1 ml syringe

A 12% wt cellulose acetate (CA) solution was made by dissolving CA (Sigma-Aldrich, Mn = 61,000 with 40% acetyl groups) in acetone and dimethylacetamide (DMAc) in a 2:1 (wt) proportion. The solution was loaded onto a 1 mL syringe (B. BRAUN) with a 21-gauge needle tip (from ITEC). Previously reported parameters^{15 (link)} were used to produce the electrospun membrane. The process was carried out under controlled environmental conditions, namely at temperature of 22 °C and relative humidity of 40%, approximately. A thermoplastic frame, with 6 cm ×6 cm of dimension, was designed using Autodesk Fusion 360 software and printed in a 3D printer (PRUSA i3) to support the carbon-based wires (inner electrodes) during the electrospinning process (Fig. SI1). For 60 minutes deposition, a CA fibres layer with a thickness of 193 ± 74.9 µm, approximately, was obtained.

The hydrogel composition (used for hydrogel characteristics, cell-material interaction, and cellular hydrogels) contained 100 mg GG (Gelzan, Sigma Aldrich, Taufkirchen, Germany) in 9 mL double distilled water and 1 mL mesenchymal stem cell growth medium (MSCGMx, PeloBiotech). Additionally, the bioink composition (used for bioprinting experiments) contained 100 mg GG in 7.5 mL double distilled water, 1.5 mL MSCGMx, and 1 mL PBS+.
For both, GG was dissolved in double distilled water in the microwave by heating the liquid several times, stirring at a vortexer and heating again until a clear solution was obtained. Afterward, the solution was tempered at 37 °C, and the additional liquids (MSCGMx or MSCGMx and PBS+ with or without cells) were carefully added by pipetting up and down while stirring simultaneously. A measure of 100 µL of the resulting hydrogel-like solution were transferred into a mold (8 mm diameter, 3 mm height) within a 24-well plate (Greiner BioOne) using a 1 mL syringe (B.Braun). Finally, for crosslinking the unstable hydrogel-like solution, the molds were rinsed with 750 µL cell culture medium and placed into an incubator (Thermo Fisher Scientific) under standard conditions (37 °C and 5% CO₂).

Each sample was collected in a 1 ml syringe (B. Braun Medical SA, Emmenbrücke, Switzerland) through a 12-cm-long catheter (needle and adaptor: LUER Lock 0.4 × 12 mm, silicone tubing 0.6 mm OD) and processed without delay. The blood was transferred into two (sample plus spare) heparinized capillary tubes (115 µl; AVL, Roche Diagnostics, Switzerland), hermetically closed with two capillary caps, mixed using a magnetic stick, labelled, kept in a thermostable obscure box and transferred to the nearby laboratory of the cantonal hospital of Fribourg for blood gas analysis within 15 min (Radiometer ABL 700, Diamond Diagnostics, MA, USA).

Two hippocampal slices were placed over the membrane in each chip. For a proper chip sealing, the PDMS block underside was previously exposed to a plasma activation, and subsequently, it was attached to the upper vinyl layer. Immediately, the lower microfluidic chamber was filled to have fresh medium using a micropipette to avoid the slice dehumidification, and in addition, inlet and outlet tubings were fitted into the PDMS perforations. Next, a 1 mL syringe (B. Braun, Melsungen, Germany) was charged with cell culture medium DMEM-high glucose, 20% Horse Serum heat-inactivated and 1% antibiotic and antimycotic solution (Thermo Fisher Scientific, Madrid, Spain) and connected to the chip by the inlet tubing. This syringe was positioned into a perfusion pump with a 1 µL/min flow rate being this flow perfused only through the lower microfluidic chamber. A CO₂ incubator (37 °C, 5% CO₂) was employed to preserve the microfluidic platform in equal temperature, humidity, and CO₂ atmosphere conditions for 10 days.