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. Author manuscript; available in PMC: 2024 Dec 1.
Published in final edited form as: Anal Chim Acta. 2023 Oct 28;1283:341963. doi: 10.1016/j.aca.2023.341963

Application of surface-layer matrix-assisted laser desorption/ionization mass spectrometry imaging to pharmaceutical-loaded poly(ester urea) films

Kayla Williams-Pavlantos a, Natasha C Brigham-Stinson b, Matthew L Becker b,c,d, Chrys Wesdemiotis a,*
PMCID: PMC10657383  NIHMSID: NIHMS1943966  PMID: 37977787

Abstract

Polymer thin films are often used in transdermal patches as a method of continuous drug administration for patients with chronic illness. Understanding the drug segregation and distribution within these films is important for monitoring proper drug release over time. Surface-layer matrix-assisted laser desorption/ionization mass spectrometry imaging (SL-MALDI-MSI) is a unique analytical technique that provides an optical representation of chemical compositions that exist at the surface of polymeric materials. Solvent-free sublimation is employed for application of matrix to the sample surface, so that only molecules in direct contact with the matrix layer are detected. Here, these methodologies are utilized to visualize variations in drug concentration at both the air and substrate interface in pharmaceutical loaded polymer films.

Keywords: Mass spectrometry imaging, Surface layer analysis, Polymer thin films, Pharmaceuticals, Poly(ester urea)

Graphical Abstract

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1. Introduction

Pain management remains one of the main challenges in health care as pain is subjective and will vary with ailment. In most cases, patients are prescribed oral or intravenous (IV) opioids to manage their pain but over time excess use of these types of medications can lead to drug dependent addictions [1–3]. Topical or transdermal administration of opioids, however, have a lower risk of addiction [4]. As a result, recent studies have focused on the development of drug loaded biomaterials using natural and synthetic polymers as an alternative source of pain management [5–7]. For proper development and application of these devices, it is crucial to understand the drug environment that exists within them, particularly the environment at the surface, which is the first layer to interact with biological tissue upon appliation.

Many of the analytical techniques commonly used to study pharmaceuticals and polymers alike probe only the bulk properties of the material. In many cases, however, the bulk properties and the surface properties differ significantly. For this reason, surface specific techniques such as neutron reflectometry (NR) [8], X-ray reflectometry (XRR) [9], and X-ray photoelectron scattering (XPS) [10], have been developed to complement these bulk techniques. These techniques utilize neutron or X-ray beams to probe differences in reflectivity, kinetic energy, and electron density across the surface of a sample. From these experiments, elemental composition, surface roughness and thickness can be calculated. While each of these techniques has been shown to be advantageous for studying polymer films [8,11–14], they also suffer from disadvantages that hinder their overall application. Specifically, NR, XRR, and XPS require isotopic labeling, suffer from poor resolution, and are unable to directly detect intact molecules at the surface. While XPS can indirectly detect elements or fragment molecules from the surface this method is still limited in its scope due to the inability to detect large oligomers or macromolecules.

The above mentioned spectroscopic methods can reveal some information, but they do not provide a complete picture of the surface environment. Mass spectrometry methods, such as time of flight-secondary ion mass spectrometry (ToF-SIMS) and desorption electrospray ionization mass spectrometry (DESI-MS), offer a more robust analysis, as they can directly detect molecular fragments and small oligomers from the surface and do not require isotopic labeling [15]. However, these methods still suffer from their own disadvantages including intense noise signal and overlapping multiply charged distributions leading to complex spectra and the need for advanced deconvolution software.

On the other hand, matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) has been shown to be a valuable technique for analyzing polymeric materials [16]. MALDI is considered a soft ionization technique that, when coupled with a suitable mass analyzer, can analyze polymers and biomolecules with large molecular weights, with little to no fragmentation. This allows for the determination of polymer repeat unit masses, copolymer compositions, and polymer end groups, which is not possible with other analysis techniques such as gel permeation chromatography (GPC). These advantages have led to numerous applications of MALDI-MS for the characterization of polymers and biomolecules [17,18].

Over the years, MALDI-MS has become one of the primary methods for imaging of tissue samples due to its selectivity and sensitivity. While MALDI-MS imaging (MALDI-MSI) is most frequently utilized for investigating biological samples and tissues [19–22], the ability to analyze a large molecular weight range using MALDI-ToF instrumentation has also led to an increase in polymer imaging studies using MALDI-MSI [23–25]. Most MALDI-MSI methods utilize spin casting or spray coating for the application of matrix and cationization salt to the samples prior to analysis. This type of application, which uses solvent can disrupt both surface and bulk species, therefore altering the overall surface environment. To properly investigate surface materials using MALDI-MSI a solvent free sample preparation method must be utilized [26,27]. One way this can be achieved is by mechanical dusting of the matrix [28]. The surface specificity (top ~2 nm) of such matrix application has been demonstrated by the analysis of two bilayer films, each composed of a polymer monolayer deposited onto the surface of another polymer [28]. The first film contained two different types of polymers, viz. poly(methyl methacrylate) (PMMA) on top of polystyrene (PS); whereas the second film was comprised of very similar polymers, viz. poly(cyclohexyl methacrylate) (PCHMA) on top of PMMA. In both cases, the resulting MALDI-MS spectra only contained ions characteristic of the surface composition (i.e., the monolayer); hence, the technique was termed surface layer (SL) MALDI-MS.

Mechanical dusting, albeit fast and convenient, leads to uneven surface coverage by matrix, causing poor sensitivity and reproducibility. These issues can be remedied by utilizing sublimation to apply the matrix [29]. Sublimation techniques have been previously employed for purification [30] as well as sample preparation for MALDI-MS analysis [31]. To utilize sublimation for SL-MALDI sample preparation, the desired matrix and/or cationizing agent are deposited into the bottom of the chamber and the sample is attached to a cold finger with the surface oriented toward the matrix powder [29]. Since this process directly coats the surface of the sample without the use of solvent, there is no disruption to the surface environment. Sublimation in conjunction with MALDI-MSI has proven to be a beneficial technique for surface specific macromolecular analysis [29,32].

Sublimation has also enabled the analysis and imaging of films with different surface compositions. This was documented by the study of bilayer films prepared by spin casting PS on top of a PMMA film and subsequently removing a portion of the top PS layer by immersing it in cyclohexane [32]. After matrix and salt deposition by sublimation, the interface between washed and unwashed surface sections was correctly identified by the detection of only PS ions in the unwashed and only PMMA ions in the washed section. These results affirmed that heterogeneous surfaces can also be adequately probed and imaged by SL-MALDI-MS.

One of the limitations of SL-MALDI-MSI is linked to the laser used. Laser frequency and spot size directly impact both the analysis time as well as the spot-to-spot resolution of the obtained images. Even with these limitations, this method has successfully analyzed mechanical defects on polymer films using both large (200 μm) and small (35 μm) spatial resolution, thus establishing the robustness of this method for surface analysis [32].

This work focuses on the application of SL-MALDI-MSI to investigate pharmaceutical loaded polymer thin films. Each film was comprised of a mixture of both high molecular weight poly(ester urea) and bupivacaine (Fig. 1) at varying concentrations and was fabricated using solvent-cast blade coating. Theoretical models for polymer blends predict that lower molecular weight species are entropically favored at interfaces, specifically the air-interface [33,34]. Recent SL-MALDI-MS studies on polystyrene films prepared by spin casting confirmed this expectation [35,36] and also showed that the nonpolar constituents of a film prepared from blends preferentially segregate to the nonpolar air-surface interface [36]. Consequently, the bupivacaine drug investigated in this study should be mainly present at the air interface in the drug coated polymer thin films. Herein, we demonstrate that SL-MALDI-MSI is a valuable technique for revealing the pharmaceutical environment not only at the air-interface, but also at the substrate-interface. These results are valuable in understanding the drug profile of the films and optimizing the formulation to provide the best drug release system.

Fig. 1.

Fig. 1.

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Chemical structure and composition of bupivacaine, a widely used local anesthetic.

2. Experimental section

2.1. Materials

The chemicals, reagents, and solvents used for the polymer synthesis and SL-MALDI-MS analysis were acquired from Sigma-Aldrich (Milwaukee, WI) or Fisher Scientific (Pittsburgh, PA). The anesthetic drug bupivacaine was acquired from Santa Cruz Biotechnology Inc. (Dallas, TX). All materials were of high purity and were used in the condition received by the respective manufacturer.

2.2. Synthesis of poly(ester urea)s

A two-step interfacial polycondensation was used to create high molecular weight (MW) polymers from the di-p-toluenesulfonic monomer salts of bis(l-valine)-octane-1,8-diester (1-VAL-8) or bis(L-valine)-decane-1,10-diester (1-VAL-10). The synthesis of these monomer salts was carried out according to methods described previously [37,38]. Briefly, in a 2 L round bottom flask, 1,8-octanediol or 1,10-decanediol (1.00 eq.), l-valine (2.25 eq.), p-toluenesulfonic acid monohydrate (2.32 eq.), and toluene (1000 mL) were added and equipped with a stir bar. A Dean-Stark trap attached with a condenser was fastened to the round bottom flask and the reaction was heated to 110 °C and refluxed for 24 h. Water production from the reaction was used to monitor progress. The reaction was cooled to room temperature, and the resulting white precipitate was isolated by vacuum filtration in a Buchner funnel. The product was dissolved in boiling water, hot vacuum filtered, and cooled to room temperature, at which point the monomer crystallized out of solution. This process was repeated three times to ensure product purity via proton nuclear magnetic resonance (1H NMR) analysis. The resulting monomers are symbolized by M(1-AA-D), where M stands for monomer, AA is the amino acid (Val), and D is the length of the mid chain diol esterified by AA. The monomer salts were dried and then used for polymerization (Fig. S1).

The synthesis of poly(ester urea)s (PEUs), P(1-VAL-8) and P(1-VAL-10), by interfacial polymerization of the p-toluenesulfonic monomer salts with triphosgene was adapted from methods described previously [37–39]. Briefly, polymerization was performed by dissolving the monomers (1.0 eq. total) with sodium carbonate (3.1 eq.) in distilled water into a 5-L three-neck round-bottom flask. The solution was equipped with an overhead mechanical stir rod and allowed to stir until the solids were dissolved. The reaction was then placed in an ice bath and cooled to 0 °C. In a 500 mL round bottom flask, triphosgene (0.4 eq.) was dissolved in chloroform and subsequently added to the reaction vessel slowly. The solution turned white upon addition of the chloroform mixture and was stirred for 24 h. The product was then transferred to a separatory funnel. The reaction mixture was precipitated into hot water to remove chloroform and starting material impurities. The polymer was collected and dried under reduced pressure to remove residual water. If impurities were present in NMR analysis, additional purification steps were performed by dissolving the polymer in acetone and reprecipitating the solution into water.

2.3. Polymer characterization

Various methods were utilized to ensure material purity and understand the properties of PEUs. Size exclusion chromatography (SEC) was performed using an EcoSEC HLC-8320GPC (Tosoh Bioscience Inc., San Francisco, CA) equipped with a TSKgel GMHHR-M 7.8mm I.D. × 30 cm mixed bed column and a refractive index (RI) detector. The number average molecular mass (Mn), weight average molecular mass (Mw), and molecular mass distribution (ĐM) for each sample was calculated against a calibration curve of poly(styrene) standards (PStQuick C and D standards, Tosoh Bioscience Inc.) with THF as the eluent (0.35 mL/min at 40 °C) (Fig. S2). Thermogravimetric analysis (TGA) was performed using a TA Discovery TGA 550 (Waters, New Castle, DE) with heating ramps of 10 °C/min in the temperature range from 0 to 500 °C. The degradation temperature (Td) was determined from 10% mass loss (Fig. S3). Differential scanning calorimetry (DSC) was performed using a TA Discovery DSC 250 (Waters, New Castle, DE) with heating and cooling cycles (10 °C/min) and temperature sweeps from 0 to 100 °C. The glass transition temperature (Tg) was determined from the midpoint of the second heating cycle curve (Fig. S4). 1H NMR spectra were obtained using a 500 MHz Varian NMR spectrometer (Varian Inc., Palo Alto, CA). Chemical shifts are reported in ppm (δ) and referenced to residual solvent resonances (1H NMR DMSO-d6 2.50 ppm). Multiplicities were explained using the following abbreviations: s = singlet, d = doublet, t = triplet, br = broad singlet, and m = multiplet (Fig. S5).

2.4. Thin film preparation

Amino acid-based PEU materials P(1-VAL-8) and P(1-VAL-10), synthesized as described by the Becker Laboratory at Duke University, were used for the preparation of drug loaded thin films. These materials were dissolved at approximately 30–40% polymer in acetone (wt/wt). Bupivacaine, ranging from 10–40 wt %, was added to the polymer solution. The combined solution was mechanically stirred for 24 hours to allow the chemicals to completely dissolve. Solutions were added to the well of a doctor blade and coated onto a polyethylene terephthalate (PET) substrate. Using an EC-100 Fixed Speed Drawdown Coaster, the blade was pushed onto the substrate drawing the solution cast film onto the substrate (Fig. 2). Films were dried in ambient conditions for at least 24 h followed by attachment to vacuum or freeze-drying and lyophilization to remove any remaining solvent and to set the films in their final state prior to analysis. The average thickness of each film was 80 (±20) μm.

Fig. 2.

Fig. 2.

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Chemical structure of PEUs used in this study and film fabrication. (a) V8 and V10 abbreviate polymers P(1-VAL-8) and P(1-VAL-10), respectively. The physical properties of the polymers can be seen in the table at the top of the figure. (b) Thin films produced via a solvent cast blade coating procedure.

The PET substrate only served as an inert template on which the drug/PEU film was formed by blade casting. The films formed in this process can easily be removed from their substrate and should have similar roughness and polarity on both sides [38,39]. Further, it is worth noting that the average molecular weights of the PEU materials P(1-VAL-8) and P(1-VAL-10), cf. Fig. 2 and S2, lie well above the upper mass limit of SL-MALDI-MS (<20 kDa) [15]. Hence, only the bupivacaine content of the film surfaces is detectable and quantifiable.

2.4. Matrix sublimation

Prior to SL-MALDI-MSI analysis, the surface of each film was coated with matrix via sublimation [29]. This procedure was carried out by depositing approximately 32 (±2) mg of α-cyano-4-hydroxycinnamic acid (CHCA) matrix (99.9% pure, Sigma-Aldrich, St. Louis, MO) into the bottom of a homemade sublimation apparatus (Fig. S6) and attaching the film (1 cm × 1 cm) to the cold finger using double sided tape. The film was oriented so that its surface was facing towards the matrix. The entire chamber was evacuated to a pressure of approximately 10−1 Torr and the cooling water flowing through the cold finger was chilled to approximately 8 °C. After completion of these steps, the apparatus was submerged into a warm silicon oil bath (160 °C). This resulted in the matrix molecules subliming into the vapor phase and then depositing onto the cool surface of the film (cf. Fig. S7). Sublimation was carried out for 30 minutes before removing the apparatus from the warm oil and allowing it to return to room temperature before venting the chamber and removing the sample. This same procedure was also carried out for the substrate-interface studies; for this, the film was peeled off the substrate and the surface previously in contact with the substrate was oriented towards the matrix in the sublimation chamber.

2.5. SL-MALDI-MSI analysis

SL-MALDI-MSI data were acquired using a Bruker UltraFlex-III MALDI-ToF/ToF mass spectrometer (Bruker Daltonics, Billerica, MA) equipped with a Nd/YAG laser (λ = 355 nm; 100 Hz repetition rate). Prior to SL-MALDI-MSI, optical images of each sample were acquired with an Epson Scanner. The imaging resolution was set to correspond to the instrument’s spatial resolution as described in the FlexImaging User Manual Guidelines. The instrument was calibrated using CHCA matrix as external standard. All analyses were performed in positive reflectron mode over a m/z range of 100–1000. A consistent laser attenuation of 50% was used for all imaging runs to remove any laser fluence bias from the analysis. For each image, approximately 400 raster positions (2.5 mm × 2.5 mm) were selected, and 500 shots were collected at each raster position.

To successfully interpret the SL-MALDI-MSI data, the protonated bupivacaine ion (m/z 289.23) was selected as a mass filter to represent the presence of bupivacaine at the analyzed surface. A mass window of ±1 Da (±1 m/z unit) was included within the mass filter to probe the isotope cluster of the bupivacaine molecule. Each image was prepared by normalizing peak intensities to correct for the noise within each acquired spectrum.

2.6. Statistical analysis

For each film, statistical plots were prepared to further evaluate the bupivacaine distribution at the interfaces. Raster positions were selected at random to remove any bias from the statistical analysis. The spectrum at each raster position was then analyzed and the intensity of both the CHCA signal and bupivacaine signal were recorded. Plots were constructed by determining the ratio of bupivacaine to CHCA signal at the selected raster positions. All ratios were then averaged to determine one average ratio per film concentration. This process was repeated in triplicate for each film concentration. An R2 value greater than 0.99 indicated a strong agreement between the experimental data and the modeled trend line.

3. Results and Discussion

3.1. SL-MALDI-MSI analysis of the air-interface

A unique and key characteristic of SL-MALDI-MS analysis is that only material in direct contact with the matrix layer is desorbed and ionized, resulting in the detection of signal from only the top 2 nm of the sample [28]. An important factor in obtaining quality images by this technique is to ensure that the matrix deposition on the surface is both uniform and of the appropriate thickness. To obtain a uniform matrix coating, preliminary experiments were performed to determine the proper sublimation conditions (Table S1). CHCA was selected as the matrix for these experiments, as it has been widely used for MALDI-MSI of biological samples [40] and there was no overlap between matrix peaks and the bupivacaine peaks in the MALDI-MS spectra of these materials (Fig. 3). No cationization salt was necessary, as bupivacaine is readily protonated by matrix ions and sodiated by adventitious Na+.

Fig. 3.

Fig. 3.

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MALDI-MS spectra of CHCA matrix (bottom) and bupivacaine standard (top). No matrix peaks are observed at m/z 289.2 or 311.2, which are the ions characteristic of bupivacaine.

The sublimation time, oil bath temperature, and amount of matrix were adjusted until a visually even matrix coating was achieved (Fig. S7). Once the sublimation conditions were optimized, SL-MALDI-MSI analysis was performed on the air-exposed side of 20% bupivacaine-loaded films (air-interface). Expectedly, an intense signal corresponding to protonated bupivacaine is observed in the spectrum acquired from the surface of the film (Fig. 4a). In addition to its high intensity, the signal is relatively uniform across the entire scanned region as shown by the consistent color in Fig. 4b. This result provides clear evidence that at a 20% drug-load concentration, the bupivacaine molecules are densely and evenly distributed at the air-interface of the film.

Fig. 4.

Fig. 4.

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a) Extracted mass spectrum from a single raster position on the bupivacaine loaded P(1-VAL-10) PEU thin film. An intense peak at m/z 289.2 confirms the presence of bupivacaine at the surface of the PEU film. b) SL-MALDI Image of m/z 289.2.

To further investigate these films, SL-MALDI-MSI analysis was carried out on samples prepared with both higher and lower bupivacaine concentrations than the initial 20% films. In all cases, protonated bupivacaine signal at m/z 289.2 was observed at the air-interface; however, variations in the signal intensity and uniformity were observed at each of the respective bupivacaine concentrations. At lower load concentrations (Fig. 5a), the signal intensity and uniformity decrease. While it is expected that the observed signal would decrease with decreasing solution concentration, the non-uniformity of the signal provides additional insight into how the bupivacaine molecules arrange within the surface of the film. The images demonstrate that at lower concentrations the bupivacaine molecules spread out across the surface of the film as opposed to aggregating in one central location.

Fig. 5.

Fig. 5.

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SL-MALDI images of m/z 289.2 at the air-interface of P(1-VAL-10) polymer films prepared using a) low and b) high bupivacaine solution concentrations. The lines that are seen in images at 10–25% are attributed to either the direction of blade coating (artifact is perpendicular to the doctor blade) or from potential drag as the blade coater is getting pushed across the substrate (artifact is parallel to the doctor blade).

While the signal at the air-interface increases proportionally with the loaded drug amount at the lower bupivacaine solution concentrations shown in Fig. 5a, it begins to level off at the higher bupivacaine solution concentrations, as indicated by the very similar color profile for the 25%, 30%, and 35% films (Fig. 5b). These results strongly suggest that the air-interface of the films is saturated with bupivacaine when high solution concentrations of the drug are used. Such surface saturation could force segregation of bupivacaine molecules to other portions of the film. To further investigate this hypothesis, the substrate-interface was analyzed using the same SL-MALDI-MSI technique.

3.2. SL-MALDI-MSI analysis of the substrate-interface

According to polymer mixture theory, it is expected that bupivacaine signal would be observed at the air-interface as smaller molecules are more entropically favored to accumulate at the surface [34]. Our SL-MALDI-MSI data of the air-interface of the bupivacaine loaded films reveal, however, that entropic favorability may not be the only factor impacting drug segregation. For more information about the drug distribution in the PEU films, the substrate-interface of the samples with the highest drug concentration (25%, 30%, and 35%) plus an additional 40% bupivacaine-loaded film were analyzed by SL-MALDI-MSI to determine if the excess bupivacaine travels to the substrate-interface or if it disperses within the bulk of the polymer film.

The SL-MALDI images of the substrate-interface of the four films with high drug concentration (Fig. 6) show a gradual increase in signal as the bupivacaine solution concentration is increased. These results indicate that if the air-interface is saturated, bupivacaine molecules begin to migrate to the interface at the substrate side. Although the air-interface is more entropically favored than the substrate-interface, the small bupivacaine molecules tend to accumulate at one of the interfaces as opposed to in the bulk, as also found for other small molecules in thin films prepared from polymer blends [41].

Fig. 6.

Fig. 6.

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SL-MALDI images of m/z 289.2 at the substrate-interface of P(1-VAL-10) polymer films prepared using 25%, 30%, 35%, and 40% bupivacaine solution concentrations.

3.3. Statistical interpretation of imaging results

While sublimation is a reliable method for sample preparation, there are some sources of error that could lead to variations in the matrix coating when using homebuilt sublimators such as the one in this work [29]. These could include changes in humidity, adding slightly higher or lower amounts of matrix into the chamber, or minor variations in vacuum pressure. These changes could impact the observed results, as the interaction between the matrix coating and the surface molecules directly dictates the detection of surface species. To verify that the trends observed in the previously described SL-MALDI-MSI experiments were not influenced by variations in matrix coating, spectra from each imaging analysis were extracted to compare the signal intensity of bupivacaine molecules to matrix molecules. By using a ratio of these peak intensities, any increase or decrease of bupivacaine signal, related to the sublimation process can be accounted for independent of variations in film composition.

As shown in the air-interface plot (Fig. 7a), the ratio of bupivacaine to matrix signal increases linearly at the three lower concentrations but levels off at higher concentrations. These quantitative trends are consistent with the results observed in the SL-MALDI-MSI analysis (cf. Fig. 5). The 2D images of the three low concentration films show an increase in bupivacaine signal from 50% to 100% (Fig. 5a) matching the linear regression of the 10%, 15%, and 20% loaded bupivacaine films in Fig. 7a. In contrast, at the three higher concentrations, 25%, 30%, and 35%, the ratio plateaus (Fig. 7a) and is consistent with the three corresponding SL-MALDI images having a fairly uniform and intense signal (Fig. 5b).

Fig. 7.

Fig. 7.

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a) Triplicate plots fit to a polynomial curve, comparing the relative signal intensity of bupivacaine to CHCA in SL-MALDI-MS spectra acquired at the air-interface. b) Triplicate plots fit to a linear curve, comparing the relative signal intensity of bupivacaine to CHCA in SL-MALDI-MS spectra acquired at the substrate-interface.

The same statistical methods were utilized to interpret the SL-MALDI-MSI results of the substrate interface of the films with 25%, 30%, 35%, and 40% drug loading (Fig. 7b). A linear increase in the bupivacaine to CHCA intensity ratio is observed as the bupivacaine solution concentration rises. This is again consistent with the trends observed in the 2D images shown in Fig. 6, where the bupivacaine signal intensity increases from 30% to 100%. In both cases the statistical interpretations verify the trends observed in the SL-MALDI-MSI experiments and validate this method for studying drug segregation profiles.

4. Conclusion and outlook

The work presented here builds on previous studies utilizing SL-MALDI-MSI as a surface specific imaging technique for the characterization of macromolecular surfaces [32]. Using a solvent-free sample preparation protocol, such as sublimation, a uniform matrix coating can be applied to the sample’s surface to provide a representative image of its composition. Similar to earlier work, this SL-MALDI-MSI method allows for detection and imaging of species only present at the top monolayer of the sample. In the present study, SL-MALDI-MSI was successfully used for the detection of a pharmaceutical drug, bupivacaine, within polymer films. Expectedly, the drug was detected at the air-interface, in quantities proportional to the amount present in the blend from which the films were prepared. At high solution concentrations of the drug, surface saturation by bupivacaine molecules occurred. Detection of bupivacaine signal at the substrate-interface in the high concentration films provided strong evidence that excess bupivacaine segregates to the substrate-interface if the air-interface becomes saturated. Collectively, these results provide important information for the continued development of transdermal or implantable patches and show the unique benefits of using SL-MALDI-MSI to study such materials.

Future work will focus on studying the drug segregation process in different copolymer systems. Additionally, quantitative SL-MALDI-MSI will be explored to assess the surface concentration of the drug, using established procedures for quantitative MALDI-MSI of drugs in biological tissues (in-solution and on-film approaches) [42–44].

Supplementary Material

1

Highlights.

  • Drug-loaded films were prepared from poly(ester urea)-drug blends by casting methods

  • The air- and substrate-interfaces of the films were imaged by SL-MALDI-MSI

  • SL-MALDI-MSI probes the molecular makeup of polymeric material surfaces (top 2 nm)

  • Drug coverage at the air-interface increased with drug solution concentration

  • When the air-interface became saturated, drug segregated to the substrate-interface

Acknowledgements

The authors acknowledge support from the National Science Foundation (CHE-1808115) and the National Institutes of Health (1R44GM140795-01A1).

Declaration of interests

Chrys Wesdemiotis reports financial support was provided by National Science Foundation. Matthew L. Becker reports financial support was provided by National Institutes of Health.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Declaration of competing interest

The authors declare no known competing interests that would influence the work reported in this paper.

CRediT authorship contribution statement

Kayla Williams-Pavlantos: conceptualization, mass spectrometry methodology, investigation, writing – original draft. Natasha C. Brigham-Stinson: conceptualization, polymer synthesis, film fabrication, investigation, writing – review & editing. Matthew L. Becker: conceptualization, synthesis and fabrication supervision, writing – review & editing. Chrys Wesdemiotis: conceptualization, mass spectrometry analysis supervision, writing – review & editing, project administration.

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