F127

The Na⁺ selective membrane cocktail consisted of Na ionophore X (1% weight by weight, w/w), Na-TFPB (0.55% w/w), PVC (33% w/w), and DOS (65.45% w/w). 100 mg of the membrane cocktail was dissolved in 660 μl of tetrahydrofuran^{17 (link)}. The K⁺-selective membrane cocktail was composed of valinomycin (2% w/w), NaTPB (0.5%), PVC (32.7% w/w), and DOS (64.7% w/w). 100 mg of the membrane cocktail was dissolved in 350 μl of cyclohexanone. The ion-selective solutions were sealed and stored at 4 °C. The solution for the PVB reference electrode was prepared by dissolving 79.1 mg PVB and 50 mg of NaCl into 1 ml methanol^{36 (link)}. 2 mg F127 and 0.2 mg of multiwall carbon nanotubes were added into the reference solution to minimize the potential drift²⁵ .
Poly(3,4-ethylenedioxythiophene) PEDOT:PSS was chosen as the ion–electron transducer to minimize the potential drift of the ISEs^{37 (link)} and deposited onto the working electrodes by galvanostatic electrochemical polymerization with an external Ag/AgCl reference electrode from a solution containing 0.01-M EDOT and 0.1-M NaPSS. A constant current of 14 μA (2 mA cm⁻²) was applied to produce polymerization charges of 10 mC onto each electrode.
Ion-selective membranes were then prepared by drop-casting 10 μl of the Na⁺-selective membrane cocktail and 4 μl of the K⁺-selective membrane cocktail onto their corresponding electrodes. The common reference electrode for the Na⁺ and K⁺ ISEs was modified by casting 10 μl of reference solution onto the Ag/AgCl electrode. The modified electrodes were left to dry overnight. The sensors could be used without pre-conditioning (with a small drift of ~2–3 mV h⁻¹). However, to obtain the best performance for long-term continuous measurements such as dehydration studies, the ion-selective sensors were covered with a solution containing 0.1-M NaCl and 0.01-M KCl through microinjection (without contact to glucose and lactate sensors) for 1 h before measurements. This conditioning process was important to minimize the potential drift further.

Patterned PDMS stamps were cast from a photoresist-patterned silicon wafer, as previously described.^{23 (link)} Stamp-off templates were cast similarly, but from negative photoresist patterns on the silicon wafer. Flat PDMS stamps were cast from an unpatterned silicon wafer. For microcontact printing, PDMS stamps were inked with protein at 50 μg ml⁻¹ in H₂O (for fibronectin and vitronectin), 50 μg ml⁻¹ in PBS (for bovine serum albumins), or 100 μg ml⁻¹ in 1% (v/v) acetic acid (for collagen type I), all for 1 h at room temperature. The stamps were then thoroughly rinsed in H₂O and blown dry with a stream of N₂. In parallel, the target substrate (a stamp-off template or PDMS cell culture substrate) was treated with ultraviolet ozone for specified times (Jelight Company, Irvine, CA). The stamp was then placed in conformal contact with the target substrate for ~1 s. F127 Pluronics was adsorbed to PDMS surfaces from a 0.2% (w/v) solution for 1 h at room temperature to prevent protein adsorbtion to non-functionalized portions of the PDMS.