Manufacturing of Three-dimensionally Microstructured Nanocomposites through Microfluidic Infiltration

1. Fabrication of 3D Microfluidic Networks

1. Melt the fugitive ink at 80 °C and load it into a 3 ml syringe barrel.
   Note: The fugitive ink is a binary mixture of a microcrystalline wax and a petroleum jelly with a weight proportion of 40:60.
2. Choose a deposition nozzle depending on the desired filament diameter (e.g. internal diameter (ID) = 150 µm).
3. Install the nozzle on the syringe barrel containing the ink material and mount it onto the syringe holder of the dispensing robot.
4. Use an Excel program to design the moving path of the dispensing robot for the fabrication of the desired 3D scaffold structure.
   Note: The overall dimensions of the 3D ink structure and filaments' spacing in a given layer can be easily programmed; in this case, the dimensions are 60 mm in length, 7.5 mm in width, and 1.7 mm in thickness with 0.25 mm horizontal spacing between each filament.
5. Set the deposition pressure on the pressure regulator and the robot dispensing speed.
   Note: The fugitive ink filament diameter varies depending on the nozzle diameter, deposition pressure, ink viscosity and dispensing speed. Here, the filament diameter is ~150 µm for a deposition speed of 4.7 mm/sec at an extrusion pressure of 1.9 MPa.
6. Start the fabrication of the microscaffold with the deposition of the ink-based filaments on an epoxy substrate, which leads to a 2D pattern (Figure 2a).
7. Deposit the subsequent layers by successively incrementing the z-position of the dispensing nozzle by an amount equal to the diameter of the filaments (Figure 2b).
   Note: Self-supported 3D structures with other geometries and many layers (e.g. a few hundreds layers) could be built.
8. Mix the two parts of epoxy (i.e. the resin and the hardener) used for the encapsulation and degas the epoxy mixture under vacuum for a defined time (here, 0.15 bar for 30 min) to remove the bubbles trapped during mixing of the epoxy components.
   Note: The degassing time may vary with the gel time of the epoxy mixture. For a different epoxy system, the required degassing time may be shorter or longer.
9. Load the epoxy resin into a 3 ml syringe barrel using a fluid dispenser by applying a negative pressure and then mount a fine nozzle (e.g. ID = 0.51 mm) into the syringe barrel.
10. Place drops of epoxy over the inclined scaffold structure at its upper end using the same fluid dispenser and mounted nozzle to minimize the risk of bubble trapping during the epoxy encapsulation.
    Note: The epoxy then flows into the empty spaces between filaments, driven by gravity and capillary forces.
11. Continue placing drops of epoxy over the scaffold until the empty space between the scaffold filaments is completely filled.
12. Let the encapsulating epoxy precure at room temperature for 24 hr and then put the structure in an oven for post-curing at 60 °C (Figure 2c).
    Note: A different cure schedule may be applied for a different epoxy system.
13. Cut the excess parts of epoxy using a precision saw after complete curing.
14. Drill two holes at two ends of the structure and insert two plastic tubes.
15. Remove the fugitive ink from the structure as following:
    1. Put the samples in an oven at 90 °C for 30 min for the ink liquefaction (Figure 2d).
    2. Shortly after taking the samples out of the oven, wash the channel network with the suction of hot distilled water through the tubes attached to the opened channels for 5 min followed by hexane for another 5 min.
       Note: The ink removal yields an interconnected 3D microfluidic network (Figure 5). Post-cleaning of the networks using hexane is performed in order to remove the possible residual traces of the ink from the channel walls.

2. Nanocomposite Preparation

Note: The nanocomposites are prepared by blending a dual cure (ultraviolet/heat curable) thermosetting resin, either an epoxy resin or a urethane-based resin and nanofillers (here, single-walled carbon nanotubes) at different loadings.

1. Add the desired amount of nanotubes to a solution of 0.1 mM of a surfactant (zinc protoporphyrin IX) either in acetone or dichloromethane¹⁴ (Figure 6).
   Note: Here, 150 mg of CNTs was added to the solution (~50 ml) in order to prepare a nanocomposite with a final nanotube concentration of 0.5% wt. It should be also mentioned that the use of high boiling temperature solvents like DMF should be avoided due to possible heat-curing of the UV-epoxy used in this study at temperatures above 60 °C during the solvent evaporation.
2. Sonicate the suspension in an ultrasonic bath for 30 min to debundle the nanotube aggregates (Figure 6).
   Note: Additional efforts such as filtration or ultracentrifugation of the nanotube solution should be made to remove the remaining big clusters before mixing with the resin.
3. Mix the resin (either epoxy or urethane) with the nanotube suspension over a magnetic stirring hot plate at a temperature slightly below the solvent boiling temperature (e.g. 50 °C for acetone solution) for 4 hr.
4. Place the nanocomposite mixture into the ultrasonication bath and simultaneously apply the sonication and heating (40-50 °C) for 1 hr (Figure 6).
5. Let the residual solvent evaporate by heating the nanocomposite at 30 °C for 12 hr and then at 50 °C for 24 hr under vacuum (~0.1 bar).
6. Shear mix the nanocomposite materials by passing them through a small gap between the rolls in a three-roll mill mixer in order to break large nanotube aggregates (Figure 6). Keep a portion of nanocomposite prior to three-roll mixing for baseline comparison.
7. Set the three-roll mixing parameters (i.e. gaps and rotating speed).
   Note: Here, a constant speed of 250 rpm is used for the apron roll. However, the gaps between rolls are reduced in three-step processing as follows: 5 passes at 25 µm, 5 passes at 10 µm, and 10 passes at 5 µm, respectively.
8. Degas the final mixture under vacuum of ~0.1 bar for 24 hr using a desiccator to remove the air bubbles trapped during the mixing.

3. Nanocomposite Infiltration (Injection)

1. Load the nanocomposites, prepared in section 2, into a 3 ml syringe barrel using the fluid dispenser by applying a negative pressure.
2. Insert a fine nozzle (e.g. ID = 0.51 mm) that fits into the plastic tubes attached to the opened channels (same tubes used for the ink removal) and mount it into the syringe barrel containing the nanocomposite materials.
3. Set the desired pressure (i.e. positive pressure) on the pressure dispenser.
   Note: Here, the nanocomposite injection pressure is set at 400 kPa.
   Note: A vacuum (i.e. negative pressure) could be applied to the other end (i.e. outlet side) to assist the network filling. Once the pressure is applied, the microfluidic network, built in Protocol 1, is filled by nanocomposite suspension, which enters the network through the plastic tubes.
4. Shortly after the injection, expose the nanocomposite-filled composite beams to UV illumination of a UV lamp for 30 min for precuring.
   Note: This precuring operation is thought to reduce the effect of Brownian motion on the CNTs possible orientation. It also reduces the heat-induced shrinkage (Figure 7)
5. Post-cure the manufactured beams in the oven at, in the case of UV-epoxy, 80 °C for 1 hr followed by 130 °C for another 1 hr (Figure 7).
6. Cut the excess epoxy parts using a saw and then polish the beams to the desired dimensions (here, ~60 mm in length, ~6.8 mm in width, and ~1.6 mm in thickness of the beams were manufactured for ease of mechanical characterization).