 20 mg/mL and 30 mg/mL) and acrylamide (1.69 M, 2.53 M and 3.38 M) were used.
