Development and adhesion evaluation of transdermal rotigotine patches utilizing 3D-printed skin-mimicking substrate, solid-state NMR, and ATR-FTIR techniques

Abstract

Transdermal rotigotine patches, used to treat parkinson’s disease, often face challenges in maintaining adequate adhesion, which is crucial for effective drug delivery. Adhesion performance is influenced by environmental conditions such as humidity and temperature, as well as skin characteristics like wrinkles and micro-delaminations that vary with age and sex. Standard adhesion tests using stainless steel (SS) substrates do not accurately mimic human skin, leading to overestimated adhesion strength. This study developed rotigotine matrix transdermal formulations with silicone pressure sensitive adhesive (PSA) and evaluated their adhesion properties at 32 ± 1°C and 75 ± 5% RH in a stability chamber. Moisture uptake over 24 hours was measured using solid-state nuclear magnetic resonance (SSNMR) spectroscopy and attenuated total reflectance-fourier transform infrared- (ATR-FTIR) spectroscopy. Adhesion tests, including probe tack and peel, were performed on SS and 3D-printed acrylonitrile butadiene styrene (ABS) substrates designed with micro-delaminations and wrinkles to simulate skin conditions. In vitro permeation testing (IVPT) studies demonstrated a flux of 10.48 ± 0.61 and 10.03 ± 0.57 μg/h/cm² for formulations with and without mannitol, respectively. SSNMR and ATR-FTIR revealed significant moisture uptake, contributing to adhesion loss. Adhesion forces were significantly lower on ABS compared to SS, with further reductions observed on wrinkled and micro-delaminated surfaces, indicating that SS substrates overestimate adhesion results. This is the first study to combine SSNMR and skin-mimetic substrates for analyzing adhesion loss in transdermal patches, highlighting the potential of moisture-resistant agents like mannitol to enhance patch performance.

Graphical Abstract

1. Introduction

Transdermal drug delivery systems represent a significant advancement in pharmaceutical technology, offering a non-invasive and convenient method for administering active pharmaceutical ingredients (APIs) through the skin. These systems provide several benefits, including sustained drug release, improved patient compliance, and reduced gastrointestinal side effects. Rotigotine, a non-ergolinic dopamine agonist, is delivered via a transdermal patch to treat parkinson’s disease (PD), a progressive neurodegenerative disorder characterized by motor and non-motor symptoms that severely impact quality of life (Sudam and Suresh 2016, Dahmash, Attiany et al. 2024). The patch provides continuous, controlled medication release over 24 hours, helping maintain stable dopamine levels and manage symptoms such as rigidity, bradykinesia, and tremors. This delivery method is particularly beneficial for patients experiencing gastrointestinal disturbances that reduce the efficacy of oral medications (Oki, Toma-Okura et al. 2006, Bagde, Patel et al. 2019, Salau, Bagde et al. 2022, Bagde, Mosley-Kellum et al. 2024).

Consistent and controlled drug delivery is crucial for effective PD symptom management. Fluctuating drug levels can lead to “off” periods of symptom resurgence and “on” periods of better control (Sanford and Scott 2011). Clinical studies demonstrate that rotigotine patches not only improve motor symptoms but also enhance non-motor outcomes, including sleep quality and overall health-related quality of life (Raeder, Boura et al. 2021). However, patch adhesion can significantly influence therapeutic outcomes. Poor adhesion, caused by skin texture variations such as wrinkles and micro-delaminations, reduces drug delivery efficiency and increases the risk of unintentional drug transfer (Wokovich, Prodduturi et al. 2006, Cilurzo, Gennari et al. 2012). Factors like age, gender, and individual skin conditions further contribute to these challenges. Conventional adhesion testing on stainless steel (SS) substrates, with high surface energy, overestimates adhesion strength because it does not replicate the mechanical and topographical complexity of human skin (Bagde, Mosley-Kellum et al. 2024).

To address these limitations, this study has developed skin-mimicking probes using acrylonitrile butadiene styrene (ABS) through advanced 3D printing techniques. These ABS substrates possess a surface energy of 42 dyne/cm, closely resembling the surface energy of human skin, which ranges from 25 to 56 dyne/cm depending on its hydrolipidic balance. This design enables a more precise evaluation of patch adhesion. Previous work demonstrated the use of polypropylene substrates for evaluating silicone pressure-sensitive adhesives (PSAs) (Bagde, Mosley-Kellum et al. 2024). However, this study marks the first application of 3D-printed skin-mimetic ABS substrates to assess rotigotine patch adhesion, aiming to bridge the gap between laboratory tests and real-world conditions for more accurate, reliable predictions of patch performance.

Environmental factors, including temperature and humidity, further influence adhesion. Elevated humidity levels and temperatures increase moisture uptake, weakening adhesive bonds and reducing patch efficacy (Cilurzo, Gennari et al. 2012). Solid state nuclear magnetic resonance (SSNMR) spectroscopy provides molecular-level insights into these effects by analyzing the structure and interactions within the patch matrix (Desai, Cormier et al. 2014, Luo, Liu et al. 2021). SSNMR has been used to characterize drug-in-adhesive systems, showing how chemical penetration enhancers and ionic liquids enhance drug solubility, permeability, and stability (Li, Xu et al. 2021). Attenuated total reflectance-fourier transform infrared (FTIR-ATR) spectroscopy complements SSNMR by non-destructively examining surface chemistry and material composition. This technique monitors molecular interactions and functional group shifts, offering valuable insights into drug-excipient dynamics and the effects of penetration enhancers on skin permeability (Kazarian and Chan 2006, Jeong, Kwon et al. 2021, Li, Liu et al. 2022). Together, SSNMR and ATR-FTIR enable a comprehensive evaluation of transdermal patch performance.

By integrating advanced analytical techniques with 3D-printed skin-mimicking substrates, this study aims to improve the understanding of how environmental factors and skin conditions affect rotigotine patch adhesion. These findings offer new strategies for optimizing transdermal drug delivery systems, enhancing both therapeutic effectiveness and patient safety.

2. Materials and Methods

2.1. Materials

ABS filaments were obtained from Ultimate 3D Printing Store, FL, USA and Silicone PSA 4302 was gifted by DuPont, CA, USA. Elcometer 3580/1 casting knife film applicator (width: 50mm) was obtained from Elcometer Inc. MI, USA, 3M Scotchpak^™ 9735 backing polyester film laminate and 3M Scotchpak^™ 1022 release liner fluoropolymer coated polyester film were gifted by 3M, MN, USA, and Jet mounter JM18 electric 18-inch pressure sensitive cold mount was procured from My Binding LLC, OR, USA. Texture analyzer (TA.XTPlus) and TA-57R SS probe were purchased from Texture Technologies Corp. and Stable Micro Systems, Ltd, MA, USA. MethodX 3D Printer was obtained from Makerbot, NY, USA, ethyl acetate was obtained from Sigma-Aldrich, MO, USA. Type 304 stainless steel test plates (200mm × 50mm × 1.2mm) were obtained from ChemInstruments, OH, USA. Franz diffusion cells were obtained from PermeGear, Inc. PA, USA. Rheometer was purchased from TA instruments, DE, USA (Model: Discovery Hybrid Rheometer (DHR) 20).

2.2. Methods

2.2.1. Rotigotine matrix transdermal formulation

Rotigotine patch formulation was developed in our laboratory by dissolving rotigotine in a solution containing ethanol and PVP-K30. Further excipients, including DL-alpha tocopherol, ascorbyl palmitate, and sodium metabisulphite were added with and without mannitol in the solution. The formulation was stirred overnight at 60 RPM using a magnetic stirrer. The resultant homogeneous, translucent formulation was then coated on a release liner (9744) with a target dry coat weight of 10 ± 0.5 mg/cm². The coated release liner was then dried at 90⁰C for 30 minutes in the oven. The dried release liner was, then, kept at room temperature for 5 min before laminating with a backing membrane (9735) using a JM18 laminator. Patches of 0.79 cm² size were then cut and weighed to confirm the dry coat weight coated on the release liner (Table 1).

Table 1:

Rotigotine formulation: Components of the rotigotine matrix formulation with and without mannitol

Formulation	Rotogotine (%w/w)	PSA-4302 (%w/w)	PVP-K30 (%w/w)	DL-alpha tocoferol (%w/w)	Ascorbyl palmitate (%w/w)	Sodium metabisulfite (%w/w)	Mannitol (%w/w)
Without mannitol	4.5	87.16	8	0.07	0.02	0.25
With mannitol 0.05 % w/w	4.5	87.11	8	0.07	0.02	0.25	0.05
With mannitol 0.1 % w/w	4.5	87.06	8	0.07	0.02	0.25	0.1

2.2.2. Viscosity measurement

Viscosity of the rotigotine formulation was measured using DHR 20 rheometer (TA instruments) with a plate–plate geometry (25 mm diameter). Approximately 0.3 mL of formulation was loaded onto the lower plate of the rheometer and flow sweep measurements with 0.5mm gap, 0.1 S⁻¹ to 100 S⁻¹ shear rate were employed in obtaining the viscosity of each formulation. The study was conducted at standard room temperature (25⁰C). Data was then collected and plotted as viscosity (Pa. s) vs shear rate (s⁻¹) (Bagde, Patel et al. 2019).

2.2.3. Drug content assay

Briefly, rotigotine transdermal formulations without mannitol, with 0.05% w/w mannitol, and with 0.1% w/w mannitol were cut into patches of 0.79 cm² size. Each patch was placed in a glass vial with acetonitrile (ACN) and subjected to bath sonication for 2 hours. After sonication, ACN samples were collected and centrifuged at 14,000 RPM to remove any undissolved particles. The supernatant was then collected and injected into an HPLC system to determine the drug content.

2.2.4. In vitro release testing (IVRT)

In vitro dissolution study was conducted using USP dissolution apparatus Type II with a small-volume adapter assembly in phosphate buffer (pH 6.2) containing 5% (w/v) Brij 20. The dissolution medium (50 mL) was equilibrated to 32 ± 1°C for 45 minutes. The patches were adhered using cello tape on the pedal, with the formulation facing the release medium. The pedal rotation speed was set to 100 rpm, and samples (0.5 mL) were collected at 0.5, 1, 2, 4, and 6 hours for analysis by HPLC. After each sampling, fresh buffer (0.5 mL) was added to the flask. The samples were diluted with the mobile phase (50:50) before HPLC analysis.

2.2.5. In vitro permeation testing (IVPT)

Dermatomed human skin [0.5±0.1 mm thickness] was purchased from New York Firefighters Skin Bank (New York Presbyterian Hospital, NY, USA) and transported in 10% glycerin solution in saline and stored at −80 °C. The skin was defrosted and rinsed with distilled water for 45 min to remove excess glycerin before all experiments. Our laboratory has previously validated the methods of storage and preparation of dermatomed human skin and has certified that the skin retains its integrity for accurate permeation. Transepidermal water loss (TEWL) was measured using the Delfin VapoMeter (Delfin Technologies Ltd., Finland) to assess skin barrier integrity. The TEWL value was recorded as 10.22±1.34 g/m²/h, which falls within the acceptance criteria outlined by FDA guidelines (USFDA 2022). The receiver compartment comprised of (pH 6.2) phosphate buffer with 5 % (w/v) Brij 20 and was maintained at 32±1 °C with continuous stirring at 300 rpm. The solubility study of rotigotine was conducted in phosphate buffer (pH 6.2) with 5% w/v Brij 20. Our results showed that at least 1 mg of rotigotine was soluble in release media containing phosphate buffer (pH 6.2) with 5% w/v Brij 20. After equilibration, the skin was dried using a kimpwipe to remove any water droplets/moisture on the skin’s surface. Rotigotine Patches were then placed on the dried skin surface. Cello tape (as a patch protector) was adhered on top of the patches to avoid any direct contact of the patch with moisture. Samples from the receiving compartment were taken at 2, 4, 6, 8 and 24 hr. At each sampling time, fresh release media was replaced in the receiving compartment (Boakye, Patel et al. 2016, Boakye, Patel et al. 2017, Salau, Bagde et al. 2022, USFDA 2022).

2.2.6. HPLC detection of rotigotine

All samples were analyzed using an HPLC (e2695, Waters corporation, MA, USA). The mobile phase consisted of 55% acetonitrile (ACN) and 45% of a 1M phosphate buffer (pH 2.5 adjusted using o-phosphoric acid) at a flow rate of 1 ml/min and injection volume of 20µl. A reverse phase C₁₈ column (Symmetry^®, 5µm, 4.6 × 250mm; Waters Technology Corporation, USA) with a guard column (Symmetry^®, reversed phase, C₁₈) was used for the elution of samples. The retention time was found to be 2.14 min at 224 nm wavelength. The calibration curve (peak area vs concentration) was generated over the range of 2–100 μg/ml and was found to be linear with a correlation coefficient of 0.9998 (Swarupa, Krishna et al. 2015).

2.2.7. 3D printing of ABS probes

ABS probes were designed in AutoCAD fusion 360 modeling software (Dimensions: 7mm diameter with a 1” radius) to match the dimensions of the TA-57R stainless-steel probe. Micro-delaminated probes were designed by making elliptical impressions (4mm major axis, 2mm minor axis, and 40µm depth) on the probe surface. A standard tessellation language (STL) file of the probe was created and imported to the MethodX 3D printer. Wrinkled probes were created by projecting a triangular array onto the probe testing surface such that the apex of each triangular wrinkle faced upward. Each wrinkle had a height and width of 1.5 mm, and the distance between the apex of adjacent wrinkles was also 1.5 mm. ABS filament was loaded into the printer, and all probes were oriented horizontally in the virtual print area. The printer was calibrated, and afterward, all probes were printed at 0.2mm print resolution, 100% infill, a chamber, and an extruder temperature of 107°C and 260°C, respectively.

2.2.8. 3D printing of ABS peel adhesion plates

Peel adhesion plates were designed in Autodesk Fusion 360 modeling software (Dimensions: 200 mm * 50mm * 2.4 mm). Micro-delaminated plates (Dimensions: 60 mm * 50mm * 2.4 mm) were designed by making elliptical impressions (4mm major axis, 2mm minor axis, and 40µm depth) on the plate surface. A standard tessellation language (STL) file of the plate was created and imported to the MethodX 3D printer. Wrinkled plates (Dimensions: 200 mm * 50mm * 2.4 mm) were created by projecting a triangular array onto the plate testing surface such that the apex of each triangular wrinkle faced upward. Each wrinkle had a height and width of 1.5 mm, and the distance between the apex of adjacent wrinkles was also 1.5 mm. The regular and wrinkled plates were printed horizontally without supports with the following print parameters: 0.2mm print resolution, 100% infill density, 260°C nozzle temperature, 107°C chamber temperature at the build plane, 70°C chamber heater temperature, and 130°C build platform temperature. Further, printing parameters to print micro-delaminated plates were: 0.02mm resolution, 50% infill density, 260°C nozzle temperature, 107°C chamber temperature at the build plane, 70°C chamber heater temperature, and 130°C build platform temperature.

2.2.9. Probe tack testing

Probe tack testing was conducted using a TA.XT Plus texture analyzer. The rotigotine transdermal patches were evaluated using the TA-57R stainless steel and 3D-printed ABS skin-mimetic probes. Before tack testing, the instrument was first calibrated for force and height. Patches were then analyzed using a pre-test speed of 0.5mm/sec, a test speed of 0.5 mm/sec, a post-test speed of 1 mm/sec, the applied force of 0.005 N, a return distance of 10 mm, a contact time of 10 sec, and a trigger force of 0.003 N. The peak adhesive force (N) was obtained from Force vs Distance graphs generated from each test (Bagde, Mosley-Kellum et al. 2024).

2.2.10. 90֯ Peel adhesion test

The TA.XTPlus texture analyzer (Texture Tech Technologies, USA) was used to perform the 90° peel adhesion test. Strips of rotigotine laminates (1.2 × 5 cm) were used for analysis. The release liner was removed, and laminates were applied to the SS/PP plates. Briefly, the 90⁰-peel rig with sled (305A; Texture Technologies, MS, USA) was connected to the TA.XTPlus texture analyzer, and the test was performed with pre-test speed: 1mm/s; test speed: 5 mm/s; post-test speed: 10.0 mm/s; test mode: tension; and target distance: 100.00 mm. A graph of force (N) vs. distance (mm) was generated using Exponent software (Texture Technologies Corp. and Stable Micro Systems, Ltd., USA), displaying the mean peel force required to detach the laminates from the SS/PP plates (Sudam and Suresh 2016).

2.2.11. Environmental conditions exposure test

Briefly, human dermatomed skin was thawed for 45 min in distilled water to wash the glycerin. Pieces of skin (from the same skin donor) were then cut and mounted on the donor compartment of the Franz diffusion cell apparatus. Phosphate buffer (pH 6.2) with 5% w/v Brij 20 was filled in the receiver compartment. The setup was then kept in the stability chamber and equilibrated for 45 min at 32 ± 1°C and 75 ± 5% RH. The skin surface temperature was measured and found to be 32 °C after equilibrating the Franz cell setup in the stability chamber at 32 ± 1°C and 75 ± 5% RH. After equilibration, the skin was dried using Kimwipes to remove any water droplets/moisture on the skin surface. Rotigotine Patches were then placed on the dried skin surface. Cello tape (as a patch protector) was adhered on top of the patches to avoid any direct contact of the patch with moisture. Patch samples were collected at 0, 4,6,8,12, and 24 hr.

2.2.11.1. Water uptake evaluation by SSNMR

The rotigotine formulation was removed from the backing membrane using a spatula and collected in an Eppendorf tube for SSNMR analysis. ¹H spectra and ¹H-¹³C cross-polarization magic-angle spinning (CPMAS) spectra were acquired at 14.1 T SSNMR spectrometer operated by Bruker Avance III console by using a NHMFL’s homebuilt ¹H-X-Y triple resonance MAS probe. Each spectrum was measured by packing the formulation sample into a 3.2 mm MAS rotor spun at 12 kHz for SSNMR measurement under 90 kHz SPINAL-64 ¹H decoupling. The ¹H (70–100 % ramp) and ¹³C pulse powers applied simultaneously during a 1 ms CP mixing block were 67 kHz and 50 kHz, respectively. The calibrated 90-degree pulse durations were 2.5 ms for the 1H channel and 5 ms for the ¹³C channel (Pines, Gibby et al. 1972, Fyfe 1983, Fung, Khitrin et al. 2000, Mehring 2012, Schmidt-Rohr and Spiess 2012).

2.2.11.2. Water uptake evaluation by ATR-FTIR

FTIR spectra were acquired using a Bruker Invenio-S FTIR spectrometer (Bruker Optics LLC.) equipped with a Platinum ATR accessory and OPUS 7.5 software. Samples were directly placed at the ATR crystal surface without any additional preparation. A uniform pressure was applied to the sample using the sample device. Spectra were recorded in the mid-infrared region (4000–400 cm⁻¹) with a spectral resolution of 4 cm⁻¹, and 32 scans per sample. Background corrections were performed by measuring a blank absorbance prior samples measurement. Data were processed and analyzed using Bruker's OPUS software (Latif, Al-Harbi et al. 2022, Nawaz, Latif et al. 2023).

2.2.11.3. Probe tack testing on environmental condition exposed patches

Patch samples were collected at the end of 24-hour study, and adhesion testing was performed using the same parameters as the probe tack testing, as described in Method Section 2.2.8. Patches were analyzed using a pre-test speed of 0.5mm/sec, a test speed of 0.5 mm/sec, a post-test speed of 1 mm/sec, the applied force of 0.005 N, a return distance of 10 mm, a contact time of 10 sec, and a trigger force of 0.003 N. The peak adhesive force (N) was obtained from Force vs Distance graphs generated from each test (Bagde, Mosley-Kellum et al. 2024).

2.2.11.4. Transepidermal water loss (TEWL) measurement of skin

Trans-epidermal water loss (TEWL) was measured using the Delfin VapoMeter (Delfin Technologies Ltd., Finland). The device was carefully placed on the skin prior to the study and at its conclusion, with the corresponding TEWL values recorded for analysis.

2.2.12. Statistical Analysis

Based on at least three repetitions, the raw data findings were displayed as the mean ± standard deviation (SD). The t-test with unpaired experimental design with Welch’s correction was used for the statistical analysis of the data. Differences between groups were considered statistically significant at p < 0.05, while p = ns indicated no significant difference. Statistical analyses were conducted using GraphPad Prism 5.0 (GraphPad Software, Inc., San Diego, CA).

3. Results

3.1. Rotigotine matrix transdermal formulation and drug content

Rotigotine matrix transdermal formulations with and without mannitol were formulated successfully and analyzed for the drug content. Drug content assay showed 97.24 ± 1.42, 97.68 ± 1.34, and 96.87 ± 1.44 % of drug in the formulation without mannitol, with 0.05% w/w mannitol and 0.1% w/w mannitol, respectively.

3.2. Viscosity measurement

Our data showed viscosities of 13.15 ± 1.29, 12.92 ± 1.14, and 10.56 ± 0.60 Pa.S for rotigotine formulation with and without mannitol (0.05% w/w) and plain PSA 4302. Furthermore, a decrease in viscosity with an increase in shear rate was observed in all samples, including rotigotine formulation with and without mannitol and plain PSA 4302 (Fig. 1a).

Figure 1:

Viscosity measurement and in vitro release study of rotigotine formulations: a) Viscosity of rotigotine formulations with and without mannitol showing no significant difference in the viscosity and b) IVRT of rotigotine formulations with and without mannitol showing no significant difference in drug release on addition of mannitol in the rotigotine formulation

3.3. IVRT

Data showed 100% drug release within 1 hour from both rotigotine patches (with 0 and 0.05%w/w mannitol), with no significant differences in drug release profiles. This indicates that addition of mannitol did not affect the release of rotigotine from the patches (Fig. 1b).

3.4. IVPT

Skin permeation study showed 13.54 ± 0.01, 13.27 ± 0.42 µg/cm² drug permeation from rotigotine formulation with and without mannitol (0.05% w/w), respectively, at the end of 2h. Furthermore, results showed 161.03 ± 9.46 and 154.14 ± 8.89 µg/cm² rotigotine permeation from rotigotine formulation with and without mannitol, respectively, at the end of 24h (Fig. 2a). Rotigotine formulation with 0 and 0.05%w/w mannitol showed average flux of 10.48 ± 0.61 and 10.03 ± 0.57 µg/h/cm² at the end of 24 h IVPT study (Fig. 2b).

Figure 2:

Skin permeation study of rotigotine formulations: a) IVPT of rotigotine formulations with and without mannitol showing no significant difference in drug permeation on addition of mannitol in the rotigotine formulation and b) Flux profile of rotigotine formulations with and without mannitol

3.5. Printing probes and plates

Uniform ABS probes and peel adhesion plates were successfully printed utilizing the MethodX FDM 3D printer. All printed probes were uniform in dimension and possessed the required testing surface stipulated in the STL design files. Elliptical impressions into the testing surfaces of the ABS probes and plates were successfully made to simulate micro-delamination of the skin mimetic substrates. Furthermore, wrinkles were designed successfully on the surface of ABS with a width and height of 1.5 mm, simulating the skin's natural folds, with an apex-to-apex distance of 1.5 mm between adjacent wrinkles.

3.6. Probe Tack and peel adhesion Testing on ABS Vs SS substrate

The objective of this study was to compare the adhesion testing data using ABS vs SS substrate. Rotigotine patches with 0, 0.05, and 0.1% w/w mannitol showed no significant differences in peak adhesive force when assessed prior to the IVPT and moisture uptake studies, as the mannitol concentration was negligible (0.05 & 0.1% w/w). Therefore, only rotigotine formulation without mannitol was selected and analyzed for the adhesion testing. Probe tack test data showed 0.59 ± 0.12 and 0.11 ± 0.01 N peak adhesive force was required to unbind the patch from the SS and ABS substrates, respectively. A significant reduction of peak adhesive force was observed in the case of ABS compared to SS substrate. Additionally, peel adhesion data also showed a significant reduction of peel force in the case of ABS substrates with a 4.12 ± 0.41 N as compared to SS substrate with a peel force of 6.81± 0.58 N (Fig 3).

Figure 3:

Probe tack and peel adhesion test: a) Probe tack test of rotigotine formulation showing significant decrease in the peak adhesive force in ABS probe as compared to SS probe and b) Peel adhesion test of rotigotine formulation showing significant decrease in the peel force in ABS plate as compared to SS plate

3.7. Probe Tack and peel adhesion Testing on ABS Vs micro-delaminated and wrinkled ABS substrate

Probe tack test data showed 0.11 ± 0.01 and 0.03 ± 0.005 N peak adhesive force was required to unbind the rotigotine transdermal formulation from the normal ABS and ABS substrates with wrinkles, respectively. A significant reduction of peak adhesive force was observed in the case of ABS probe with wrinkles compared to normal ABS substrate (Fig 4). Additionally, probe tack data also showed a significant reduction of peel force in the case of ABS substrates with micro-delamination (0.05 ± 0.01 N) as compared to normal ABS substrate (0.11 ± 0.01 N) (Fig 5). Wrinkled ABS plate also showed significant (p<0.01) decrease in peel adhesion force (0.07 ± 0.01 N) as compared to normal ABS plate (4.12 ± 0.41 N) in peel adhesion testing (Fig 6). Similarly, data showed significant (p<0.01) decrease in peel adhesion force (0.05 ± 0.01 N for micro-delaminated plate as compared to normal ABS plate (4.12 ± 0.41 N) (Fig 7).

Figure 4:

Probe tack test using wrinkled substrate: a) 3D CAD model created and used to simulate micro-delamination conditions in the 3D-printed skin-mimetic substrate; b) Probe tack test of rotigotine formulation showing significant decrease in the peak adhesive force in micro-delaminated ABS probe as compared to normal probe

Figure 5:

Probe tack test using micro-delaminated substrate: a) 3D CAD model created and used to simulate wrinkle conditions in the 3D-printed skin-mimetic substrate; b) Probe tack test of rotigotine formulation showing significant decrease in the peak adhesive force in wrinkled ABS probe as compared to normal probe

Figure 6:

Peel adhesion test using wrinkled substrate: a) 3D CAD model created and used to simulate micro-delamination conditions in the 3D-printed skin-mimetic substrate; b) Peel adhesion test of rotigotine formulation showing significant decrease in the peel adhesion force in micro-delaminated ABS plate as compared to normal plate

Figure 7:

Peel adhesion test using micro-delaminated substrate: a) 3D CAD model created and used to simulate wrinkle conditions in the 3D-printed skin-mimetic substrate; b) Peel adheion test of rotigotine formulation showing significant decrease in the peel adhesion force in wrinkled ABS plate as compared to normal plate

3.8. Environmental conditions exposure test

3.8.1. Water uptake evaluation by SSNMR

Results revealed that rotigotine transdermal formulation without mannitol showed a significantly high concentration of moisture as compared to formulation with mannitol in all samples, including 2,4, 6, 8, 16 and 24 h samples. ¹H SSNMR analysis revealed a time-dependent increase in moisture uptake within the rotigotine patch upon exposure to the 75 ± 5% relative humidity (RH) and 32 ± 1°C conditions. The patch sample at the 24-hour time point exhibited the highest moisture content compared to all other time points. This was evidenced by the progressive increase in signal intensity observed at the ~4.7 ppm region of the spectra, corresponding to the presence of water molecules (H₂O). Additionally, after 24 hours of experiment, rotigotine patches developed air pockets within the patch formulation (Fig 8). On the other hand, rotigotine patches with mannitol (0.05% w/w) showed a significant reduction in moisture absorption in all samples, including 2,4, 6, 8, 16, and 24 h samples as compared to patches without mannitol (Fig 9).

Figure 8:

Environmental conditions exposure test: a) Stability chamber setup utilized to conduct permeation studies with Franz diffusion cells under controlled environmental condition and b) SSNMR data acquired by analyzing rotigotine patches after being exposed to 75% RH and 32° C temperature for 0, 4, 6, 8, 16, and 24 h

Figure 9:

SSNMR spectra comparison of moisture absorption in rotigotine patches with and without mannitol: SSNMR spectra of rotigotine formulation with and without mannitol after exposure to 75% RH and 32° C temperature showing decrease in the moisture absorption in the patches containing 0.05% mannitol at 4, 6, 8, 16, and 24 h

Furthermore, the objective of this study was also to determine whether increasing the concentration of mannitol affects moisture uptake in the rotigotine transdermal formulation. Our SSNMR results indicated that rotigotine patches with a mannitol concentration of 0.1% w/w exhibited a significant reduction in moisture absorption across all samples, including those taken at 8, 16, and 24 hours, compared to patches with a mannitol concentration of 0.05% w/w. It was observed that doubling the mannitol concentration significantly reduced moisture uptake (Fig 10).

Figure 10:

SSNMR spectra of rotigotine patches with 0.05 and 0.1% mannitol: SSNMR spectra showing a decrease in the moisture absorption in rotigotine formulation with 0.1% mannitol as compared to 0.05% after exposure to 75% RH and 32°C temperature

3.8.2. Water uptake evaluation by ATR-FTIR

¹H SSNMR results were corroborated by the findings from ATR-FTIR spectroscopy, where a progressive intensification of the broad peak exceeding 3000 cm⁻¹ was observed in the case of rotigotine patches without mannitol. This spectral signature, characterized by the broad absorption of the O-H stretching mode in water (H₂O), is indicative of hydrogen bonding among water molecules. It confirms the time-dependent moisture uptake within the patch (Fig 11). In the magnified ATR-FTIR spectra of Rotigotine formulation with mannitol, some peaks are still observed above 3000 cm⁻¹. These peaks correspond to water vapor, as indicated by the very narrow, multiple peaks associated with the rotational motions of water molecules in the gas phase. Since these peaks did not originate from water deposited on the sample surface, they were not considered moisture peaks, unlike those observed in the spectra of the rotigotine formulation without mannitol (Fig 12).

Figure 11:

ATR-FTIR spectra of rotigotine patches without mannitol: ATR-FTIR spectra showing increase in the moisture absorption in the patches at 4, 6, 8, 16, and 24 h after exposure to 75% RH and 32° C temperature

Figure 12:

ATR-FTIR spectra of rotigotine patches with 0.05% mannitol: ATR-FTIR spectra showing decrease in the moisture absorption in the patches at 4, 6, 8, 16, and 24 h after exposure to 75% RH and 32° C temperature

3.8.3. Probe tack testing on environmental condition exposed patches

The peak adhesive force for rotigotine patches with and without mannitol was found to be 0.132± 0.009 N and 0.054±0.007 N, respectively at the end of 24h study. The data showed a significant decrease in the adhesive properties of rotigotine patches without mannitol compared to those containing mannitol in 24 h study (Fig. 13a). This suggests that moisture exposure had a considerable impact on adhesion in patches without mannitol. Conversely, the presence of mannitol in the formulation provided protection against adhesion loss, enhancing the patch's overall stability in humid conditions.

Figure 13:

Adhesion testing of patches and TEWL measurement of skin: a) Probe tack test showing significant decrease in peak adhesive force in rotigotine patches with 0% as compared to 0.05%w/w mannitol and b) TEWL of skin used for rotigotine patches with 0 and 0.05%w/w mannitol showing no significant difference at 0 and 24 h.

3.8.4. TEWL measurement of skin

The data indicated that trans-epidermal water loss (TEWL) from rotigotine patches containing 0% w/w mannitol measured 11.23±1.2 g/m²/h before the study (0 h) and decreased to 9.45±0.93 g/m²/h at the end of the study (24 h). Similarly, rotigotine patches with 0.05% w/w mannitol exhibited TEWL values of 12.67±0.78 g/m²/h at 0 h and 10.19±1.46 g/m²/h at 24 h. Although a slight decrease in TEWL was observed after 24 hours of exposure to 75% relative humidity (RH), the reduction was not statistically significant (Fig. 13b).

4. Discussion

The present study explores the limitations of existing transdermal patch adhesion methods using rotigotine matrix patches currently used for Parkinson’s disease. Transdermal patches provide a convenient and non-invasive way to deliver medications through the skin (Boakye, Patel et al. 2016, Bagde, Patel et al. 2019, Kundu, Arnett et al. 2020, Bagde, Dev et al. 2023). However, their efficacy largely depends on reliable adhesion to the skin's surface. For rotigotine patches, which deliver dopamine agonists to manage Parkinson's symptoms, consistent adhesion is critical. Various skin conditions such as micro-delamination, wrinkles, and differences in roughness that vary by age, gender, and individual skin characteristics affect adhesion strength and, thus, the drug's delivery (Cilurzo, Gennari et al. 2012). Poor adhesion can lead to suboptimal dosing, reducing therapeutic effectiveness and potentially causing side effects if the patch comes loose and contacts clothing or other people, posing risks, especially if children are involved (Wong, Ang et al. 2023). Conventionally, adhesion testing for these patches uses stainless steel (SS) probes, which have high surface energy and lack similarity to human skin, leading to less predictive testing outcomes (Bagde, Mosley-Kellum et al. 2024). To address this gap, the study proposes the development of skin-mimicking probes made from acrylonitrile butadiene styrene (ABS) to provide a more accurate assessment of patch adhesion. These ABS substrates, produced through advanced 3D printing, are engineered to match the surface energy and physical properties of human skin more closely, offering a realistic alternative to SS probes in predicting patch performance on actual skin. The study’s objective goes beyond developing new substrates; it also aims to understand how environmental factors—particularly temperature and humidity—affect the adhesive properties of rotigotine patches. Given that high humidity or temperatures can weaken adhesion, this study employs sophisticated analytical techniques like SSNMR and ATR-FTIR spectroscopy. By integrating these new substrates and analytical methods, the study seeks to improve the reliability, safety, and effectiveness of rotigotine transdermal patches, ultimately benefiting patients by ensuring stable and consistent drug delivery across diverse environmental and skin conditions. To the best of our knowledge, this is the first study to investigate the adhesion properties of rotigotine transdermal patches using a 3D-printed skin-mimicking substrate combined with analytical techniques such as SSNMR and ATR-FTIR.

The surface energy of Acrylonitrile Butadiene Styrene (ABS) is 42 dyne/cm, which is noteworthy as it closely aligns with the surface energy of human skin, typically ranging from 25 to 56 dyne/cm depending on its hydrolipidic balance. This similarity is vital for accurately assessing the adhesion properties of transdermal patches. To simulate skin-like adhesion conditions for rotigotine transdermal patches, we developed 3D-printed ABS probes and plates with uniform dimensions. Our previous research has shown that these 3D-printed substrates effectively mimic both the surface energy and texture of human skin, which is essential for realistic adhesion testing (Dahmash, Attiany et al. 2024). However, to capture the full diversity of human skin surfaces, further studies are necessary.

In our future research, we plan to 3D print probes and substrates using materials with surface energies ranging from 25 to 56 dyne/cm. This will include polymers such as polyethylene (PE) with a surface energy of 31 dyne/cm, polycarbonate (PC) at 42 dyne/cm, nylon at 46 dyne/cm, and polyimide at 50 dyne/cm (Bagde, Mosley-Kellum et al. 2024). By exploring these various materials, we aim to enhance the relevance and accuracy of adhesion testing for transdermal applications.

In our study, we also incorporated micro-delaminated and wrinkled surfaces on 3D-printed ABS substrates to simulate the complex texture of human skin, particularly in aging. Micro-delamination was simulated by incorporating elliptical impressions with dimensions of 4 mm major axis, 2 mm minor axis, and 40 µm depth, effectively mimicking the natural microstructural features of aged skin, which often exhibits peeling or flaking due to reduced moisture retention and elasticity. The wrinkles were designed with a 1.5 mm width and height, reflecting the skin's natural folds and creases that significantly influence the adhesion properties of transdermal patches. Additionally, the distance between the apex points of adjacent wrinkles was also 1.5 mm. To further enhance the simulation of skin aging conditions, future improvements could include varying wrinkle and micro-delamination patterns to explore different depths and orientations, which may provide insights into how these variations affect adhesion. Wong et al. stated that while ultrathin, flexible polymer-based devices offer high flexibility and conductivity, they often fail to conform to the skin's irregularities, such as microscale wrinkles. This results in air gaps that cause adhesion loss, compromising the accuracy of biosignal measurements and health monitoring. Overcoming these adhesion issues is critical for improving device performance in biomedical applications (Wong, Deen et al. 2023). Therefore, incorporating microstructural features such as wrinkles and micro-delaminations in the skin mimetic substrates is crucial to enhance the predictive accuracy of adhesion tests, making them more representative of in vivo conditions. This approach addresses the limitations of traditional methods that often utilize rigid substrates like stainless steel, which fail to capture the complexities of human skin (Bagde, Mosley-Kellum et al. 2024). To the best of our knowledge, this is the first study to investigate the adhesion properties of transdermal patches using 3D-printed substrates designed with micro-delaminations and wrinkling features.

The drug release study demonstrated that rotigotine patches, with and without mannitol, achieved 100% drug release within 1 hour, with no significant differences in their release profiles. This finding indicates that the presence of mannitol did not influence the release of rotigotine from the patches. The rapid drug release can be attributed to the formulation strategy, where rotigotine was initially solubilized in ethanol, followed by the incorporation of PVP-K30, a well-established immediate-release polymer (Thakur, Tekko et al. 2016). Mannitol was added at a very low concentration (0.05–0.1%), with no impact on the drug’s solubility. Furthermore, mannitol was uniformly dispersed only after the complete solubilization of rotigotine in ethanol, along with PVP-K90, PSA, and other excipients, ensuring homogeneity in the formulation. In vitro permeation testing demonstrated a similar flux of rotigotine from formulations with and without mannitol, indicating that adding mannitol does not impact rotigotine permeation. For the development of the rotigotine formulation, excipients from the marketed Neupro patch were used, including amine-compatible PSA 4302, PVP-K30 (polymer), DL-alpha-tocopherol, ascorbyl palmitate (permeation enhancer), and sodium palmitate. These excipients are also present in the Neupro patch, along with ascorbyl palmitate, povidone, silicone adhesive, sodium metabisulfite, and DL-alpha-tocopherol (USFDA 2006). All excipient concentrations in this study were aligned with the US FDA Inactive Ingredient Database. Liu et al. investigated the impact of deep eutectic formation on drug-polymer miscibility and skin permeability in a rotigotine drug-in-adhesive transdermal patch. Their study focused on enhancing drug permeation by converting rotigotine into a deep eutectic “liquid co-crystal.” The results demonstrated a 75% reduction in the administration area and a minimized risk of crystallization, confirming the potential for improved drug-polymer compatibility and skin permeability. Using a solvent evaporation method, patches containing rotigotine-organic acid deep eutectics were prepared which showed skin permeation rate of 216.8 ± 14.2 μg/cm². Excipients such as lactic acid, butyric acid, and others, along with acrylic PSAs as adhesives, were also used to optimize the formulation (Liu, Qu et al. 2020). In contrast, our study utilized excipients present in the commercially available Neupro patch to maintain formulation consistency. While Liu et al. reported higher permeation rates, our optimized formulation achieved a comparable rotigotine skin permeation of 161.03 ± 9.46 μg/cm², supporting its feasibility for transdermal drug delivery with a clinically relevant approach. It is important to mention that there are significant diferences in the IVPT protocols between our study and that of Liu et al. In our study, the receiver compartment contained 5 mL of phosphate buffer (pH 6.2) with 5% (w/v) Brij 20. In contrast, Liu et al. used pH 6.5 phosphate-buffered saline (PBS, 3 mL) and stirred it at approximately 600 rpm. Furthermore, their study utilized rat skin for permeation analysis. These methodological differences, including skin type, receptor medium composition, and experimental conditions, have a substantial impact on drug permeation outcomes.

The viscosity data of rotigotine transdermal formulations, both with and without mannitol, indicated a distinct shear-thinning behavior characterized by decreased viscosity at higher shear rates. This observation aligns with the findings reported by Mendes et al., who have extensively reviewed the analytical methods and rheological properties of rotigotine formulations (Mendes, Pinto et al. 2021). Furthermore, Frampton et al. highlighted that the shear-thinning behavior is a common trait among non-Newtonian fluids, supporting the observed rheological properties of rotigotine patches (Frampton 2019). Additionally, Benitez et al. have discussed the development of rotigotine transdermal systems, noting that such formulations often exhibit shear-thinning behavior due to their complex molecular interactions and compositions. These characteristics are particularly advantageous in transdermal systems, as they facilitate ease of application and uniform drug delivery across the skin (Benitez, Edens et al. 2014).

Probe tack and peel adhesion tests revealed that ABS substrates exhibited significantly lower adhesive forces than stainless steel (SS), with even lower forces observed on micro-delaminated and wrinkled ABS, reflecting the influence of ABS microtexture on adhesion. This observation aligns with the findings reported by Cilurzo et al., who emphasized the critical role of adhesive properties in transdermal patch development and the impact of surface characteristics on adhesion performance (Cilurzo, Gennari et al. 2012). Additionally, Wong et al. highlighted the challenges posed by skin microtexture which can significantly affect the adhesive forces of medical devices (Wong, Deen et al. 2023). These insights underscore the importance of considering skin microtexture in the design and evaluation of transdermal patches to ensure optimal adhesion and therapeutic efficacy.

¹H SSNMR analysis revealed that rotigotine patches without mannitol absorbed more moisture under humid conditions than those with mannitol, resulting in air pocket formation and increased water signal intensities over time. SSNMR has been previously employed in pharmaceutical research to investigate molecular dynamics and interactions within formulations, including transdermal patches (Geppi, Mollica et al. 2008, Li, Xu et al. 2021). Our study introduces a novel application of SSNMR to analyze moisture uptake and adhesion properties in rotigotine transdermal patches, particularly under conditions that simulate real-world humidity and temperature.

While SSNMR provides significant advantages in analyzing molecular interactions and moisture dynamics, its adoption in the pharmaceutical industry may face challenges due to high costs and specialized equipment requirements. However, the potential industrial applications of SSNMR are substantial. For instance, its ability to deliver detailed insights into the structural integrity of transdermal patches can enhance adhesion testing, ensuring that products meet stringent quality standards (Sharma, Sharma et al. 2024, Song, Cai et al. 2024). This capability is crucial for optimizing formulations, improving patient compliance and therapeutic efficacy. Moreover, SSNMR can facilitate moisture testing, allowing manufacturers to assess how formulations respond to varying environmental conditions. This is particularly important in developing moisture-sensitive products, as it helps mitigate risks associated with adhesion loss and product degradation. By integrating SSNMR into routine quality control processes, pharmaceutical companies can achieve more reliable and consistent product performance, benefiting manufacturers and patients.

ATR-FTIR analysis confirmed the increased moisture uptake in patches without mannitol, as evidenced by the broad O-H absorption peak (water) above 3000 cm⁻¹. This aligns with the research by Guo et al., which characterized water adsorption in cellulose nanofiber films using FTIR spectroscopy and highlighted the role of excipients in controlling moisture content (Guo, Liu et al. 2018). Furthermore, the review by Sivadasan and Madkhali on transdermal drug delivery systems underscores the critical role of formulation components in maintaining the stability and efficacy of transdermal patches under varying environmental conditions (Sivadasan and Madkhali 2024). Therefore, including mannitol in rotigotine patches significantly improved their moisture resistance under humid conditions. FTIR spectroscopy is a valuable alternative in the pharmaceutical industry, offering critical insights into the chemical composition and moisture content of formulations (Tiernan, Byrne et al. 2020). While it may not provide the same level of molecular detail as SSNMR, FTIR is widely used for its efficiency and cost-effectiveness in routine quality control. Its ability to quickly assess moisture levels is particularly significant, as moisture can greatly impact the adhesion properties of transdermal patches. By identifying moisture content and chemical interactions, FTIR helps manufacturers optimize formulations to enhance adhesion and stability, ultimately improving product performance.

Our results indicated no significant differences in TEWL values of skin before and after the environmental condition exposure study, which may be attributed to the adherence of the cell tape as a patch protector on the skin, potentially limiting humidity absorption. However, Because of the occlusion of the skin, the patch led to increased skin hydration. This hydration may have influenced moisture migration into the rotigotine patch formulations. Our adhesion study results indicate significant reduction in adhesion for rotigotine patches without mannitol compared to those containing mannitol, demonstrating the protective effect of mannitol against moisture-induced adhesion loss. While the peak adhesive force for rotigotine patches containing 0.05% mannitol decreased at the end of the 24-hour study compared to the initial (0-hour) data, these patches still exhibited significantly higher adhesion than those without mannitol. This confirms that despite reduction over time, mannitol plays a crucial role in maintaining better adhesion under humid conditions. To further explore this effect, our future studies will investigate adhesion at varying concentrations of mannitol. However, it is reported that incorporating drugs and excipients, such as penetration enhancers and crystallization inhibitors, can alter the rheological and adhesive properties of pressure-sensitive adhesives (PSAs) (Mehdizadeh, Ghahremani et al. 2006, Cilurzo, Gennari et al. 2012). For example, isopropyl myristate improved drug permeation in silicone-based PSAs, increasing peel strength but reducing cohesion, while it decreased peel adhesion in PAMA-based matrices (Ko 1996). Similarly, oleic acid reduced adhesion in acrylic PSAs but had the opposite effect in PAMA-based systems (Lin, Lee et al. 1991).

Moisture-protecting agents are vital for improving the performance and stability of transdermal patches. In our study, the incorporation of mannitol into rotigotine patches significantly enhanced their moisture resistance and stability, particularly in humid conditions. Notably, this is the first study to demonstrate the use of mannitol as a moisture-protecting agent in transdermal rotigotine matrix patches. Supporting this, Natarajan et al explored transdermal patches formulated with cefixime, where various polymers served as effective moisture barriers, enhancing the patches' performance by reducing moisture-related degradation. Further, chitosan and sodium alginate were utilized as the primary polymers in their formulations. These polymers were chosen mostly for their ability to enhance the patches' performance by minimizing moisture-related degradation, thereby improving the stability and efficacy of the drug delivery system. The combination of chitosan and sodium alginate not only provided structural integrity but also facilitated controlled drug release, making the patches more effective for therapeutic use (Natarajan and Rajendran 2014). Additionally, Latif et al studied the development of methotrexate-loaded transdermal patches, employing a combination of hydrophobic and hydrophilic polymers to effectively manage moisture levels and optimize drug release profiles. They used hydroxypropyl methylcellulose (HPMC), known for its hydrophilic properties, which aids in controlled drug release, and Eudragit S-100, a hydrophobic polymer that provides a moisture barrier and enhanced stability of the formulation. This strategic blend of hydrophilic and hydrophobic polymers allowed for improved drug permeation and retention, making the patches more effective for therapeutic use (Latif, Nawaz et al. 2022).

Overall, this study demonstrated a novel technique for adhesion testing for rotigotine transdermal patches used in Parkinson's treatment by developing skin-like ABS substrate through 3D printing, which better mimic human skin than conventional SS substrate. Our results demonstrated that ABS substrates, particularly those featuring micro-delaminations and wrinkles, exhibited lower adhesive forces, which might better represent the adhesion behavior observed with actual human skin. Additionally, SSNMR and ATR-FTIR analyses revealed that patches without mannitol absorbed more moisture, which could impact adhesion in humid conditions. This approach aims to ensure more reliable, safe, and effective drug delivery through enhanced adhesion testing methods.

Summarily, this research underscores the potential for 3D-printed skin-mimetic substrates and advanced analytical techniques to improve in vitro adhesion testing of transdermal patches. By bridging the gap between traditional methods and real-world conditions, our findings pave the way for more patient-centered innovations in transdermal drug delivery systems.

5. Conclusion

This study demonstrated that environmental conditions and substrate surface features significantly impact the adhesion and moisture stability of rotigotine transdermal patches. By successfully developing and employing 3D-printed ABS substrate with micro-delaminations and wrinkles, we developed a more realistic skin-mimetic surface for testing adhesion. The in vitro permeation testing revealed that the addition of mannitol did not compromise drug delivery, maintaining comparable rotigotine flux between formulations with and without mannitol. Rheological analyses further confirmed the shear-thinning behavior of these formulations, facilitating consistent spreadability across different patch compositions. Adhesion testing revealed that ABS substrates, particularly those with microstructural features like wrinkles and micro-delaminations, exhibited significantly lower adhesive forces compared to SS substrates. This highlights the inadequacy of conventional SS-based adhesion testing in replicating real-world conditions. Advanced analytical methods, including SSNMR and ATR-FTIR spectroscopy, were integral to understanding the impact of environmental factors on patch performance. These techniques revealed the critical role of mannitol as a moisture-protecting agent in reducing adhesion loss and air pocket formation under high humidity conditions. The inclusion of mannitol significantly improved the moisture resistance of rotigotine patches, particularly in challenging environmental conditions. This study marks the first demonstration of mannitol as a moisture-protecting agent in rotigotine matrix patches, highlighting its potential to enhance patch performance. The findings also underscore the necessity of designing adhesion testing methods that incorporate realistic substrate properties and environmental variables to improve the reliability and therapeutic efficacy of transdermal patches. In conclusion, this research advances the development of transdermal drug delivery systems by providing novel insights into the interaction between formulation components, environmental factors, and substrate features. By integrating 3D-printed skin-like substrates with advanced analytical methods, this work sets the stage for improved design, testing, and performance of transdermal patches tailored to diverse patient needs and environmental challenges.

Highlights.

• Developed 3D-printed skin-mimetic substrates for rotigotine transdermal patch adhesion testing.

• Combined SSNMR and ATR-FTIR to analyze moisture absorption in rotigotine patches.

• Demonstrated the impact of mannitol on reducing patch moisture absorption.

• Simulated skin microtextures with wrinkles and micro-delaminations for realistic testing.

• 3D-printed skin-mimetic substrates can be used over conventional stainless-steel substrate for in vitro adhesion testing of transdermal patches

Data availability

Data will be made available on request.