Design and Vat-Photopolymerization 3D Printing of Customized Micromolds for Polymeric Microneedle Fabrication

Summary

This protocol details CAD design, 3D-printed master fabrication, elastomer reverse mold preparation, and single-fill casting to create polymeric microneedle patches to achieve reproducible manufacturing of customizable arrays.

Abstract

This protocol presents a generalized, application-agnostic workflow for manufacturing polymeric microneedle (MN) patches using computer-aided design (CAD), vat-photopolymerization master fabrication (e.g., stereolithography or digital light processing, SLA/DLP), elastomer reverse molding, and single-fill casting. The method begins with parametric CAD of the MN geometry and array layout, including a thin patch base and an integrated reservoir to facilitate uniform cavity infiltration and limit thick rim formation at the patch perimeter. 3D-printing parameters are selected to preserve micro-feature integrity, followed by a brief solvent wash and controlled post-curing to remove residuals and stabilize features. The 3D-printed master is then replicated in an elastomer to yield a flexible reverse mold suitable for repeated casting cycles. Casting employs polymer solutions that are infiltrated into the mold via centrifugation or vacuum, enabling consistent cavity filling across a range of geometries. Representative quality assessments include inspection of the printed master and reverse mold for cavity completeness and defects, along with dimensional checks of needle height, base width, and tip quality at multiple array positions. The workflow accommodates multiple MN array model designs and is readily adapted to alternative polymer formulations by adjusting solution solids, viscosity, and drying conditions, enabling rapid, low-cost, and reproducible fabrication of polymeric MN patches suitable for research and prototyping.

Introduction

Microneedles (MNs) provide a practical means to access the skin, eye, or model tissues with minimal discomfort while enabling precise control over geometry and array layout^1,2,3,4. For laboratory manufacturing, replication from a high-fidelity master is efficient: once a suitable master is available, soft-lithographic copies can be produced repeatedly to screen materials, dimensions, and array configurations with consistent quality^5,6. Traditional microneedle fabrication methods, such as silicon micromachining, laser ablation, and injection molding, can achieve high precision but often require specialized facilities, complex tooling, or high initial costs, limiting their flexibility for rapid design iteration in research settings⁷.

Desktop vat-photopolymerization 3D printers, including stereolithography (SLA) and digital light processing (DLP), are well-suited for generating mold masters due to short design-to-part cycles and micrometer-scale feature control at relatively low cost⁸. In these systems, a liquid photopolymer resin is contained in a vat and selectively cured layer-by-layer by either a scanned laser beam (SLA) or a projected light pattern (DLP), enabling fine control over micro-feature geometry^8,9. Polymeric MNs are particularly attractive because they can be formulated to dissolve or swell as hydrogels; common dissolving matrices include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), hyaluronic acid (HA), and sodium carboxymethyl cellulose (NaCMC), while hydrogel-forming examples include crosslinked PVA, poly(ethylene glycol) diacrylate (PEGDA), and poly(methyl vinyl ether-co-maleic anhydride; PMVE/MA)^10,11.

Achieving accurate micro-features at sub-millimeter needle heights requires attention to print parameters and post-processing. Compared with filament extrusion approaches, vat-photopolymerization leverages photopolymerization governed by laser spot size, voxel shape, layer thickness, and exposure time; practical choices, including modest part tilt and support placement on the non-functional face, can improve effective resolution on limited-resolution printers^5,12. After master fabrication, a flexible reverse mold, for example, the silicone elastomer polydimethylsiloxane (PDMS), can be used for single-fill casting, where aqueous polymer solutions are driven into the cavities by centrifugation (using appropriate carriers for swinging bucket rotors) or by vacuum^5,6,12.

This design-to-manufacture pipeline can accelerate production and customization. Parametric CAD enables rapid edits to height, base, pitch, and array count without retooling; masters can be printed rapidly on a benchtop system; and a single master supports many replicas, reducing changeover time^5,13,14. Switching between designs becomes a matter of updating parameters and reprinting the master, while the same workflow and casting conditions are retained. As a result, laboratories can iterate designs, compare variants, and scale batches faster with minimal downtime, enabling quick manipulation of geometry for study-specific needs^5,14,15.

Geometry and layout parameters, including needle height, base width, pitch, and array count, directly influence insertion reliability, patient comfort, and material use. Choosing appropriate heights, bases, and spacing promotes consistent performance and manufacturability^3,5,16,17. To make these trade-offs explicit and reproducible, this protocol uses a conical MN model as an illustrative case, while remaining fully compatible with alternative shapes (e.g., pyramidal or hybrid), sizes, pitches, and needle counts. The result is a general manufacturing workflow that spans CAD, desktop vat-photopolymerization 3D printing of the master, reverse molding, and single-fill casting, which laboratories can adapt to rapidly and consistently produce polymeric MN patches.

Protocol

 NOTE: An overview of the complete CAD-to-manufacture workflow is shown in Figure 1.

1. CAD design

1. Use CAD software to design the MN master. Define the MN unit cell (e.g., conical or pyramidal) and set dimensions to meet defined targets (e.g., height 1,000 µm; base 500 µm).
2. Pattern the array (e.g., 6 x 6). Choose the pitch to achieve the desired inter-needle spacing (e.g., 1.1–1.2 mm pitch to keep ≥0.5 mm between needles).
3. Create a parametric patch base (e.g., 10 x 10 mm) and set the base thickness (e.g., 0.25 mm).
4. Add a round or oval reservoir beneath the base (~3 mm depth) to hold excess solution and promote uniform single-cast filling.
5. Add edge fillets (e.g., 1.0-1.5 mm) to the reservoir perimeter to improve flow during casting (Figure 2).
6. Export design as. STL.
   NOTE: Ensure the master geometry matches the intended final patch geometry.

2. Slice and print

1. Import the. STL into the slicer software. Set layer thickness to (e.g., 25-50 µm), orientation, and supports on the non-functional/back face.
2. Review the sliced layers to confirm continuous walls, no unsupported overhangs on the MN face, and adequate support.
3. Send to the 3D printer, load resin, and print. Use the smallest reliable layer thickness; thinner layers improve detail but increase print time.
   NOTE: Use the smallest reliable layer thickness supported by the printer and resin system to minimize the stair-step effect.

3. Wash and post-cure

1. Rinse the print in a suitable wash solvent (e.g., isopropyl alcohol) with gentle agitation; allow to drain and air-dry. Use a small soft-bristle brush to clear residual resin from inter-needle spaces.
2. UV post-cure in a curing device as per the resin manufacturer’s guidance; avoid overheating or over-curing.
3. Inspect under a light microscope (e.g., 4x); proceed only if features are complete and clean across the array.

4. Reverse mold

1. Mix the PDMS base and curing agent according to the manufacturer’s instructions.
2. Place the 3D printed master MN-side up in a casting dish (secure the base with a small amount of glue or double-sided tape); pour elastomer to cover by ≥3–5 mm.
3. Degas if required; cure at room temperature for 24 h or mild heat as specified until elastomeric.
4. Gently peel the elastomeric mold from the master; trim edges; store dust-free.
   NOTE: Some photopolymerizable resins inhibit platinum-catalyzed silicone curing. Improve curing by holding the fully post-cured master at ~40-50 °C overnight (dust-free) before pouring the elastomer. Ensure the surface is clean, dry, and free of uncured resin.

5. Preparation of polymeric solution

1. Weigh the polymer to the desired concentration (e.g., 5%). Add solvent (e.g., deionized water) with stirring; heat if required until fully dissolved.
2. Cool to room temperature; degas (vacuum or rest) to remove entrained air.
   NOTE: Other polymer systems (e.g., PVA, PVP, HA) can be used; adjust solids, viscosity, and drying conditions accordingly.

6. Filling the mold

1. Dispense polymeric solution to cover the array and reservoir. Choose one of the following to drive the solution into cavities.
2. Vacuum route: Place the mold in a desiccator or vacuum oven (no heat). Apply vacuum for 2–5 min. Slowly vent to draw the solution into the cavities. Repeat if needed.
3. Centrifuge route: Place the mold in a swinging-bucket (horizontal or 90°) rotor. Centrifuge at 1,500–3,000 x g for 5–10 min to promote complete filling of the cavities. If incomplete filling is observed, adjust time or g-force within this range.
   NOTE: For small patches, design the mold to fit 50 mL conical tubes (seated on a lightweight cured silicone support) or use a plate-style carrier so the mold remains level and cannot shift during acceleration.

7. Drying, release, and basic QC

1. Dry the filled mold at ambient conditions (or controlled RH/temperature) until the polymeric solution fully solidifies.
2. Initiate release by gently flexing the elastomeric mold, then peel the MN patch free. Trim to remove excess polymer at the edges.
3. Inspect under a digital microscope for needle yield (% intact). Measure height and base at multiple positions across the array and report mean ± SD (and % deviation from CAD, if applicable).

Results

The workflow yields a parametric CAD design for an MN array, a 3D printed master that is replicated in PDMS to form a flexible reverse mold, and demolded polymeric MN patches produced by a single fill (Figure 1). Because all geometric variables are defined parametrically, the same protocol can be flexibly adjusted to change MN dimensions (height, base), needle shape (e.g., pyramidal, conical, hybrid), pitch, and the total number of needles per array without altering downstream steps.

Discussion

This protocol establishes a CAD-to-vat-photopolymerization-to-PDMS workflow for fabricating polymeric MN patches as a general platform that is not tied to a specific drug or tissue model. Within this workflow, several process steps play a disproportionate role in determining dimensional fidelity, mold quality, and final patch integrity. The most critical steps are producing a dimensionally reliable 3D printed master, creating a fully cured flexible reverse mold, and ensuring complete single-fill infiltration before dryin...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Analytical balance (±0.1 mg)	Mettler Toledo		For accurately weighing polymers.
Clear photopolymer resin	Formlabs		Resin used to print the microneedle master; choose the actual resin you used.
Desktop stereolithography (SLA) 3D printer Form 4	Formlabs		Used to print the microneedle master; specify the exact model you used.
Isopropyl alcohol (IPA), ≥99%	Merck Sigma Aldrich		Used for washing SLA prints. Flammable; use in fume hood.
Magnetic stirrer / hot plate	IKA		For preparing polymer solutions.
Microscopy/image analysis software	ImageJ		Used to measure needle height, base width, and yield from micrographs.
Optical microscope or stereomicroscope	Leica		For inspecting needle height, base width, and tip quality.
Parametric CAD software	Autodesk Fusion 360		Used for microneedle master design and parametric array layout.
PDMS elastomer kit (base + curing agent), Sylgard 184	Dow		Used to fabricate the flexible reverse mold.
Polyvinyl alcohol (PVA)	Merck Sigma Aldrich		Example polymer for dissolving MN patches (10% w/v in protocol).
SLA slicer software PreForm	Formlabs		Used to orient, support, and slice the STL files for printing.
Sodium carboxymethyl cellulose (NaCMC), pharmaceutical/analytic grade	Merck Sigma Aldrich		Example polymer for dissolving MN patches (10% w/v in protocol).
UV curing chamber for SLA prints Form Cure	Formlabs		For post-curing the printed master according to resin instructions.
Vacuum oven	Hanyang		Used for vacuum-assisted filling.
Wash station or wash container for SLA parts	custom container		Used with IPA to remove uncured resin from the master.