Process of Making Three-dimensional Microstructures using Vaporization of a Sacrificial Component

Summary

The Vaporization of a Sacrificial Component (VaSC) process is used to fabricate microvascular structures. This procedure uses sacrificial poly(lactic) acid fibers to form hollow microchannels with precise 3D geometric positioning provided by laser micromachined guide plates.

Abstract

Vascular structures in natural systems are able to provide high mass transport through high surface areas and optimized structure. Few synthetic material fabrication techniques are able to mimic the complexity of these structures while maintaining scalability. The Vaporization of a Sacrificial Component (VaSC) process is able to do so. This process uses sacrificial fibers as a template to form hollow, cylindrical microchannels embedded within a matrix. Tin (II) oxalate (SnOx) is embedded within poly(lactic) acid (PLA) fibers which facilitates the use of this process. The SnOx catalyzes the depolymerization of the PLA fibers at lower temperatures. The lactic acid monomers are gaseous at these temperatures and can be removed from the embedded matrix at temperatures that do not damage the matrix. Here we show a method for aligning these fibers using micromachined plates and a tensioning device to create complex patterns of three-dimensionally arrayed microchannels. The process allows the exploration of virtually any arrangement of fiber topologies and structures.

Introduction

Natural systems use extensive vascular networks to facilitate many biological functions. Mass transport can be achieved efficiently in such systems due to high surface area to volume ratios and optimized packing structures. While many synthetic fabrication techniques can produce microvascular structures, none can produce large-scale microvasculature while maintaining complexity and compatibility with existing manufacturing methods^1-5. Structures such as the avian lung provide an inspiration. How do we fabricate structures of this complexity for enhancing mass transport?

The Vaporization of a Sacrificial Component (VaSC) can produce large-scale, complex microvascular structures^6-7. This method uses the thermal depolymerization and evaporative removal of poly(lactic) acid fibers to form hollow channels that are the inverse of the fiber template. This is a sacrificial technique compatible with existing manufacturing methods. Meter long, cylindrical microchannel patterns can be formed using this fabrication process. This can be used to create vascularized devices such as self-healing polymers and 3D microvascular carbon capture units^7-10.

The carbon capture units were inspired by the avian lung that provides an efficient gas-exchange-to-weight ratio owing to its use in flight. The parabronchus is composed of hexagonally patterned microchannels, which provides high gas exchange rates and structurally stable gas exchange units. In order to create exchange units with microscale features aligned in three-dimensions, we developed a method of independently tensioning fibers using a custom designed tension board with guitar tuners and laser-micromachined plates. Each fiber is held in place by external tension and the pattern is set by the placement of holes in the plate through which the fibers run.

Protocol

1. Catalyzing Sacrificial Fibers

1. Wrap the desired amount of poly(lactic) acid fibers around the lower ¾ of customized spindle. Reduce fiber overlap to provide the maximum surface area exposure.
2. Mix deionized H₂O with 40 ml of Disperbyk 130 in a closed bottle and shake until a homogenous solution is obtained. Then place a 1,000 ml beaker in a water bath at 37 °C and pour trifluoroethanol into the beaker. The amount of H₂O and TFE to use depends on the PLA fiber diameter used.

   Fiber Diameter	Amount of H2O (ml)	Amount of TFE (ml)
   200	400	400
   300	360	440
   500	320	480

3. Add the H₂O/Disperbyk 187 solution to the beaker and stir until uniform.
4. Add 1 g of Malachite Green to the mixture and stir until dissolved.
5. Place the custom spindle with fibers in the beaker ½ inch from the bottom and attach the spindle to a digital mixer. Then start the digital mixer at 400 rpm.
6. Slowly add 1.3 g of tin (II) oxalate (SnOx) catalyst to the mixture. The addition of SnOx must be gradual in order to prevent large agglomerations of material from crashing out of the solution.
7. Adjust the pH in the mixture using NaOH until the pH is ~6.8-7.2.
8. Secure a lid to the beaker and increase the spindle rotation to 500 rpm for 24 hr. If an agglomeration of SnOx is observed, manually break it up within the first 2 hr.
9. Remove spindle and dry in oven at 35 °C overnight.
10. Unwrap and remove excess catalyst from the catalyzed PLA fibers.

2. Microvascular Gas Exchange Unit Fabrication

1. Obtain a pair of laser-cut brass patterning brass plates with the desired microvascular pattern and affix the plates on clip holders.
2. Cut a 10 inch length of catalyzed fiber per microchannel and remove any remaining catalyst using a thicker plate cut to the fiber diameter (draw plate).
3. Taper the edges of the fibers by using the tip of a hot glue gun to slowly extrude the fiber tips.
4. Thread the fibers through matching holes in the brass patterning plate pairs.
5. Screw the plates onto a molding box. Make sure the fibers are not twisted when attaching the plates.
6. String the fiber tips through the tuning pegs of the custom tensioning board.
7. Tension the PLA fibers until taut. Be careful not to over-tension and snap the fibers.
8. Remove excess particulates from the fiber pattern using compressed air.
9. Mix polydimethylsiloxane (PDMS) base with curing agent in a 10:1, v:v ratio.
10. Degas the mixture under vacuum in a desiccator jar for 10 min.
11. Pour the PDMS mixture into the mold box. Do not pour directly over the fibers in order to reduce the trapping of air bubbles.
12. Using a 26 G needle, remove any bubbles within the molding box or between the fibers.
13. Cure the PDMS mixture at 85 °C for 30 min.
14. Unfasten the brass plates from the mold box, making sure not to bend the plates or pull too hard. Remove the cured 1^st stage from the mold box.
15. Thread the fibers through an RTV end-cap by puncturing holes in the end-cap with a hypodermic needle. Depending on fiber size, use a needle gauge that has at least 2x the inner diameter of the outer diameter of your fiber. Maintain a similar pattern as the brass patterning plate, but more widely spread out.
16. Fasten the end-caps to the ends of a larger mold box and pour a 2^nd stage of PDMS.
17. Remove any remaining gas bubbles and cure at 85 °C for 30 min.
18. Cut any excess PLA fibers from the sample and place in a vacuum oven at 210 °C for 24 hr, or until the PLA fibers have been mostly evacuated.
19. If any PLA cannot be removed, gently dissolve out of the microchannels using an injection of 1 ml of chloroform.

Results

This procedure provides a method of fabricating microvascular structures embedded within a resin. These structures can conform to a variety of patterns (Figure 2). The structure of the microvascular network is only limited by the structures that can be formed with the sacrificial fibers.

Using a parallel arrangement of microvascular channels, gas transport between fluid streams is facilitated as gases traverse a permeable inter-channel membrane. These devices can be fabricated...

Discussion

The introduction of the SnOx catalyst into the PLA fibers allows the fibers to depolymerize at a lower temperature. This prevents the degradation of the embedding resin, in this case PDMS. A custom spindle is required to properly mix the treatment solution (Figure 5A). The spindle is composed of six supporting rods surrounding a central core which attaches to a digital mixer. The fibers are wrapped around the support rods so that the surface area of the wrapping fibers in contact with the catalytic solut...

Materials

Name	Company	Catalog Number	Comments
Reagent
Tin (II) oxalate	Sigma-Aldrich	402761
Disperbyk 130	BYK Additives Instruments
Trifluoroethanol	Halocarbon
Malachite Green (technical grade)	Sigma-Aldrich	M6880
Sodium hydroxide (≥98%, pellets)	Sigma-Aldrich	S5881
Polydimethylsiloxane (PDMS)	Dow Corning	3097358-1004	Distributed from Ellsworth Adhesives
Poly(lactic) acid fibers	Teijin Monofilament
Material
RW 20 Digital Mixer	IKA	3593001
Desiccator Jar	Pyrex
Vacuum Oven	Fisher Scientific
Third Hand	Jameco Electronics	26690	Plate holder
Glue Gun	Stanley	GR20L
PLA Spindle			Custom made
Tensioning Board			Custom made