The Protective Effect of a New Absorbent Incontinence Design Against an Alkaline pH Challenge on the Epidermal Barrier

Incontinence—the involuntary leakage of urine, feces, or both—is age-associated and is a result of multiple underlying diseases and pathologies. Incontinence poses considerable problems for physical, social, and psychological health and results in impaired quality of life.^1,2 According to studies published in the past 10 years, 48% to 83.7% of nursing home residents experience incontinence, with 40.0% to 66.8% experiencing urinary incontinence, 26.4% to 38.1% fecal incontinence, and 23.2% to 36.6% both urinary and fecal incontinence.^3,4 Chronic or repeated exposure of the perineal or perigenital skin to moisture from urine, enzymes from stool, or both can result in incontinence-associated dermatitis (IAD).

Clinically, IAD presents as cutaneous inflammation, sometimes with blisters and superficial erosions, and occasionally with secondary infection.^5,6 Incontinence-associated dermatitis is reported to affect 5% to 50% of patients with incontinence in the acute care setting.⁵ Incontinence-associated dermatitis needs to be differentiated from sacral pressure injuries, which share several clinical characteristics and can be difficult to distinguish clinically.^5,7,8 While ischemia is thought to cause tissue necrosis and underlie pressure injuries, IAD follows an outside-in mechanism, with initial disruption of the epidermal barrier and subsequent inflammation and superficial skin breakdown.

Prolonged contact of the skin with moisture results in stratum corneum damage.⁹ The excessive moisture of the skin surface leads to keratinocyte hyperhydration and lipid layer disruption, resulting in impairment of mechanical integrity, biochemical composition, and barrier function of the stratum corneum.^10-12 The barrier function of the skin can be further impaired by an alkaline environment relative to the normally occurring acidic surface pH. The pH of the so-called acid mantle of the stratum corneum ranges from 4 to 6 and is generated by several constituents, such as fatty acids from sebum and stratum corneum lipids, amino acids in sweat, and the release of metabolites of filaggrin breakdown.^13-15 The acidic pH of the skin surface helps prevent colonization by pathogenic bacteria.¹⁶

With aging, the stratum corneum becomes less acidic¹⁷ due to reduced sebum production and epidermal lipid synthesis.¹⁸ Occlusion and moisture lead to profound changes, such as increased skin pH, increased bacterial density, and impaired epidermal barrier function, which are reflected by increased transepidermal water loss (TEWL) readings.¹⁹

In addition to being managed with physical, pharmacological, and surgical therapies, incontinence is managed with absorbent incontinence products. The use of these products and the risk of leakage severely affect the lifestyle of affected people.^20,21 In addition, IAD is a long-term complication of incontinence that further compromises well-being. The use of skin care regimens has been found to be effective in reducing the prevalence of IAD.²²

The introduction of superabsorbent polyacrylate polymers in baby diapers has been found to significantly reduce diaper dermatitis in babies,²³ and this technology was also introduced to incontinence products for adults. Additional design improvements and the use of citrate-modified cellulose enabled the creation of incontinence diapers that maintained an acidic environment toward the skin for several hours and after 3 fluid challenges with synthetic urine to mimic multiple micturition.^24,25

The current study reports on further improvements in the so-called dry feel of the diaper surface, which is known as rewetting, while maintaining the surface pH of the diaper at 5.5. The new design was then tested to determine if it could prevent skin irritation as assessed by erythema and TEWL readings in 75 healthy volunteers when the skin of the volar forearm was challenged with an alkaline synthetic urine solution (pH 10.7) for 6 hours.

Subjects. This was an exploratory, monocentric study with a 1-time application, in which intraindividual comparisons (before and after treatment) were made in 75 female and male volunteers. The volunteer demographics are listed in Table 1. Subjects were informed about the study and their rights. Before trial participation, written informed consent was obtained from all subjects. To participate in the study, volunteers had to be at least 18 years of age, with uniform skin color and no erythema or dark pigmentation in the test area. Individuals were excluded if they had severe or chronic skin inflammation, severe allergies or any serious side effects to cosmetics, active skin disease, wounds, moles, tattoos, scars, irritated skin, or excessive hair growth at the test area that could influence the erythema quantification or TEWL measurements. Topical medication on the test area was prohibited 3 days before the start of the study and/or throughout the study. In addition, females who were pregnant or breastfeeding and individuals who were participating in other studies were excluded from this study.

The current study followed a previously published article by Larner et al,²⁶ with slight modifications. The composition of each solution used in this study is listed in Table 2. Control solutions (0.9% physiological saline solution [NaCl] on Finn chambers (SmartPractice; patch test system) [negative control 1]; synthetic urine solution on cellulose pads [negative control 2]; 2% sodium dodecyl sulfate [SDS] solution on Finn chambers [positive control]) as well as alkaline synthetic urine on cellulose pads and alkaline synthetic urine on new diaper design pads were placed on the volar aspect of both forearms under occlusion for 6 hours, for a total of 5 tests sites per forearm per patient. The cellulose and diaper pads (cut from the absorbent core of the diaper) had a diameter of 1.8 cm and were placed in a distance holder ring, while the pads were kept in place with minimal pressure from the occlusive dressing. After exposure, the skin was gently patted dry and left to recover for 1 hour, after which TEWL was measured using a Tewameter TM 300 device (Courage + Khazaka electronic GmbH) and skin erythema was measured using a handheld spectrophotometer (CM-700d; Konica Minolta Sensing) in each exposed field. Instrumental measurements were carried out before and after the application of patches on the same spots of the volar forearm in an air-conditioned room at a mean (standard deviation [SD]) temperature of 22 (2) °C and mean relative humidity of 50% (7.5%). The ∆TEWL and ∆erythema score measurements for each condition and measurement were calculated as follows:

∆TEWL = TEWL reading after exposure – TEWL reading at baseline,

                        ∆erythema score = erythema score reading after exposure – erythema score reading at baseline.

Ethical compliance. All study procedures were conducted in accordance with the principles of the Declaration of Helsinki and were approved by the Ethics Committee of the Landesärztekammer Baden-Württemberg, Stuttgart, Germany (F-2021-145). Patients provided written informed consent before inclusion. This trial was registered retrospectively (registration number: DRKS-ID: DRKS00031504; date of registration: March 16, 2023).

Development of a dry-feel and skin-friendly incontinence diaper design. By modifying the design features and materials of the absorbent core, the authors of the current study achieved an incontinence diaper that promotes dryness of skin and maintains acidic pH at the surface of the diaper, resulting in protection of skin from damage (Figure 1). A key element of the new design is the acquisition distribution layer (ADL) (11040WC0A, Airten CP PHIL, Fiberweb Terno d’Isola S.r.l, Berry Global Inc). This nonwoven material is placed underneath the top sheet. After liquid enters the top sheet of the diaper, the ADL distributes the liquid to a larger area and facilitates the penetration of the liquid into the absorbent core below. The ADL consists of coarse polyester fibers forming relatively large cavities able to effectively distribute larger fluid volumes, even under pressure. The physical characteristics of the ADL are listed in Table 3. The superabsorbent polymer (SAP) particles dispersed in the cellulose fluff lock the moisture inside the polymer and prevent the secondary distribution of the liquid to the diaper surface. Superabsorbent polymer can absorb 10 to 100 times its weight in water and 20 to 50 times its weight in salt-containing fluids such as urine.^{24, 27} The inclusion of an additional cellulose layer below the absorbent core helps to utilize the full amount of SAP inside the diaper. Monosodium citrate is dispersed inside the product. Another key design feature is a rectangular-shaped channel impression at the most likely urine entry point of the diaper. This feature further accelerates large-volume moisture distribution and absorption into the diaper. Figure 1 illustrates the 3-dimensional diaper design.

Characterization of the ADL. The strike-through test was performed according to Nonwovens Standard Procedures (NWSP) 070.3 (Edana). A 125-mm × 125-mm ADL was placed on top of 3 to 4 layers of blotting paper, and 5 mL of 0.9% NaCl solution was applied through a funnel. The time (in seconds) to complete penetration of the liquid through the ADL was recorded.

The runoff test (NWSP 080.9) was performed by delivering a mean (SD) of 25 (0.5) g of 0.9% NaCl in a continuous stream via a glass tube within a mean of 4 (0.1) seconds onto a mean 140 (2)-mm × 280 (2)-mm ADL placed on 1 layer of blotting paper (absorbent medium) on a surface with a 25° incline. Two layers of blotting paper (receiver pad) were placed at the lower end of the inclined surface to collect the runoff liquid from the ADL. The mass of liquid collected by the receiver pad is reported as runoff.

Liquid absorptive capacity was measured according to NWSP 10.1. Five ADLs (mean 100 [1] mm × 100 [1] mm) were weighed and fastened on stainless steel mesh. The mesh with the attached ADLs was immersed in the liquid for a mean of 60 (1) seconds. After removal from the liquid and draining of the unabsorbed liquid for 120 (3) seconds, the soaked ADLs were weighed. The absorbed liquid was calculated as follows:

            (weight_wet – weight_dry)/weight_dry × 100

and is reported as a percentage of dry weight.

The liquid wicking rate (NWSP 10.1) was measured by recording the height of capillary rise of the liquid after 10 seconds, 30 seconds, and 60 seconds within the ADL samples (mean, 30 [1] mm wide × 250 [1] mm long in both directions [machine direction and cross direction]).

Stiffness was measured according to ASTM D4032 (ASTM International). A 3.75-mm × 3.75-mm sample was placed on a rectangular surface centered on a hole (diameter, 17.75 mm), and the force (N) was measured to compress the ADL sample by 10 mm. The data are reported as force in newtons.

Rewet and fluid acquisition time determination. For the rewetting test, a rectangular weight (mean [SD] 3500 [70] g) with a central hole a mean of 22 (0.2) mm in diameter was placed on the central crotch area of the product (mean absorption capacity, 1428 [59] g, according to ISO 11948-1 method that measures maximum absorption capacity of the absorbent incontinence products), and 70 mL of 0.9% NaCl was delivered through the hole. Using a stopwatch, time was measured until all liquid was absorbed into the diaper (fluid acquisition time). After 30 minutes, the weight was removed, and filter paper sheets weighing 80 g were placed on top of the fluid-exposed part of the diapers. The weight was placed at the same location for 1 minute. The soaked filter papers were removed and weighed. Rewet was calculated by calculating the difference of the wet minus the dry filter paper based on the following formula: Rewet (g) = W_wet – W_dry. The rewet value is reported in grams.

The procedure was repeated immediately to obtain the second fluid acquisition time and rewet value. The weight was placed at the same spot as in the first test, and the diaper was loaded with another 70 mL of 0.9% NaCl. Both tests were conducted on 30 different samples, and the mean values are reported.

pH determination of the new incontinence diaper over time. The pH on the surface of the diaper was further measured using a pH meter (CG 841; Schott) combined with a glass electrode (Mettler Toledo). The instrument was calibrated with standard solutions before usage. The diapers, which have a mean (SD) fluid absorption capacity of 1428 (59) g according to ISO 11948-1, were loaded with 180 mL of a synthetic urine solution within 30 seconds through a cylindrical hole (40-mm diameter). The surface pH was determined after 5, 10, 15, and 20 minutes at room temperature at the same measuring spot. After 20 minutes, pH measurements were impossible because the surface of the absorbent products was completely dry. After 2 hours, the incontinence products were loaded with another 70 mL aliquot of synthetic urine at the same spot as the initial loading. The pH measurements were repeated. After 5 hours, the incontinence products were loaded a third time, this time with 40 mL of synthetic urine solution, after which pH measurements were obtained as described previously. The experiments were conducted in triplicate. For visualization of the pH gradient inside the incontinence diaper, 0.9% NaCl solution (a pH indicator solution) was added (2 drops/5 mL solution [Unisol 410; Macherey-Nagel]). After 20 minutes, the diaper was cut and photographed. Distinct colors illustrated different pH in various parts of the diaper.

Data analysis. All statistical analyses for the human alkaline synthetic urine challenge study were performed using R software (version 4.1.3; The R Project for Statistical Computing). For continuous variables, descriptive statistics including frequency, mean, SD, median, minimum, and maximum values were calculated. For categorical variables, the frequency and percentage of each category were calculated. To compare the results of the efficacy assessment between skin exposed to the alkaline synthetic urine on the new diaper pads vs the cellulose pads, a paired t test was applied in the case of normal distribution of the data, and the Wilcoxon signed rank test was used in the case of nonnormally distributed data. Continuous variables were tested for normality using the Shapiro-Wilk test. In addition, 1-way analysis of variance for repeated measures with post hoc pairwise comparisons using paired t tests with Bonferroni correction was applied to assess the differences between each pair out of the 5 fields. All inferential analyses were 2-sided, and the statistical significance level was set at .05.

Rewet and fluid acquisition time measurement. Thirty minutes after the application of 70 mL of 0.9% NaCl solution, the mean (SD) rewet value was 1.2 (0.2) mg/cm² (Table 4). After 31 minutes, a second 70 mL aliquot of 0.9% NaCl solution was applied at the same spot and the rewet measurements were repeated. The mean rewet value increased to 2.9 (1.7) mg/cm². The acquisition time measurement after application of 70 mL of 0.9% NaCl solution on a 3.8-cm² surface spot of the diaper was a mean of 44 (7) seconds for complete absorption. When repeated 31 minutes later at the same spot, the mean absorption time of another 70 mL of 0.9% NaCl solution was 27 (5) seconds.

pH surface measurements of the new diaper design. Without citrate modification, the surface pH of a diaper containing polyacrylate SAP is reportedly between 6 and 7.²⁴ In the current study, the introduction of monosodium citrate in an exemplary product changed the surface pH measurement to 5.2 (0.1) (Figure 2A). Even after repeated fluid loading, which mimicked a typical urination pattern, the new design maintained an acidic surface pH. After the initial synthetic urine solution application (180 mL), the mean surface pH was rapidly reduced to 4.5 (0.0), remaining almost stable within the next 20 minutes. After 2 hours, loading the same incontinence product at the same spot with 70 mL of the synthetic urine resulted in a mean pH reading of 5.6 (0.1), which remained stable at a mean of 5.5 (0.1) within the next 20 minutes. After 5 hours, the final loading (40 mL) showed a mean surface pH of 5.6 (0.1) over the 20-minute measurement time (Figure 2A). Longer measurement times were impossible because the surface became dry. Absorption of pH indicator solution containing 0.9% NaCl showed the emergence of an internal pH gradient within the parts of the diaper (Figure 2B).

TEWL and erythema measurements. To explore the effect of the monosodium citrate-containing diaper design on protecting the epidermal barrier of the skin against an alkaline pH challenge, the volar forearm skin of 75 healthy probands was exposed for 6 hours to an alkaline urine solution absorbed by the new diaper design absorbent core vs that absorbed by a cellulose pad control. The TEWL and erythema scores were measured to quantify the corrosive damage of the alkaline urine solution to the epidermal barrier. The 4 conditions comprised 0.9% NaCl (negative control 1), synthetic urine (pH 7.9 [negative control 2]), 2% SDS (positive control), and alkaline synthetic urine solution absorbed to cellulose patches (without SAP and monosodium citrate). The ΔTEWL values are reported for each condition and measurement. Skin exposed to 0.9% NaCl showed a mean 4.17 (4.30) g/m²/h increase in TEWL readings. For the synthetic urine solution, the mean increase in TEWL was 3.33 (3.06) g/m²/h. In the positive control group (2% SDS), the mean increase in TEWL was 14.86 (11.44) g/m²/h. The alkaline synthetic urine replacement solution absorbed into the plain cellulose patch resulted in a mean increase in TEWL of 8.38 (5.67) g/m²/h, while the same solution applied to the absorbent core of the new diaper design resulted in a mean increase in TEWL of only 3.43 (4.67) g/m²/h (Figure 3).

The spectrophotometry erythema measurements showed that 0.9% NaCl caused no skin redness (mean [SD] ΔErythema = 0 [0.72]), synthetic urine caused a mean minimum erythema score increase to 0.24 (0.92), and the positive control (2% SDS) led to a significant mean erythema increase of 2.29 (1.59). The highest mean erythema value, 2.56 (1.24) g/m²/h, was for the alkaline synthetic urine solution. The erythema readings of the alkaline synthetic urine loaded into the new diaper pad showed a decrease in the mean erythema value to 1.18 (1.30) g/m²/h (Figure 4).

Epidermal barrier function is multifaceted and depends on the complex interplay between several physiological processes. The acidic skin pH further protects and regulates the integrity and cohesion of the stratum corneum.^13,19An acidic pH is created by amino acids, fatty acids, and metabolites of filaggrin breakdown.¹³ With age, the pH of the skin surface tends to increase, making the skin more susceptible to external attacks. Moisture and alkaline skin pH are considered to be the most critical risk factors for the development of IAD.²⁸ Moisture from occlusion increases the vulnerability of the skin to friction and overhydration of the stratum corneum and leads to an increase in skin surface pH and impaired epidermal barrier function.¹⁹

In baby diaper dermatitis, prolonged urine exposure and the decomposition of urea by bacteria can generate ammonium, increasing the skin surface pH.^16,29 Contact with fecal matter and the remaining digestive enzymatic activity in stool, as well as other stool constituents, can further impair the protective function of the epidermal barrier. With cumulative damage, the skin becomes inflamed, and this condition is clinically referred to as IAD.

There is consensus that in individuals with incontinence, the skin should be kept as dry as possible and should be protected from corrosive factors such as alkaline pH and digestive enzymes on the surface.³⁰ Thus, preventive measures for IAD include avoidance of excessive skin moisture through skin care regimens and absorbent incontinence products with as little rewetting as possible.⁴

One engineering and design breakthrough was the introduction of polyacrylate-based SAPs in diaper designs to reduce skin wetness and diaper rashes.^31,32 Moreover, the fast distribution and guidance of urine to deeper layers of a diaper can help minimize skin exposure to direct wetness. The quick absorption and distribution of liquid is a feature of the ADL and is quite dependent on the performance characteristics of the materials used. Ideally, an ADL is soft, yet still retains the porosity of the inner structure and guides and distributes liquids. These technical features result in a perception of dryness on the surface. The fluid absorption into the polyacrylate-based SAP particles is much slower and takes several minutes. The current study reports the key characteristics of the ADL in detail, and these features may have had some effect on the protective properties of the diaper design in the clinical provocation test. The basic design of a diaper can be improved by technologies that provide an acidic surface pH adapted to the physiology of the epidermal barrier.²⁴Two of the authors of the current study (RK, HS) previously reported the use of a citrate-modified cellulose type curled fiber that was interposed between the outermost top sheet and the absorbent core.²⁴ This resulted in a stable pH gradient over 5 hours within the diaper when loaded 3 times with 0.9% NaCl. However, this specialty cellulose significantly increases cost and is a challenge to introduce in the manufacturing process, and it has not gained widespread application.

In the current study, when the normal skin of healthy probands was exposed to the new diaper design loaded with an alkaline urine solution, a clear protective effect was observed concerning TEWL and skin erythema development. The pH challenge model was adapted from Larner et al.²⁶ Although that was originally a cumulative irritation model for IAD, in the current study the skin was exposed to the pH challenge only once, after which TEWL and erythema scores were measured. In Larner et al,²⁶ multiple exposures did not lead to a gradual increase in TEWL readings with exposure; rather, measurement readings returned to baseline levels during the recovery period between exposures. Erythema scores increased during the 5 exposures, but whether this increase was significant was not reported. In the current study, a single exposure was sufficient to demonstrate that the corrosive effects of the alkaline synthetic urine solution were ameliorated and that TEWL readings were indistinguishable from those produced by the non-alkaline synthetic urine solution (negative control 2).

The authors of this study believe that technical diaper design optimization can have practical clinical relevance. There is a trend toward less frequent diaper changes due to staff shortages, cost constraints, and patient-centered benefits, which can make frequent changes difficult in daily practice. Moreover, from a patient-centered benefit perspective, fewer diaper changes during the sleep period at night are less disruptive.³³ Unfortunately, this also results in exposure of the skin to urine-loaded diapers for a longer time period. Adequate absorbency to reduce leakage and minimize rewetting can help maintain skin health and prevent IAD.

From a care perspective, absorbent incontinence products are only part of an overall strategy to reduce IAD. Skin care has been shown to reduce IAD occurrence²² and is included in most, if not all, recommendations to avoid IAD.³⁴ In a large study, 4 different care regimens were tested, and the results suggest that although the difference between the regimens was not significant in terms of preventing IAD, the implementation of a skin care regimen was essential and beneficial.³⁵However, the potential incompatibility of some skin care with absorbent incontinence products needs to be addressed. The transfer of highly hydrophobic care products such as petroleum jelly or skin oils liberally applied to so-called at risk areas of the skin can block urine acquisition into the diaper, increasing the risk of leakage and soiling of clothes and linen.

This study has some limitations. As mentioned previously, acidic surface pH of the absorbent incontinence product can markedly help strengthen and support the aged epidermal barrier. However, additional research is required to prove this claim. Another limitation is the collection of data from a single-center clinical trial with 1-time application of the test materials on the volar forearms of the healthy subjects. Further clinical studies are needed to assess whether this new absorbent incontinence design reduces skin irritation and subsequently the incidence of IAD.

The findings of the current study suggest that optimization of absorbent incontinence product designs can contribute to and support other measures, including intensive skin care regimens, to prevent IAD. Design optimizations help reduce rewetting and provide an acidic surface pH of 5.5 over several hours and multiple micturition. Even a harsh alkaline pH challenge to the epidermal barrier can be ameliorated, as shown in this study. These new design features enhance the functionality of the absorbent core, and the skin-friendly and pH-balanced surface can reduce the damaging effects of incontinence on epidermal barrier health. Future research is required to investigate skin symptoms in patients with IAD after switching to the skin-adapted diaper design.

Authors: Olga Vechter, MSc¹; Yana Arlouskaya, MSc¹; Niuosha Sanaeifar, MSc¹; Rüdiger Kesselmeier, PhD¹; Pia Beer, MSc²; Janina Tiemann, PhD²; Agnieszka Segiet, PhD, MD³; Daniel Rabczenko, PhD³; Robert Garcia, MSc⁴; Luca Ferrandi, BSc⁵; and Hans Smola, MD^1,6

Affiliations: ¹PAUL HARTMANN AG, Heidenheim, Germany; ²Dermatest GmbH, Muenster, Germany; ³Clean Data Labs, Warsaw, Poland; _⁴PGI Spain S.L., Tarragona, Spain; ⁵Fiberweb Terno d’Isola S.r.l., Bergamo, Italy; ⁶Department of Dermatology, University of Cologne, Cologne, Germany

Acknowledgment: We gratefully acknowledge Dr. Ivan Ferlez for his assistance in early discussions of the study document preparations.

Ethics approval and consent: All study procedures were conducted in accordance with the principles of the Declaration of Helsinki and were approved by the Ethics Committee of the Landesärztekammer Baden-Württemberg, Stuttgart, Germany (F-2021-145). Patients provided written informed consent before inclusion. This trial was registered retrospectively (registration number: DRKS-ID: DRKS00031504; date of registration: March 16, 2023).

Data availability: All data generated or analyzed during this study are included in this published article.

Competing interests: YA, NS, OV, RK and HS are full-time employees of PAUL HARTMANN AG. RG and LF are full-time employees of Berry Global Inc, the manufacturer of the acquisition and distribution layer. The other authors declare no conflicts of interest.

Funding: This study was funded by PAUL HARTMANN AG.

Author contributions: PB, JT, and HS performed the proband study. NS, OV, and RK performed the design work and the testing of the new diaper absorbent core. RG and LF performed the analysis of the acquisition and distribution layer. AS and DR performed the statistical analysis. HS and YA conceptualized the study. All authors reviewed the manuscript.

Correspondence: Hans Smola, MD; Department of Dermatology, University of Cologne, Kerpener Str. 62, D-50937 Cologne, Germany; hsmola@uni-koeln.de.

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