A Comprehensive Design-to-skin Pipeline to Fabricate Polymeric Microneedles Using Ultrahigh-resolution 3D Printing

Abstract

Objective: Microneedle technologies have emerged as a promising approach to improve intradermal drug delivery. This study presents a comprehensive workflow for fabricating polymeric microneedle arrays utilising ultrahigh-resolution 3-dimensional (3D) printing and silicone mould fabrication.

Methods: In this work, an extensive toolbox with over 75 distinct microneedle designs was created and sequentially fabricated from acryl using our workflow based on ultrahigh-resolution 3D printing.

Results: The microneedle design parameters included obelisk and cone-like shapes, various lengths, base and tip diameters, and different densities. We systematically assessed the optimal design parameters for effective penetration of ex vivo human skin explants.

Conclusion: Our workflow, combined with application in an ex vivo human skin model, allows systematic comparison of multiple microneedle design parameters for efficacy. This work demonstrates the potential of this systematic modelling and ultrahigh-resolution 3D printing approach to optimize microneedles for intradermal biomedical applications, including therapeutic cancer vaccination.

Keywords: (intra)dermal drug delivery; 2-photon polymerisation; Microneedle array; Moulds; PDMS; Skin penetration.

Fig. 1

Schematical representation of the 2-photon polymerisation (2PP) and the trichloro (1H,1H,2H, 2H-perfluorooctyl) silane (TPFS) evaporation steps. (a) Before the 2PP step, the silicon substrate is cleaned with acetone and isopropyl alcohol (IPA) and undergoes oxygen treatment for better functionalisation with 3-(trimethoxy silyl)propyl methacrylate (MAPTMS) via the induction of silanol groups. (b) About 150/200 µl of resin is placed on top of the Si sample, and after the 2PP step, the non-polymerised resin is removed in a double-development step in PGMEA and IPA. (c) After drying, the master mould is functionalised with TPFS to prevent silicone adhesion.

Fig. 2

Representation of polydimethylsiloxane (PDMS) moulding and assembly. (a) PDMS moulding of the master mould. i PDMS solution is first poured and degassed, and (ii) after a heating step, it is solidified. (b) The second part of the final mould is moulded starting from a 3D-printed trapezoidal prism-shaped backplate. (c) (i) Air plasma-activated surfaces that (ii) once undergo the compressing and (iii) heating step, form a strong covalent bond.

Fig. 3

Master microneedle template dimensions and design

Fig. 4

Master microneedle template and design parameters. (a) Master template designs for solid microneedle arrays created with the SolidWorks software. (b) Stereomicroscopic images of 2PP master template of different designs. i Master structure of conical microneedle arrays with tip diameter of 30 µm, pitch distance of 400 µm, length of 500 µm and microneedle diameter of 200 µm. ii Master structure of obelisk microneedle arrays with tip diameter 3 µm (pitch distance of 400 µm, length of 500 µm and microneedle diameter of 200 µm. iii Master structure of conical microneedle arrays with a needle diameter of 200 µm, pitch distance of 400 µm, length of 250 µm and tip diameter of 3 µm. iv Master structure of obelisk microneedle arrays with variations in microneedle length of 350 µm, pitch distance of 400 µm, tip diameter of 3 µm and microneedle diameter of 200 µm. Scale bar = 1 mm.

Fig. 5

(a) Inverse polydimethylsiloxane (PDMS) replicate of the acrylic microneedle master template. (b) PDMS mould for the generation of a microneedle backplate. (c) Complete inverse PDMS microneedle mould. (d) Stereomicroscopic images of single microneedle array PDMS mould’s detail. (e) Stereomicroscopic image template’s detail.

Fig. 6

Stereomicroscopic fluorescence images of polymeric microneedle arrays with different geometries (cones and obelisks) and shapes. All microneedle arrays (5 × 5 mm) include 25 microneedles of each shape (circular, tetragonal, hexagonal and octagonal), resulting in a total of 101 needles (including a reference microneedle). Microneedle arrays were imaged from a lateral view with an 80-degree angle. (a) Conical microneedle arrays with variations in tip diameter (3, 10, 20, 30 and 50 µm) (pitch distance of 400 µm, length of 500 µm and microneedle diameter of 200 µm). (b) Obelisk microneedle arrays with variations in tip diameter (3, 10, 20, 30 and 50 µm) (pitch distance of 400 µm, length of 500 µm and microneedle-diameter of 200 µm). (c) Conical microneedle arrays with variations in diameter (50, 100, 150 and 200 µm) (pitch distance of 400 µm, length of 250 µm and tip diameter of 3 µm). (d) Obelisk microneedle arrays with variations in microneedle length (250, 350, 500 and 650 µm) (pitch distance of 400 µm, tip diameter of 3 µm and microneedle diameter of 200 µm).

Fig. 7

Piercing efficiency of polymeric microneedle arrays with varying microneedle tip diameter parameters. Schematic representation of the microneedle arrays (a) designs representing the conical and obelisk microneedles with varying tip diameters. (b) Representative micrograph of trypan blue assays on ex vivo human skin to determine piercing efficiency for the conical (i) and obelisks (ii) arrays. (c-d) Piercing efficiency for the conical microneedle arrays with different shapes (circular, tetragonal, hexagonal and octagonal) and tip diameter: (i) impact velocity was 1.4 m/s, (ii) impact velocity was 1.0 m/s, and (iii) impact velocity was 0.4 m/s. (e–f) Piercing efficiency for the obelisk microneedle arrays with different shapes (circular, tetragonal, hexagonal and octagonal) and tip diameter: (i) impact velocity was 1.4 m/s, (ii) impact velocity was 1.0 m/s, and (iii) impact velocity as 0.4 m/s.

Fig. 8

Piercing efficiency of polymeric microneedle arrays with varying microneedle-base diameter and length parameters. Schematic representation of the microneedle arrays (a) designs representing the conical microneedles with varying base diameter and obelisk microneedles with varying length. (b) Representative micrographs of trypan blue assays on ex vivo human skin to determine piercing efficiency for the conical (i) and obelisks (ii) arrays. (c-d) Piercing efficiency for the conical microneedle arrays with different shapes (circular, tetragonal, hexagonal and octagonal) and base diameter: (i) impact velocity of 1.4 m/s, (ii) impact velocity of 1.0 m/s, and (iii) impact velocity of 0.4 m/s. (e–f) Piercing efficiency for the obelisk microneedle arrays with different shapes (circular, tetragonal, hexagonal and octagonal) and microneedle length: (i) impact velocity of 1.4 m/s, (ii) impact velocity of 1.0 m/s, and (iii) impact velocity of 0.4 m/s.

Fig. 9

Polymeric microneedle arrays with different microneedle densities. (a) Representative stereomicroscopic fluorescence images of microneedle arrays with different densities. All microneedle arrays (5 × 5 mm) consist of conical microneedles with a circular shape (length of 500 µm, tip diameter of 3 µm, base diameter of 200 µm). Microneedle arrays were imaged from a lateral view with an 80-degree angle. (b) Trypan blue assays on ex vivo human skin to determine piercing efficiency for each density microneedle array. (c) Piercing efficiency for the different microneedle arrays applied on the skin with an impact velocity of 1.4 m/s. (d) The surface of the skin opening per microneedle after patch application. (e) The total surface of the skin opening per array after microneedle application. Scale bar = 1 mm.