Fabricating Superhydrophobic Polymeric Materials for Biomedical Applications

Summary

Two- and three-dimensional superhydrophobic polymeric materials are prepared by electrospinning or electrospraying biodegradable polymers blended with a lower surface energy polymer of similar composition.

Abstract

Superhydrophobic materials, with surfaces possessing permanent or metastable non-wetted states, are of interest for a number of biomedical and industrial applications. Here we describe how electrospinning or electrospraying a polymer mixture containing a biodegradable, biocompatible aliphatic polyester (e.g., polycaprolactone and poly(lactide-co-glycolide)), as the major component, doped with a hydrophobic copolymer composed of the polyester and a stearate-modified poly(glycerol carbonate) affords a superhydrophobic biomaterial. The fabrication techniques of electrospinning or electrospraying provide the enhanced surface roughness and porosity on and within the fibers or the particles, respectively. The use of a low surface energy copolymer dopant that blends with the polyester and can be stably electrospun or electrosprayed affords these superhydrophobic materials. Important parameters such as fiber size, copolymer dopant composition and/or concentration, and their effects on wettability are discussed. This combination of polymer chemistry and process engineering affords a versatile approach to develop application-specific materials using scalable techniques, which are likely generalizable to a wider class of polymers for a variety of applications.

Introduction

Superhydrophobic surfaces are generally categorized as exhibiting apparent water contact angles greater than 150° with low contact angle hysteresis. These surfaces are fabricated by introducing high surface roughness on low surface energy materials to establish a resulting air-liquid-solid interface that resists wetting^1-6. Depending on the fabrication method, thin or multilayered superhydrophobic surfaces, multilayered superhydrophobic substrate coatings, or even bulk superhydrophobic structures can be prepared. This permanent or semi-permanent water repellency is a useful property that is employed to prepare self-cleaning surfaces⁷, microfluidic devices⁸, anti-fouling cell/protein surfaces^9,10, drag-reducing surfaces¹¹, and drug delivery devices^12-15. Recently, stimuli-responsive superhydrophobic materials are described where the non-wetted to wetted state is triggered by chemical, physical, or environmental cues (e.g., light, pH, temperature, ultrasound, and applied electrical potential/current)^14,16-20, and these materials are finding use for additional applications^21-25.

The first synthetic superhydrophobic surfaces were prepared by treating material surfaces with methyldihalogenosilanes²⁶, and were of limited value for biomedical applications, as the materials used were not suitable for in vivo use. Herein we describe the preparation of surface and bulk superhydrophobic materials from biocompatible polymers. Our approach entails electrospinning or electrospraying a polymer mixture containing a biodegradable, biocompatible aliphatic polyester as the major component, doped with a hydrophobic copolymer composed of the polyester and a stearate-modified poly(glycerol carbonate)^27-30. The fabrication techniques provide the enhanced surface roughness and porosity on and within the fibers or the particles, respectively, while the use of a copolymer dopant provides a low surface energy polymer that blends with the polyester and can be stably electrospun or electrosprayed^27,31,32.

Aliphatic biodegradable polyesters such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(lactic acid-co-glycolic acid) (PLGA), and polycaprolactone (PCL) are polymers used in clinically-approved devices and prominent in biomedical materials research because of their non-toxicity, biodegradability, and ease of synthesis³³. PGA and PLGA debuted in the clinic as bioresorbable sutures in the 1960’s and early 1970’s, respectively^34-37. Since then, these poly(hydroxy acids) have been processed into a variety of other application-specific form factors, such as micro-^38,39 and nanoparticles^40,41, wafers/discs⁴², meshes^27,43, foams⁴⁴, and films⁴⁵.

Aliphatic polyesters, as well as other polymers of biomedical interest, can be electrospun to produce nano- or microfiber mesh structures possessing a high surface area and porosity as well as tensile strength. Table 1 lists the synthetic polymers electrospun for various biomedical applications and their corresponding references. Electrospinning and electrospraying are rapid and commercially-scalable techniques. These two similar techniques rely on applying high voltage (electrostatic repulsion) to overcome surface tension of a polymer solution/melt in a syringe pump setup as it is directed toward a grounded target^46,47. When this technique is used in conjunction with low surface energy polymers (hydrophobic polymers such as poly(caprolactone-co-glycerol monostearate)), the resulting materials exhibit superhydrophobicity.

To illustrate this general synthetic and materials processing approach to constructing superhydrophobic materials from biomedical polymers, we describe the synthesis of superhydrophobic polycaprolactone- and poly(lactide-co-glycolide)-based materials as representative examples. The respective copolymer dopants poly(caprolactone-co-glycerol monostearate) and poly(lactide-co-glycerol monostearate) are first synthesized, then blended with polycaprolactone and poly(lactide-co-glycolide), respectively, and finally electrospun or electrosprayed. The resulting materials are characterized by SEM imaging and contact angle goniometry, and tested for in vitro and in vivo biocompatibility. Finally, bulk wetting through three-dimensional superhydrophobic meshes is examined using contrast-enhanced microcomputed tomography.

Protocol

1. Synthesizing Functionalizable poly(1,3-glycerol carbonate-co-caprolactone)²⁹ and poly(1,3-glycerol carbonate-co-lactide)^27,28.

1. Monomer synthesis.
   1. Dissolve cis-2-Phenyl-1,3-dioxan-5-ol (50 g, 0.28 mol, 1 eq.) in 500 ml dry tetrahydrofuran (THF) and stir on ice under nitrogen. Add potassium hydroxide (33.5 g, 0.84 mol, 3 eq.), finely crushed with a mortar and pestle. Place flask in ice bath.
   2. Add 49.6 ml benzyl bromide (71.32 g, 0.42 mol, 1.5 eq.) drop-wise with stirring on ice. Allow the reaction to warm to room temperature with stirring for 24 hr, under nitrogen.
   3. Add 150 ml distilled water to dissolve potassium hydroxide and remove the THF by rotary evaporation.
   4. Extract the remaining material with 200 ml dichloromethane (DCM) in a 1-L separatory funnel. Repeat the extraction twice.
   5. Dry the organic phase on sodium sulfate.
   6. Crystallize the product by adding 600 ml absolute ethanol to the solution, mixing well, and storing overnight at -20 °C. The product can be stored at -20 °C for several days before performing subsequent steps.
   7. Isolate product by vacuum-filtration through a Büchner funnel and dry on high-vacuum. The product can be stored for several days before performing subsequent steps. A typical yield for this step is ~80%.
   8. In a 1-L round-bottom flask, suspend the product obtained in step 1.1.7. in methanol (300 ml). Add 150 ml of 2 N hydrochloric acid. Reflux at 80 °C for 2 hr.
   9. Evaporate solvent and place under high-vacuum for 24 hr. The yield for this step is typically >98%.
   10. Dissolve product of 1.1.9 in THF (650 ml) and transfer to a 2-L round-bottom flask. Place flask on ice bath and stir under nitrogen. Add 22.4 ml ethyl chloroformate (25.6 g, 0.29 mol, 2 eq.) to flask under nitrogen.
   11. Add 32.8 ml triethylamine (0.29 mol, 2 eq.) to an addition funnel. Mix with an equal volume of THF. Place addition funnel on round-bottom flask and keep under nitrogen.
   12. With vigorous stirring, carefully dispense triethyamine/THF mixture drop-wise to the round-bottom flask on ice. CAUTION: this is an exothermic reaction. To prevent rapid temperature increase, add the triethylamine/THF solution no faster than 1 drop per second. After adding the full volume, stir the reaction for 4 hr, warming up to room temperature, or for 24 hr.
   13. Filter out the triethylamine hydrochloride salt using a Büchner funnel. Evaporate the solvent on a rotary evaporator.
   14. Add dichloromethane (200 ml) to the flask and heat gently until the residue is dissolved. Add 120 ml of diethyl ether while swirling. Store at -20 °C overnight to crystallize the product.
   15. Filter monomer crystals and re-crystallize before polymerizing. The monomer product can be stored sealed at room temperature for 2 weeks or at -20 °C indefinitely. Confirm product by ¹H NMR, mass spectrometry, and elemental analysis. A typical yield for this final step in the monomer synthesis is between 40-60%.
2. Copolymerization of D,L-lactide/ε-caprolactone with 5-benzyloxy-1,3-dioxan-2-one.
   1. Heat silicone oil bath to 140 °C.
   2. Measure 2.1 g of 5-benzyloxy-1,3-dioxan-2-one (prepared in 1.1) and add it to a dry 100-ml round-bottom flask. If copolymerizing D,L-lactide, measure out 5.7 g and add to the flask now. Add a magnetic stir bar and seal the flask with a rubber stopper.
      1. Also measure 240 mg (an excess) of tin(II) ethylhexanoate in a small pear-shaped flask. This polymerization will result in a 20 mol% glycerol carbonate monomer composition. Adjust the masses of monomers to achieve different monomer compositions.
   3. Flush both flasks with nitrogen on a Schlenk manifold for 5 min and add 4.24 ml ε-caprolactone under nitrogen. Evacuate flasks’ atmosphere by applying high-vacuum (300 mTorr) for 15 min to remove trace water.
   4. Recharge the flasks’ atmosphere with nitrogen; repeat this cycle twice more.
   5. Mix 500 µl dry toluene with the tin catalyst under nitrogen.
   6. Place the monomer flask in the 140 °C oil bath and add catalyst once all solids have melted. The total volume of catalyst mixture delivered should be ~100 µl. Keep at 140 °C for no more than 24 hr, then cool the molten polymer to room temperature. Perform the subsequent steps immediately or at least 24 hr later.
   7. Dissolve the polymer in dichloromethane (50 ml) and precipitate into cold methanol (200 ml). Decant supernatant and dry under high-vacuum. The subsequent steps can be performed immediately or at any point. Store polymers in the freezer until further use. The typical polymerization yield/conversion is between 80-95%.
   8. Perform ¹H NMR analysis to determine the co-monomer molar ratios. Dissolve polymer in deuterated chloroform (CDCl₃) and integrate the benzylic proton shift of the carbonate monomer at 4.58-4.68 ppm; compare this peak area with that of the methylene peak at 2.3 ppm (PCL) and methyne peak at 5.2 ppm (PLGA).
3. Polymer modification: deprotection and grafting.
   1. Dissolve polymer (~7 g) in 120 ml tetrahydrofuran (THF) in a high-pressure hydrogenation vessel. Weigh and add palladium-carbon catalyst (~2 g).
   2. Add hydrogen to the vessel using a hydrogenation apparatus. Hydrogenate at 50 psi for 4 hr. CAUTION: hydrogen gas is extremely flammable. Seek assistance from persons familiar with this procedure and always inspect the supply lines for possible leaks before performing this experiment.
   3. Filter out palladium-carbon catalyst using a packed bed of diatomaceous earth. Concentrate the polymer to ~50 ml under rotary evaporation and precipitate into cold methanol. CAUTION: Dry palladium particulates can spontaneously ignite. Keep a wet towel nearby in the event of a flare-up for smothering the flames. Add water to the palladium/carbon filter cake to keep it clumped and to prevent its ignition. Seek assistance from persons familiar with this procedure.
   4. Decant the supernatant and dry under high-vacuum. Confirm total conversion to free hydroxyl by noting the peak disappearance at 4.65 ppm (¹H NMR in CDCl₃). These polymers can be used immediately or saved for later use. Yields for this step are >90%.
   5. Dissolve the polymer and stearic acid (1.5 eq.) in 500 ml dry dichloromethane (DCM). Add N,N’-dicyclohexylcarbodiimide (DCC, 2.0 eq.) and 3 flakes of 4-dimethylaminopyridine. Stir under nitrogen at room temperature for 24 hr.
   6. Remove insoluble N,N’-dicyclohexylcarbourea through a series of repeated filtrations and concentrations. At the end, concentrate the solution to 50 ml.
   7. Precipitate polymer into cold methanol (~175 ml) and decant the supernatant. Dry the polymer under high-vacuum overnight. Subsequent use of these polymers can be performed at any time, but keep polymers in the freezer for long-term storage. The yield for this final modification step is generally between 85-90%.

2. Characterizing the Synthesized Copolymers

1. Weigh out ~10 mg polymer (record the actual mass) and add to aluminum sample pan, then hermetically seal it. Load sample pan and an unloaded (reference) pan into the differential scanning calorimeter.
2. Program a temperature ramp and cooling (“heat/cool/heat”) cycle: 1) heat from 20 °C to 225 °C at 10 °C/min, 2) cool to -75 °C at 5 °C/min, 3) heat to 225 °C at 10 °C/min.
3. Determine melting point (T_m), crystallization (T_c) and glass transition temperatures (T_g), and heat of fusion (ΔH_f) from the thermal traces (if applicable).
4. Dissolve each synthesized copolymer in THF (1 mg/ml) and filter through a 0.02-µm PTFE filter. Inject the solution into a gel permeation chromatography system and compare retention time versus a range of polystyrene standards.

3. Preparing Polymer Solutions for Electrospinning/electrospraying^27,31

1. Dissolve polymer(s) at 10-40 wt% in suitable solvent, such as chloroform/methanol (5:1) for PCL or tetrahydrofuran/N,N-dimethylformamide (7:3) for PLGA, overnight. The mass of polymer required for this step will depend on the dimensions of the desired mesh.
   Note: For example, to produce a 10 cm x 10 cm mesh of approximately 300-micron thickness, 1 gram will typically be required. It is worth noting that material losses may occur in subsequent steps of this protocol, such as during solution transfer to the syringe (especially for viscous solutions), and from dead volumes present in the optional connector tubing and the needle housing itself, which will reduce the yield of the electrospinning process. These reductions in yield may result in up to 20% loss of material, and it is recommended to scale up 1.5-fold to anticipate these losses, and also those losses associated with optimizing the electrospinning parameters when attempting this procedure for the first time.
   1. Control fiber size by varying the total polymer concentration, with larger fibers expected from more concentrated solutions. For a modest enhancement of hydrophobicity, use 10% (by total polymer mass) superhydrophobic dopant. For extremely hydrophobic/superhydrophobic materials, use 30-50% dopant and/or reduce the total polymer concentration (i.e., reduce fiber size). Subsequent work with these solutions may be performed the next day or within one week thereafter.
   2. For electrospraying, prepare solutions at lower concentrations (i.e., 2-10%) in a suitable solvent such as chloroform. Like electrospinning, modulate particle size by varying the polymer concentration.
2. Vortex polymer solution to thoroughly mix. Allow large air bubbles to subside (5 min).
3. Load solution into a glass syringe. Depending on solution viscosity, it may be easiest to remove the plunger and pour the solution directly into the syringe. A piece of inert, flexible tubing may aid maneuverability within the electrospinning setup. Invert the syringe to displace air through the hose/needle assembly.

4. Electrospinning/electrospraying Polymer Solutions

1. Load syringe onto syringe pump, set total volume (e.g., 4.5 ml) and the rate (e.g., 5 ml/hr) at which to dispense this solution.
2. Cover the collector plate with aluminum foil to ease subsequent removal and transport. Secure the foil with masking tape along the outer edges.
3. Attach the high voltage DC (HVDC) supply wire to needle tip. The distance of this needle tip to the collector is an important variable to consider because it 1) affects the electric field at a given voltage, and 2) impacts the evaporation of solvent and consequent drying of fibers during their collection.
   1. As a first attempt, use a tip-to-collector distance of 15 cm. CAUTION: High voltages and flammable solvents are involved in electrospinning/electrospraying. Provide adequate ventilation to outside exhaust, and never touch the syringe/needle or open the enclosure until absolutely certain the HVDC supply is off.
4. If electrospinning/electrospraying a large area of coverage, turn on rotating and translating collector drum. Otherwise, proceed to the next step.
5. Start the syringe pump.
6. Turn on and adjust the high voltage source to achieve an acceptable Taylor Cone. If the solution at the needle tip is sagging, increase the voltage. If multiple jets are forming, reduce the voltage. In addition to these adjustments, it may be necessary to adjust the tip-to-collector distance if the fibers/particles appear wet or if adjusting the voltage does not adequately solve a dragging droplet at the needle tip.
   Note: For detailed troubleshooting, see the comprehensive electrospinning optimization process by Leach and co-workers⁴⁷. Electrospraying will generally involve higher voltages and lower solution concentrations than electrospinning.
7. Turn off the high voltage source and then the syringe pump and motorized drum (if applicable). Allow the electrospinning enclosure to continue ventilating for 30 min.
8. Remove meshes/coatings from collector. Allow trace solvents to evaporate in a hood overnight. Materials can be stored at room temperature for at least two weeks (PLGA) or two months (PCL). Steps 4.5-4.8 may be performed in any order.

5. Characterizing Fiber and Particle Size by Light and Scanning Electron Microscopy

1. Light microscopy
   1. If producing an electrospun mesh, cut and mount thin portions of it on a glass slide.
   2. Observe fiber diameter, node characteristics (blobs or discrete), and fiber shape (i.e., beaded, flat, straight/wavy). Ideal electrospun mesh fibers are uniform, straight or wavy, and bead-free.
2. Scanning electron microscopy (SEM)
   1. Cut and mount meshes or coated surfaces on aluminum SEM stubs using conductive copper tape. Electrospun fibers and electrosprayed coatings can also be observed by SEM by directly depositing fibers/particles onto the tape in advance.
   2. Coat the meshes/coatings with a thin (~4 nm) layer of Au/Pd through sputter coating.
   3. Load stubs into SEM chamber and observe at 1-2 keV. A 250X magnification provides a general topographical assessment of the material, while higher magnifications reveal additional fiber and particle features such as hierarchal patterns for extremely superhydrophobic fibers and interconnectivity for particle coatings.

6. Determining Non-wetting Properties

1. Advancing and receding water contact angle measurements using the volume variation method
   1. Cut thin (0.5 cm x 5 cm) strips of mesh or coated material (if possible) and place on the stage of a contact angle goniometer.
   2. Capture the water drop profile while dispensing it (from a 24 AWG syringe needle) on the material surface.
      1. To do this, start with an approximate 5-µl drop, and make contact with the material surface. Continue to slowly add volume (20-25 µl) and capture the droplet image, which represents the advancing water contact angle. The needle tip should be small compared to the droplet, and the capillary length should be greater than the droplet to minimize distortion of droplet shape.
   3. Withdraw this same drop while simultaneously capturing its drop profile. Repeat on discrete surface locations of several samples to report an average value—typically, 10 measurements of both advancing and receding contact angles are sufficient to characterize these materials.
2. Determine critical surface tension of materials by modifying probing liquids.
   1. Prepare solutions varying in ethanol, propylene glycol, or ethylene glycol content, as these mixtures have known surface tensions^99-101.
      1. Alternatively, use solvents with varying surface tensions—for example, water (72 mN/m), glycerol (64 mN/m), dimethyl sulfoxide (44 mN/m), benzyl alcohol (39 mN/m), 1,4-dioxane (33 mN/m), 1-octanol (28 mN/m), and acetone (25 mN/m). It is important to use solvents that will not dissolve the polymers, as these will confound results. Additionally, it is important to note that, in addition to surface tension, these liquids have different viscosities, which may impact contact angle measurements and is a limitation of this technique.
      2. Measure the contact angle of these solutions probed on the material surface. Plot contact angle as a function of surface tension.

7. Detecting Bulk Wetting of Meshes³¹

1. Observe water infiltration into 3D meshes using micro-computed tomography (µCT).
   1. Prepare an 80 mg/ml solution of Ioxaglate (an iodinated contrast agent) in water.
   2. Submerge meshes in these solutions and incubate at 37 °C; periodically measure contrast agent (water) infiltration by µCT (18 µm³ voxel resolution) using a 70 kVP tube voltage, 114 µA current, and a 300 msec integration time.
   3. Using image processing software, measure pixel intensity throughout the thickness of the mesh, where bright pixels represent water infiltration. Select a pixel threshold value (~1500) for which higher intensity represents water infiltration.

8. Testing the Mechanical Properties of Meshes

1. Cut meshes to 1 cm x 7 cm and place between the grips of a tensile testing apparatus. Measure the exact width, length, and thickness.
2. Perform a ramp test of extension on three samples. Plot a stress-strain curve using these data to determine the elastic modulus, ultimate tensile strength, and elongation-at-break.

Results

Through a series of chemical transformations, the functional carbonate monomer 5-benzyloxy-1,3-dioxan-2-one is synthesized as a white crystalline solid (Figure 1A). ¹H NMR confirms the structure (Figure 1B) and mass spectrometry and elemental analysis confirm the composition. This solid is then copolymerized with either D,L-lactide or ε-caprolactone using a tin-catalyzed ring opening reaction at 140 °C. After purification by precipitation, the polymer compos...

Discussion

Our approach to constructing superhydrophobic materials from biomedical polymers combines synthetic polymer chemistry with the polymer processing techniques of electrospinning and electrospraying. These techniques provide either fibers or particles, respectively. Specifically, polycaprolactone and poly(lactide-co-glycolide) based superhydrophobic materials are prepared using this strategy. By varying the hydrophobic copolymer composition, percent copolymer in the final polymer blend, fiber/particle size, overall...

Materials

Name	Company	Catalog Number	Comments
Silicone oil	Sigma-Aldrich	85409
Cis-2-Phenyl-1,3-dioxan-5-ol	Sigma-Aldrich	13468
Benzyl bromide	Sigma-Aldrich	B17905	Toxic, lacrymator/eye irritant, use in chemical fume hood
Potassium hydroxide	Sigma-Aldrich	221473	Corrosive
Rotary evaporator	Buchi	R-124
High-vacuum pump	Welch	8907
Nitrogen, ultra high purity	Airgas	NI UHP300	Compressed gas
Tetrahydrofuran, stabilized with BHT	Pharmaco-Aaper	346000	Flammable. Dried through column of XXX
Dichloromethane	Pharmaco-Aaper	313000	Flammable, toxic.
Separatory funnel (1 L)	Fisher Scientific	13-678-606
Sodium sulfate	Sigma-Aldrich	239313
Ethanol, absolute	Pharmaco-Aaper	111USP200	Flammable, toxic.
Buchner funnel	Fisher Scientific	FB-966-F
Methanol	Pharmaco-Aaper	339000ACS	Flammable, toxic.
Hydrochloric acid	Sigma-Aldrich	320331	Corrosive. Diluted to 2N in distilled water.
Ethyl chloroformate, 97%	Sigma-Aldrich	185892	Toxic, flammable, harmful to environment
Triethylamine (anhydrous)	Sigma-Aldrich	471283	Toxic, flammable, harmful to environment
Diethyl ether	Pharmaco-Aaper	373ANHACS	Highly flammable. Purified through XXX column.
3,6-Dimethyl-1,4-dioxane-2,5-dione (D,L-lactide)	Sigma-Aldrich	303143
Tin (II) ethylhexanoate	Sigma-Aldrich	S3252	Toxic.
ε-caprolactone (97%)	Sigma-Aldrich	704067
Toluene, anhydrous	Sigma-Aldrich	244511	Flammable, toxic.
Glass syringe	Hamilton Company	1700-series
Deuterated chloroform	Cambridge Isotopes Laboratories, Inc.	DLM-29-10	Toxic
Nuclear magnetic resonance instrument	Varian	V400
Palladium on carbon catalyst	Strem Chemicals, Inc.	46-1707
Hydrogenator unit	Parr	3911
Hydrogenator shaker vessel	Parr	66CA
Hydrogen	Airgas	HY HP300	Highly flammable.
Diatomaceous earth	Sigma-Aldrich	22140
2H,2H,3H,3H-perflurononanoic acid	Oakwood Products, Inc.	10519	Toxic.
Stearic acid	Sigma-Aldrich	S4751
N,N’-dicyclohexylcarbodiimide	Sigma-Aldrich	D80002	Toxic, irritant.
4-(dimethylamino) pyridine	Sigma-Aldrich	107700	Toxic.
Hexanes	Pharmaco-Aaper	359000ACS	Toxic, flammable.
Gel permeation chromatography (GPC) system	Rainin
GPC column	Waters	WAT044228
Differential scanning calorimeter	TA Instruments	Q100
Chloroform	Pharmaco-Aaper	309000ACS	Toxic.
N,N-dimethylformamide	Sigma-Aldrich	227056	Toxic, flammable.
Polycaprolactone, MW 70-90 kg/mol	Sigma-Aldrich	440744
Poly(lactide-co-glycolide), MW 136 kg/mol	Evonik Industries	LP-712
10 ml glass syringe	Hamilton Company	81620
18 AWG blunt needle	BRICO Medical Supplies	BN1815
Electrospinner enclosure box	Custom-built	N/A	Made of acrylic panels
High voltage DC supply	Glassman High Voltage, Inc.	PS/EL30R01.5	High voltages, electrocution hazard
Linear (translating) stage	Servo Systems Co.	LPS-12-20-0.2	Optional
Programmable motor & power supply	Intelligent Motion Systems, Inc.	MDrive23 Plus	Optional
24V DC motor & power supply	McMaster-Carr	6331K32	Optional
Aluminum collector drum	Custom-built		Optional
Syringe pump	Fisher Scientific	78-0100I
Inverted optical microscope	Olympus	IX70
Scanning electron microscope	Carl Zeiss	Supra V55
Conductive copper tape	3M	16072
Aluminum SEM stubs	Electron Microscopy Sciences	75200
Contact angle goniometer	Kruss	DSA100
Propylene glycol	Sigma-Aldrich	W294004	Toxic.
Ethylene glycol	Sigma-Aldrich	324558	Toxic.
Ioxaglate	Guerbet
Fetal bovine serum	American Type Culture Collection	30-2020
Micro-computed tomography instrument	Scanco
Image analysis software (Analyze)	Mayo Clinic
Tensile tester	Instron	5848
Micrometer	Multitoyo	293-340
Calipers	Fisher Scientific	14-648-17