Probing Skin for Metabolites and Topical Drugs with Hydrogel

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Probing Skin for Metabolites and Topical Drugs with Hydrogel Micropatches Ewelina Dutkiewicz, Hsien-Yi Chiu, and Pawel L Urban Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04276 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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Analytical Chemistry

Feature Article

Probing Skin for Metabolites and Topical Drugs with Hydrogel Micropatches

Ewelina P. Dutkiewicz,1 Hsien-Yi Chiu,2 Pawel L. Urban1,3*

1

Department of Applied Chemistry, National Chiao Tung University, 1001 University Rd., Hsinchu, 300, Taiwan

2

Department of Dermatology, National Taiwan University Hospital Hsin-Chu Branch, 25 Jingguo Rd., Hsinchu, 300, Taiwan

3

Institute of Molecular Science, National Chiao Tung University, 1001 University Rd., Hsinchu, 300, Taiwan

* Corresponding author. E-mail: [email protected]

1 ACS Paragon Plus Environment


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Sampling the skin surface is a convenient way to obtain biological specimens bearing clinically relevant information. Hydrogel micropatches enable non-invasive collection of skin excretion specimens, which can subsequently be subjected to rapid mass spectrometric analysis providing insights into the skin metabolome.

Introduction

Skin is the outer protective layer in the higher animal organisms. It has numerous functions including thermoregulation.1 Sweat is secreted by specialized glands to decrease the body temperature. Apart from water and minerals, many natural and xenobiotic compounds are released with sweat.2,3 Sebum (an oily substance) is secreted by another type of specialized glands in order to moisturize skin and protect it from dehydration.4 Furthermore, the skin surface is covered with the compounds produced during the breakdown of proteins in the epidermis (the outermost layer of skin).5 The complex mixture of chemicals present on the surface of human skin (including sweat, sebum, and protein degradation products) carries clinically relevant information. Changes in the metabolic profiles of skin excretions can be related to different physiological and pathological states of the organism. For example, sweat testing is a “gold standard” in the diagnosis of cystic fibrosis in newborn children.6,7 Moreover, recent studies suggest that skin excretion analysis can find various other applications in clinical studies, toxicology, and forensics.8 In fact, the excretion of drugs of abuse with sweat and sebum has been of interest for many years now.9 Analysis of skin excretions can be useful while investigating skin diseases10,11 as well as disorders of inner organs.12,13 The results of skin excretion analyses are complementary to those of the standard blood and urine tests. Most importantly, skin excretion


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specimens can be collected in a non-invasive manner, and they are not prone to adulteration. However, under normal conditions, only small amounts of skin excretions are present on the skin surface and these are at very low concentrations. Thus, sensitive analytical tools are needed to sample and detect metabolites or drug residues present on the skin surface. Hydrogel micropatch sampling has been developed and used in combination with mass spectrometry (MS) for analysis of metabolites in skin excretion specimens. Due to the usefulness of this analytical approach, it is interesting to review the most recent progress in its development, and consider how it could improve future diagnostic procedures.

Conventional skin sampling and analysis

One of the reasons that skin excretions have not extensively been used in clinical analysis is the lack of convenient methods for hassle-free specimen collection. For instance, for the diagnosis of cystic fibrosis, relatively large volumes of sweat (∼ 50 µL) need to be collected. In the midtwentieth century, patients were placed in plastic suit bags for 30-90 minutes while an absorbing material was applied to the patient’s back to collect sweat specimens (Figure 1).14 Overall, this proved to be a cumbersome approach.15 Nowadays it is common to collect sweat using the socalled Macroduct device.6,7 This method requires the administration of an alkaloid drug into the skin via an electric field to locally induce sweating. In another approach, a vessel (e.g., glass cylinder) filled with an organic solvent is brought into contact with skin surface to extract metabolites.16 Subsequently, the extract can be introduced into an analytical instrument. A major disadvantage of that approach is that organic solvents are not biocompatible, and they may cause skin irritation.



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Less invasive methods for collecting small amounts of skin excretions have also been proposed (Table 1). Typically, specimens are collected using a liquid-absorbing material. The most widely used collector of this type is semipermeable sweat patch developed by Sudormed in 1990 and available commercially (PharmCheck).17 Other examples include isopropyl alcohol swab,18 cotton pad19 or textile material probes.20 These materials can be used to wipe the surface of skin to collect the excretion residues. Alternatively, the collecting materials can be affixed onto skin for a longer period of time if necessary to collect relatively large amounts of sweat. Small amounts of skin excretions can also be imprinted on very thin materials such as a silica plate10 or special adhesive tape.21,22 These types of materials are affixed onto skin for very short periods of time. The so-called tape stripping is a technique commonly used in dermatology and cosmetology to investigate sebum and the upper layer of the epidermis—the stratum corneum. Other types of collectors, based on polydimethylsiloxane (PDMS) film have been introduced to capture volatile compounds emanating from skin.23,24 In all of these protocols, after the specimens were collected, metabolites had to be recovered from the collection devices via extraction (usually off-line) with a solvent. Subsequently, the extracts were subjected to chemical analysis. The off-line extraction and any other specimen treatment (e.g., analyte preconcentration) are time-consuming. These steps may also introduce systematic and random errors into the analysis results. For those reasons, on-line extraction of metabolites from the collecting device— combined with a fast and sensitive detection method—is desirable. In one outstanding demonstration, metabolites and drugs were sampled in vivo from the surface of a fingertip and directly subjected to MS detection.25 Such analysis could be very convenient and fast. However, a risk of electric shock and the need for a bulky mass spectrometer on the site of sampling have probably prevented its wide-spread implementation.


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Hydrogel micropatches

Hydrogels are a hydrophilic polymeric network composed of one or several types of monomers. It is a three-dimensional material that can “swell” and thus retain large amounts of water (typically, > 90% by weight). Hydrogels can be classified according to different criteria such as origin (natural, synthetic), polymeric composition (homopolymers, copolymers, multipolymers), configuration (non-crystalline, semi-crystalline, crystalline), type of cross-linking (chemical, physical), physical appearance (matrix, film, microsphere), or network electrical charge (nonionic, ionic, ampholytic, zwitterionic).26 Hydrogels have already found numerous biomedical applications. The features that make hydrogels attractive to biomedical research include their biocompatibility, adjustable shape, simple tuning of physicochemical properties, and low cost of production. Numerous hydrogel applications have been described in the areas of sensing, fluid control, drug delivery, fabrication of arrays, artificial muscles, and nerves.27 Importantly, hydrogels are used to prepare wound dressings.28 When applied to the wound site, the hydrogel gently attaches to the injured skin and moisturizes its surface. Nonetheless, a major shortcoming of hydrogels in medicine is their tendency to desiccate in the open air. Therefore, precautions must be taken to avoid excessive evaporation of water while handling hydrogels. Agarose is a natural homopolymer derived from red algae. Its monomer consists of Dgalactose and 3,6-anhydro-L-galactopyranose moieties.29 The molecular weights of the linear agarose molecules extend to ∼ 120 kDa. Agarose powder dissolves in boiling water, and it forms a gel while cooling to ∼ 35 °C. In this process, agarose molecules form double helical structures, which subsequently form bundles and coalesce forming a three-dimensional matrix.30 Importantly,



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agarose exhibits thermal hysteresis—its melting temperature is higher than its gelation temperature.29 Agarose is a suitable hydrogel for sampling hydrophilic skin metabolites because it is biocompatible, polar, highly hydrated, and inexpensive. It can be produced industrially in high purity. This feature is important because contaminant residues could interfere with detection by sensitive instrumental platforms such as MS. Considering the advantages of agarose, our laboratory designed a probe for skin sampling that uses agarose as a sorbent.31 In this probe, agarose hydrogel micropatches are embedded within polytetrafluoroethylene (PTFE; Figure 2a). The specimen size is fixed by the surface area (∼ 3-5 mm2) and the volume (∼ 3-5 µL) of hydrogel micropatch that has contact with the skin.11,31 Hydrogel micropatch probes are affixed onto the skin surface with medical adhesive tape (Figure 2b). We note that in most cases the PTFE support does not play any active role during collection of skin excretions. On the contrary, the carbohydrate moieties are rich in hydroxyl groups that can readily form hydrogen bonds with water molecules. In fact, the—nanometer scale—pores within the agarose gel are completely filled with water. When the probe is in contact with the skin, metabolites diffuse into the water within the pores. The water held by cohesive forces—and the water molecules weakly interacting with the carbohydrate moieties—provide a highly hydrophilic environment that readily accepts polar metabolites (e.g., those excreted with sweat). The transfer of molecules is also enhanced by the temperature gradient developed in the proximity of skin surface. Considering this mechanism, the type of aqueous phase sampling is somewhat different from the extraction of analytes into the solid-phase micro-extraction probes, where surface functionalization and chemical nature of the sorbent often play a key role.


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Mass spectrometric detection

Mass spectrometry is a universal detection platform in analytical chemistry. It provides high sensitivity and selectivity with fast turnaround times (typically, few seconds or less).32 MS combined with a separation technique such as liquid or gas chromatography has been used in several studies to detect and identify numerous skin metabolites (e.g., amino acids, carboxylic acids, fatty acids, lipids, sugars, and exogenous metabolites).33-35 In those reports, larger volumes of sweat were collected using the Macroduct device or smaller volumes were collected on special collectors and further extracted off-line. In the approach developed by our team, the hydrogelbased skin excretion sampling probe11,31,36 was combined on-line with a popular extractionionization technique called nanospray desorption electrospray ionization (nanoDESI).37 In the setup used to scan the hydrogel micropatches (Figure 2c), a solvent mixture (acetonitrile, water, and ammonia solution) is continuously delivered through a solvent capillary to an agarose micropatch. Another capillary acts as a nanoESI emitter and comes into contact with the solvent eluted from the first capillary. It then transports the solvent along with metabolites extracted from the agarose micropatch towards the orifice of the mass spectrometer.31 The surface of the PTFE support is hydrophobic, and it is not easily wetted with the solvent mixture. Thus, the liquid junction between the two capillaries can be sustained. Using the MS system described above, the chemical fingerprints of individual micropatches can be obtained in just a few seconds after loading a probe. During the typical operation, metabolites are extracted from every micropatch for 1-2 min, and the recorded time-dependent ion currents are averaged. In a recent study, we investigated the skin metabolic profiles of 100 patients suffering from psoriasis (a common skin disease) and 100 healthy individuals.11 The



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hydrogel micropatch probes were successfully affixed onto lesional (L) and non-lesional (NL) psoriatic skin areas without causing any irritation or side effects. Careful control of sampling conditions was important to assure high repeatability, and to enable result comparisons for the large groups of subjects. Special measures were always in place while collecting specimens for analysis: the subjects were adapted to the controlled sampling environment (temperature and humidity) for 15 min. In the raw mass spectra we observed numerous signals related to skin metabolites (Figure 2d-i). Further chemometric analysis (based on over 200 metabolite features) revealed major differences between the metabolic profiles of psoriatic and healthy skin.11 Importantly, we did not observe a major influence of skin temperature and transepidermal water loss on the metabolite levels, which could obscure the clinically relevant alterations to skin metabolite composition. Tiny amounts of metabolites (down to picomole level) can be picked up from the skin surface in a short period of time (1-20 min) and detected by MS. Notably, the sampling time does influence the amounts of extracted skin metabolites, and—indirectly—MS signals (Figure 3). Initially, signal intensities related to skin metabolites increase while increasing sampling time (110 min). However, during longer collection times (60 min), larger amounts of salts are also extracted causing ion suppression. The limited chemical selectivity of hydrogel micropatch sampling is not necessarily a disadvantage because—in untargeted metabolomics—it is necessary to sample and analyze many metabolites with diverse properties.38 While low-molecular-weight (typically, < 500 u) polar compounds are mainly extracted by hydrogel matrix and detected by nanoDESI-MS, less polar compounds (e.g., fatty acids) can also diffuse into hydrogel. Moreover, low-polarity species—present on skin—are preferentially adsorbed on the hydrophobic surface of the PTFE support.31 Therefore, the hydrogel micropatch probe can be used for simultaneous collection of polar and non-polar compounds for analysis. Although the probe is not specific to 8 ACS Paragon Plus Environment

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any individual metabolite, high analytical selectivity is achieved in the MS detection step when individual signals are matched with individual metabolites. This condition can be fulfilled to the greatest extent via high-resolution mass spectrometers (ion cyclotron resonance cell or orbital ion trap) scanning hydrogel micropatch probes exposed to skin.

Micropatch-arrayed pads

In one variant of the hydrogel micropatch probe, the micropatches were arranged in an array of spots within larger (28 × 28 mm) PTFE chips.36 These are referred to as micropatch-arrayed pads (MAPAs). A single MAPA probe contains 25 cavities filled with the agarose hydrogel (Figure 4a). MAPA probe is affixed onto skin similarly to standard hydrogel micropatch probes (Figure 4b) to acquire spatial distribution of a chemical (drug, metabolite). After sampling, the probe is installed on the top of a motorized xyz-positioning system, and an automated tapping-mode nanoDESI-MS scan of each micropatch can be performed. Because scanning MAPA probes is time-consuming (45 min), it was necessary to incorporate a miniature humidity chamber (Figure 4c) into the scanning device to protect the hydrogel micropatches from drying. In one embodiment, the MAPA was placed on the skin next to a nicotine patch, and dispersion of nicotine in the surrounding area was monitored over the period of several hours (Figure 4b). Lateral distribution of the drug was visualized by constructing heat maps based on normalized ion intensities (Figure 4d). Sampling was repeated several times to provide information on the spatiotemporal dynamics of nicotine.36



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Clinical considerations on metabolic profiling of skin

Modern metabolomic approaches can improve the understanding of physiological processes involved in various diseases including inborn errors of metabolism, cancer, obesity, diabetes, and cardiovascular disease.39 Most metabolomic studies are conducted using tissue, serum, plasma, or urine specimens. However, metabolomic analysis of skin excretions, such as sweat, has recently received much attention.3,11,12,31,40 In addition to the use of sweat testing in cystic fibrosis, recent studies have shown the usefulness of sweat specimens in detection of biomarkers of major depressive disorder40 and lung cancer.13 By employing the hydrogel micropatch sampling approach in combination with MS, we identified the metabolic signatures which could discriminate between the subjects with and without psoriasis, and thus fulfill the role of disease severity-associated biomarkers.11 Clinically, the use of hydrogel micropatches to collect skin excretions for metabolic profiling presents several advantages. Most importantly, the non-invasive sampling reduces the patients’ stress during blood specimen collection. The non-invasive character of clinical tests is crucial when diagnosing hemophiliacs, neonates, or elderly individuals. Blood sampling of these patients is either difficult or dangerous. Moreover, avoiding venipuncture decreases the risk of infection. While conventional protocols involving blood or urine analysis require complex specimen processing, skin excretion specimens obtained with the hydrogel micropatch probes do not require any post-collection processing. Overall, the metabolic profiling of skin excretions provided by the hydrogel micropatch sampling method could further improve pathophysiological understanding of disease. This opens new avenues to the discovery of predictive, diagnostic or


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prognostic biomarkers. Such skin biomarkers could then be used to monitor the progression of disease on a regular basis and can help predict the patient response to treatment. Metabolic profiling of skin excretions is also useful when monitoring the fates of drugs in human body.41 One recent study demonstrated the utility of sweat patches in combination with liquid chromatography coupled with tandem MS to evaluate the excretion of therapeutic drugs and metabolites in patients with attention deficit/hyperactivity disorder.42 Likewise, the MAPA method can be used to assess the spatiotemporal kinetics of topical drugs.36 The time-consuming analyte separation step is eliminated in this application. The knowledge gained from such studies can be useful when introducing new topical drugs with unknown skin surface diffusivities. One impediment on the way to the widespread implementation of skin excretion sampling and analysis is the lack of knowledge on the possible correlations between metabolite concentrations in skin excretions and the conventional clinical matrices such as blood, urine or structural tissue. Such correlations need to be carefully investigated for a large number of subjects and different physiological states (health and disease) using quantitative analytical methods. Several studies have been conducted to correlate the presence of various xenobiotics (such as drugs of abuse or pesticides) in skin excretions and other biological specimens.18,19,43 In one such study, metabolic profiles of urine and skin excretion specimens—obtained from 10 cocaine users with an isopropyl alcohol swab and sweat patch—were compared.18 In most cases, results from the tests were in a good agreement. However, in some cases, positive skin and negative urine results were recorded, what may be attributed to contamination of skin surface with drug residues. In another study, blood, urine, saliva and skin excretion specimens (collected with cotton pad) were obtained simultaneously from almost 200 subjects to test for the presence of cannabis-related compounds.19 The study revealed the presence of such compounds in the



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urine specimens obtained from 22 tested subjects. Interestingly, skin excretion specimens obtained from 16 of those 22 subjects also revealed the presence of cannabis-related compounds.

Conclusions and perspective

We believe that the share of skin-based assays in clinical practice will increase as soon as the new screening approaches are introduced and verified in the clinical setting. The hydrogel micropatch method is useful because it provides a way to collect skin excretion samples quickly and without prior sweat stimulation. Subsequent research efforts should focus on improving the metabolite/drug-trapping efficiency of the hydrogel micropatches. Certainly, apart from agarose, other hydrogels have favorable properties in this application. For example, the recently introduced technology of binding hydrogels with elastomers to prepare hydrogel hybrids that do not dry out44 may be used to prepare robust hydrogel micropatches for skin metabolite sampling. On the other hand, aerogels45 can possibly be used to trap volatile compounds released by skin. Nonetheless, biocompatibility and compatibility with MS detection are the prerequisites for using these new sorbents as a replacement for agarose. However, clinical approval of new biomaterials may be difficult and expensive; therefore, repurposing well-known biomaterials is suggested.46 For many clinical applications, it is essential to obtain quantitative or semi-quantitative results. In fact, limited quantitative capabilities of the hydrogel micropatch sampling combined with MS have been demonstrated in the previous work.31,36 Nevertheless, further improvements in quantification of skin metabolites are necessary. For example, a big challenge in skin excretion analysis is to compensate for interpersonal differences in excretion rates. This challenge can be addressed by incorporating normalization steps into the analytical workflow. Such a


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normalization is critical when conducting absolute quantitative analysis of skin metabolites. We are certain that with further development the hydrogel micropatch methodology will find applications in other industries beyond clinical analysis including the development/testing of cosmetics and forensic science.

Biographies Ewelina P. Dutkiewicz obtained her PhD in chemistry from the Department of Applied Chemistry at the National Chiao Tung University. She works on analysis of skin metabolites by means of hydrogel micropatches combined with mass spectrometry. Hsien-Yi Chiu is an attending physician of dermatology department and research scientist at the National Taiwan University Hospital, Hsin-Chu Branch. He has contributed significantly to a number of important studies on psoriasis. Pawel L. Urban obtained his MSc degree in biology from the University of Warsaw and PhD in chemistry from the University of York. Following a postdoctoral stay in the ETH Zurich, he established his research laboratory dedicated to biochemical analysis in the National Chiao Tung University.

Acknowledgements We thank Mr Gurpur Rakesh D. Prabhu for help with graphic design. We also acknowledge the National Chiao Tung University and the Ministry of Science and Technology, Taiwan (MOST 104-2628-M-009-003-MY4) for the financial support.



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(42) Marchei, E.; Papaseit, E.; Garcia-Algar, O.; Bilbao, A.; Farré, M.; Pacifici, R.; Pichini, S. Drug Test. Anal. 2013, 5, 191-195. (43) Genuis, S. J.; Lane, K.; Birkholz, D. Biomed. Res. Int. 2016, 2016, 1624643. (44) Yuk, H.; Zhang, T.; Parada, G. A.; Liu, X.; Zhao, X. Nat. Commun. 2016, 7, 12028. (45) Aegerter, M. A.; Leventis, N.; Koebel, M. A. (eds.) Aerogels Handbook; Springer, New York, 2011. (46) Peppas, N. A.; Khademhosseini, A. Nature 2016, 540, 335-336. (47) Cone, E. J.; Hillsgrove, M. J.; Jenkins, A. J.; Keenan, R. M.; Darwin, W. D. J. Anal. Toxicol. 1994, 18, 298-305. (48) Muramoto, S.; Forbes, T. P.; Van Asten, A. C.; Gillen, G. Anal. Chem. 2015, 87, 5444-5450. (49) Minematsu, T.; Horii, M.; Oe, M.; Sugama, J.; Mugita, Y.; Huang, L.; Nakagami, G.; Sanada, H. Adv. Skin Wound Care 2014, 27, 272-279. (50) Peng, R.; Sonner, Z.; Hauke, A.; Wilder, E.; Kasting, J.; Gaillard, T.; Swaille, D.; Sherman, F.; Mao, X.; Hagen, J.; Murdock, R.; Heikenfeld, J. Lab Chip 2016, 16, 4415-4423.



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Figure captions

Figure 1. Mode of attachment of filter papers and plastic strips to child (a), and plastic bag in position (b). Reproduced from Archives of Disease in Childhood (B. W. Webb, P. T. Flute, M. J. H. Smith, 32, 82-84, copyright 1957)14 with permission from BMJ Publishing Group Ltd.

Figure 2. Hydrogel micropatch probe for skin metabolite analysis: a) layout; b) skin sampling cross-sectional view; c) nanoDESI-MS setup used to screen the probes; d-i) mass spectra revealing substantial differences in metabolic profiles observed in the lesional (L) and nonlesional (NL) skin of psoriatic patients and healthy controls. Mass spectra recorded in positive (+) and negative (-) ion modes. Reprinted with permission from the American Association for Clinical Chemistry, 2016.11

Figure 3. Influence of sweat sampling time on MS peak intensity. Skin metabolite-related MS features are highlighted. This figure is based on the data collected in the previous study.31

Figure 4. Micropatch-arrayed pads: a) MAPA probe (the scale bar corresponds to 10 mm); b) sampling nicotine in the proximity of a nicotine patch; c) humidity chamber for MAPA; d) two ion maps (nicotine, m/z 163) constructed based on a MS scan of MAPA following sampling of the skin next to the nicotine patch at different time points. Adapted with permission from the Wiley, 2015.36


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Table 1. Examples of available techniques for skin metabolite sampling. Technique/probe*

Invasiveness

Sampling time

Macroduct6,34

invasive

5 + 30 min 5 + 15 min

Solvent extraction16

invasive

∼ 2 min

Semipermeable skin patch47

moderately invasive

0-7 days

Tape stripping22

moderately invasive

∼ 1 min

Isopropyl alcohol swab18

moderately invasive

∼ 1 min

Cotton pad19

little or noninvasive

∼ 1 min

Textile20


n.d.

Silica plate10

little or noninvasive little or noninvasive

PDMS film23,24

Sample preparation dilution; dilution or hydrolysis, evaporation, dilution or SPE, evaporation, dilution preconcentration by evaporation solvent extraction, filtration, dilution, SPE, evaporation, derivatization

Analytes

Amounts detected

chloride ion untargeted study

7-136 mmol L-1 n.d.

VOCs

n.d.

Pros

Cons

- several tens of microliters of sweat can be collected

- use of drug (pilocarpine) and electricity to induce sweating

- collection and extraction at the same time

- skin irritation

- enables collection of larger bulk amounts of specimens collected over long periods of time - extended time of monitoring

- worn up to a fortnight - skin irritation, contamination and decomposition of analytes during the prolonged sampling - extraction of metabolites from the probe required

heroin, cocaine, and their metabolites

up to 441 ng

proteins, peptides, amino acids

n.d.

- fast sampling

- low specimen load - skin irritation after repeated sampling - extraction of metabolites from the probe required

cocaine, benzoylecgonine

up to 22 μg

- fast sampling

- extraction of metabolites from the probe required

THC

up to 152 ng

- fast sampling


methamphetamine and its metabolites

∼ 1-6000 ng

- fast sampling


solvent extraction

lipids

n.d.

- fast sampling

20 min 30 min

none

VOCs

n.d.

- trapping volatile analytes

cocaine, methamphetamine, and heroin

1-50 ng

1 min

washing, solvent extraction at elevated T, precipitation, (hydrolysis, derivatization) solvent extraction, SPE, evaporation, derivatization solvent extraction, evaporation derivatization solvent extraction, evaporation, derivatization

Nanostructured silicon48


< 1 min

printing drug molecules on the surface of a fingerprint

Skin blotting with nitrocellulose membrane49


1-10 min

immunostaining

proteins

0.1-10 µg µL-1

Oil/micro-porous membrane50


2-8 min

none

n.d.

n.d.

Hydrogel micropatch probe11,31


1-180 min 20 min

none

72 ng µL-1 (threonine)

Micropatch-arrayed pads36


10 min

none

polar low-molecular weight metabolites nicotine, scopolamine, and polar low-molecular weight metabolites

down to 2.1 ng (nicotine)

- low specimen load - extraction of metabolites - possible decomposition of labile metabolites during thermal desorption

- fast sampling - spatially resolved quantification

- long preparation of wafers

- investigation of protein distribution

- antibody-based technique

- nL volumes of sweat can be collected - separation of sweat gland excretions from other skin excretions - fast sampling - on-line extraction - fast sampling - on-line extraction - imaging drug distribution on skin

- possible diffusion of lipophilic analytes into oil - drying of hydrogel - long scan - drying of hydrogel

* Only representative references are cited. n.d. – no data, SPE – solid-phase extraction, T – temperature, VOCs – volatile organic compounds, THC – ∆9-tetrahydrocannabinol



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Figure 1

a)

b)

ACS Paragon Plus Environment

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Figure 2



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Figure 3


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Figure 4



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Graphical abstract


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Cover art


Probing Skin for Metabolites and Topical Drugs with Hydrogel

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