Wearable porous PDMS layer of high moisture permeability for skin trouble reduction

The present research proposes the present porous polydimethylsiloxane (PDMS) layer for the skin trouble reduced daily life skin attachable devices. The present research proposes the new pores forming method in the PDMS by crystallization and dissolution of the citric acid in the PDMS for fabricating high uniform and small size pores. The present porous PDMS layer (i) decreases the pore size 93.2%p and increases the pore size uniformity 425%p compared to the conventional porous PDMS layer of mixing sugars and PDMS; (ii) is able to be fabricated in the thickness of 21–101 µm by spin-coating; (iii) has the 2.2 times higher water vapor transmission rate (947 ± 10.8 g/day•m2) compared to the human skin water vapor transmission rate. The present porous PDMS layer reduces the skin trouble effectively by having the high water vapor permeability, therefore is applicable to the human daily-life skin attachable devices.


Scientific Reports
| (2021) 11:938 | https://doi.org/10.1038/s41598-020-78580-z www.nature.com/scientificreports/ sugar or salt. However, the porous PDMS structures formed by aqueous solutes are difficult for applications to the human skin attachable devices because the large centrifugal forces of the spin-coating process result in the large and non-uniform size (70-400 µm) of sugar.
In this research, we propose a novel porous PDMS layer fabrication including the special process of the citric acid crystallization and dissolution to solve the problems of the conventional porous PDMS layer fabrication methods, as follows. The present porous PDMS layer use citric acids as pore forming material, therefore requiring the 0.16% of the material cost of the conventional methods mixing PDMS and beads for the fabrication of the same porosity porous PDMS layer. The present porous PDMS layer fabrication method is able to acquire higher porosity (< 60%) than the conventional method mixing PDMS and liquid, by controlling the mixing ratio of the PDMS and the citric acids. The present porous PDMS layer form pores inside the PDMS by using ethanol and citric acid, instead of water and sugar of the conventional method mixing PDMS and aqueous solute. The ethanol has higher PDMS permeability than water 13 and the citric acid have higher ethanol solubility 14,15 than sugar. The present porous PDMS layer form pores by citric acid crystallization in PDMS followed by the citric acid crystal removal using ethanol, thereby having 93.2%p decreased and 425%p increased pore size and pore size uniformity, respectively, compared to those of the conventional method using sugar. The present porous PDMS layer is applicable to the skin attachable device by distributing the uniform and the small size pores in the spin-coated porous PDMS layer thin membrane in the thickness range of 21-101 μm.
Human skin compatibility of the present porous PDMS layer is evaluated based on the water vapor transmission rate. The water vapor occupies the dominant amount of the human skin secretion, thereby the water vapor transmission rate is adaptable to evaluate human skin compatibility. Surface roughness of the present porous PDMS layer is characterized for the attachment reliability of the human physiological sensors on the present porous PDMS layer.
The present research proposes the novel porous PDMS structure fabrication process to enhance the water vapor transmission rate of the skin attachable devices by citric acid crystallization process. The present porous PDMS layer fabrication process enables the spin-coated porous PDMS layer for the skin attachable device applications by not only reducing the pore sizes but also improving the uniformity of the pore sizes. The present porous PDMS layer is experimentally verified having the skin trouble reduction effect by having the water vapor transmission rate over the water vapor evaporation rate of the human skin, thereby applicable to the daily life physiological signal monitoring human skin attachable device.

Experimental results and discussion
Fabrication process. The present porous PDMS fabrication use four materials of PDMS, citric acid, ethanol, and toluene. PDMS is the structure material. Toluene is used to dilute PDMS for easy mixing of PDMS and ethanol-citric acid solution. Citric acid is pore forming material of the present porous PDMS layer fabrication. Ethanol plays two roles of the citric acid solvent and the crystallized citric acid removal. Figure s1 shows the fabrication process of the present porous PDMS layer. The first step of the present porous PDMS layer fabrication is mixing the four materials of the PDMS, the toluene, the citric acid, and the ethanol (Fig. s1a). The toluene and the ethanol solvent mixture is evaporated on the hot plate to crystallize the citric acid in the PDMS (Fig. s1b). The citric acid crystals and PDMS mixture (Fig. s1c) is molded and cured using the PDMS curing agency (Fig. s1d). The PDMS curing agency is added in the weight ratio of 1:10 with PDMS. The cured citric acid crystals and PDMS mixture is dipped in the ethanol to remove the citric acid crystals (Fig. s1e). The present porous PDMS (Fig. s1f.) is fabricated followed by drying the ethanol. The present porous PDMS layer is fabricated to have different pore size, pore size uniformity, and porosity depending on the two major fabrication conditions of: (i) the toluene and ethanol solvent mixture evaporation temperature (t e ); (ii) the citric acid and the PDMS weight mixing ratio (WMR). t e controls the evaporation speed of the toluene and the ethanol. The toluene and the ethanol evaporation speed decide the sizes of the citric acid crystals. The sizes of the citric acid crystals affect the pore size and the pore size uniformity of the present porous PDMS layer. The size of the citric acid crystals decides the centrifugal force on each citric acid crystal during the spin-coating membrane fabrication process. Larger citric acid crystals are easy to be removed during the spin-coating process due to the larger centrifugal force. Thus, t e affects the pore size and the pore size uniformity of the present porous PDMS layer, thereby deciding the spin-coating possibility of the porous PDMS layer with the uniformly distributed smallsized pores. t e is controlled in 60, 70, 120, 130, 140, and 150 °C for the present porous PDMS layer fabrication. t e in the range of 80-110 °C are not adaptable to fabricate the present porous PDMS layer. The details about the t e ranges are explained in the method section. WMR decides the porosity of the present porous PDMS layer. The porosity of the porous PDMS layer is decided by the amount of the citric acid crystals removed by the ethanol. The porosity is a dominant factor to decide the water vapor transmission rate of the present porous PDMS layer. Thus, WMR is the key factor to decide the water vapor transmission rate of the present porous PDMS layer. WMR is calculated by the division of the citric acid mass and the PDMS mass. WMR is controlled in the 4 different types of 0.5, 1.0, 1.5, and 2.0 for the present porous PDMS layer fabrication. The specific amount of the materials used in the fabrication process is explained in the method section. Figure 1 shows the present porous PDMS layer, fabricated at t e = 150 °C and WMR = 0.5.
Volumetric contraction. The present porous PDMS layer is volumetrically contracted during the fabrication process of the citric acid crystal removal. The volumetric contraction of the present porous PDMS layer is measured for varying t e and WMR to verify the effect of the volumetric contraction on the pore size, pore size uniformity, and porosity. The volumetric contractions are dramatically decreased from 0.21 ± 0.037 µm to 0.04 ± 0.035 µm, fabricated at t e in the range of 60-70 °C and 120 ~ 150 °C, respectively (Fig. 2a) toluene (boiling point = 110.6 °C), are both boiled over the temperature of 120 °C. t e over 120 °C increases the evaporation speed of the solvent mixture of the ethanol and the toluene by boiling, thus reducing the pore size and increasing the pore size uniformity. The pore sizes are 160 ± 67 µm and 20 ± 2.9 µm, fabricated at t e in the range of 60-70 °C and 120-150 °C, respectively (Fig. 2b). The pore size uniformity based on coefficient of vari-   Table 1). The pore shape of the present porous PDMS layer, fabricated at t e below 70 °C, which is the temperature under the boiling point of the ethanol and the toluene solvent mixture, is irregular due to the slow evaporation speed. The pore shape of the present porous PDMS layer, fabricated at t e over 120 °C, which is the temperature over the boiling point of the ethanol and the toluene solvent mixture, is spherical due to the fast evaporation speed (Fig. s2). The present porous PDMS layer requires the uniform and small size citric-acid crystals for the formation of the porous PDMS membrane with the uniformly distributed small-sized pores, by spin-coating. The porous PDMS layer, fabricated at t e = 150 °C, shows the smallest pore size and the pore size CV as 19 ± 17 µm and 0.08, respectively, thereby enabling the formation of the uniformly distributed small-sized pores in the spin-coated porous PDMS layer. t e = 150 °C is decided as the optimal fabrication condition for the spin-coating process of the present porous PDMS layer. The surface of the spin-coated porous PDMS layers, fabricated at WMR = 0.5 for varying t e are shown in Fig. 4.
The pore size of the present porous PDMS layer is dominantly affected by not WMR but t e. The pore size of the present porous PDMS layer is similar for varying WMR. The pore sizes of the present porous PDMS layers, fabricated at t e = 150 °C for WMR = 0.5, 1.0, 1.5, and 2.0 are 21 ± 2.9 µm, 26 ± 5.6 µm, 20 ± 10 µm, 20 ± 13 µm, respectively ( Table 2). The pore size uniformity of the present porous PDMS layer is affected by the pore shape distortion. The porous PDMS layer with high WMR is easy to be collapsed due to the low volume of PDMS, mechanical supports in the present porous PDMS layer, thereby the higher WMR leads higher volumetric contraction in the present porous PDMS layer fabrication. The pore shape (Fig. s3) of the present porous PDMS layer is distorted by increment of WMR due to the volumetric contraction. The pore distortion level is quantified  www.nature.com/scientificreports/ by the division of the pore dimensions of x and y directions (Fig. s4). The pore size CV of the present porous PDMS layers, fabricated at t e = 150 °C for WMR = 0.5, 1.0, 1.5, and 2.0 are 0.13, 0.22, 0.42, and 0.79, respectively ( Fig. 3b), thereby the pore size uniformity is decreased depending on WMR. The pore size uniformity is affected by the distortion of the pore shape resulted from the volumetric contraction of the present porous PDMS layer.
Porosity. The porosity of the present porous PDMS layer is affected by the volumetric contraction. The porosity is the ratio of the volume of the PDMS and the volume of the pores in the present porous PDMS layer. The volumetric contraction reduces the volume of the pores, thereby affecting the porosity of the present porous PDMS layer. The experimental porosity of the present porous PDMS layer, fabricated at WMR = 0.5 for varying t e is shown in Fig. 2c with the theoretical porosity of neglecting and considering the volumetric contraction (Table s1). The experimental porosity of the present porous PDMS layer, fabricated at t e = 150 °C for varying WMR is shown in Fig. 3c with the theoretical porosity of neglecting and considering the volumetric contraction (Table s2). The experimental porosity of the present porous PDMS layer follows the theoretical porosity considering the volumetric contraction. Thus, the porosity of the present porous PDMS layer is affected by the volumetric contraction.
Water vapor transmission rate. The present porous PDMS layer requires the water vapor transmission rate over 432 g/day·m 23 to reduce the skin trouble. The theoretical water vapor transmission rate of the present porous PDMS layer, fabricated at WMR = 0.5, 1.0, 1.5, and 2.0 is 774, 652, 571, 573 g/day·m 2 , respectively, in the thickness of 27.6 µm, thereby showing the highest water vapor transmission rate at the fabrication condition of WMR = 0.5 (Fig. s5). The specific theoretical water vapor transmission rate calculation method is shown in the methods section. The experimental water vapor transmission rates of the conventional PDMS, the present porous PDMS layers fabricated at WMR = 0.5, and the present porous PDMS layers fabricated at WMR = 2.0 are 296 ± 2.7 g/day·m 2 , 947 ± 8.7 g/day·m 2 and 751 ± 6.9 g/day·m 2 16 . The present porous PDMS layer enables the skin trouble reduction compared to the conventional water vapor permeable membranes by the improvement of the water vapor transmission rate. The water vapor transmission rate (947 ± 8.7 g/day·m 2 ) of the present porous PDMS layer, fabricated at WMR = 0.5 and t e = 150 °C, with no skin attachment is 2.2 times higher than that of the human skin requirement (432 g/day·m 23 ), therefore the attachment of the present porous PDMS layer on human skin do not disturb the water vapor evaporation of the human skin.
Optimal fabrication condition. WMR = 0.5 and t e = 150 °C is decided as the optimal fabrication condition considering the water vapor permeability and the spin-coating possibility for the uniformly distributed small-sized pores.
Pore size comparison of the present porous PDMS layer and Non-crystalized porous PDMS layer. The conformal skin contact of the porous PDMS layer is important for the accurate daily-life physiological signal monitoring. The conformal skin contact of the porous PDMS layer is enabled by the fabrication of the spin-coated porous PDMS layer with uniform and small size citric acid crystals. The uniform and small size citric acid crystals prevents the removal of the citric acid particles due to the centrifugal force during the spin-coating process, thereby forming the uniform and small size pores in the porous PDMS layer. The pore size and the pore size CV of the present porous PDMS layer fabricated at t e = 150 °C and WMR = 0.5 are compared to those of the non-crystallized porous PDMS layer, fabricated at WMR = 0.5 to verify the present porous PDMS layer's applicability for the conformal skin attachable material. The non-crystallized porous PDMS layer is fabricated by the citric acid mixing and dissolution without the crystallization process. The pore size (19 ± 1.7 µm) of the present porous PDMS layer is 93.2%p decreased compared to that (280 ± 117 µm) of the non-crystallized porous PDMS layer. The pore size uniformity (pore size CV = 0.008) of the present porous PDMS layer is 425%p increased compared to that (pore size CV = 0.42) of the non-crystallized porous PDMS layer ( Table 3). The surface of the spin-coated present porous PDMS layer and non-crystallized PDMS layer, fabricated at the spincoating RPM of 1000, 2000, and 4000 are shown in Fig. 6. The present porous PDMS layer is applicable to the skin attachable device by distributing the uniform and small size pores in the porous PDMS layer in the thickness range of 21-101 µm (Fig. 7).   Fig. s8a,b. The thickness of the present porous PDMS layer and the conventional PDMS layer with holes array is designed as 31.2 µm. The conventional PDMS layer with holes array has the same water vapor transmission rate with the present porous PDMS layer at the hole radius of 83.4 µm. The holes occupy the 54.6% of the overall area, thereby reducing the device integration efficiencies. Young's modulus of the conventional PDMS layer with 83.4 µm radius holes array is 307.5 kPa which is 65% of that of the present porous PDMS layer (Fig. s9), thereby having lower durability than the present porous PDMS layer.
Surface roughness. The surface roughness of the present porous PDMS layer is characterized for the reliability verification of the human physiological sensor integration on the present porous PDMS layer. The surface roughness of the present porous PDMS layer fabricated at WMR = 0.5 for varying t e is shown in Fig. 2d. The surface roughness of the present porous PDMS layer fabricated at t e = 150 °C for varying WMR is shown in Fig. 3d. The surface roughness of the present porous PDMS layer is lowest as 5.1 ± 0.19 µm in the fabrication conditions of t e = 150 °C and WMR = 0.5. Thereby the fabrication condition of t e = 150 °C and WMR = 0.5 has the highest reliability for the human physiological sensor integration on the present porous PDMS layer.
Endurance on the human skin use. The endurance of the present porous PDMS layer is expected by applying strains in the human skin strain range of 0.3 17 . The stress-strain curve of the porous PDMS layers (Fig. s5) are linear in the human skin strain range, thereby the present porous PDMS layers are elastic material  Human skin attachment test. The skin trouble reduction ability of the present porous PDMS layer is verified by the human skin test. The present porous PDMS layer and the conventional non-porous PDMS layer are attached on the same human skin subject (n = 1) for 7 days. The skin attaching the conventional non-porous PDMS layer changes into red color. However, the skin attaching the present porous PDMS layer does not show the severe color change ( Table 4). The dermatological diagnosis of skin redness by the attachment of the conventional non-porous PDMS layer is 'contact dermatitis' which is coming from the lack of water vapor evaporation on human skin. Thereby, the improvement of the water vapor transmission rate of the present porous PDMS layer reduces the skin troubles of the human skin attachment. In summary, the present research experimentally verifies the novel porous PDMS layer fabricated by controlling the citric acid crystallization conditions of t e and WMR for: i) enabling the spin-coated porous PDMS layer with the uniformly distributed small-sized pores for the conformal skin contact; ii) improving the water vapor transmission rate for the skin trouble reduction at the daily life skin contact.
The present porous PDMS layer shows the smallest pore size and the pore size CV at the fabrication condition of t e = 150 °C. The present porous PDMS layer shows the highest water vapor transmission rate at the fabrication condition of WMR = 0.5. The present porous PDMS layer fabricated at t e = 150 °C and WMR = 0.5 is effective for the conformal contact and the skin trouble reduction of the skin attachable devices. The present porous PDMS layer fabricated at t e = 150 °C and WMR = 0.5 decreases the pore size 93.2%p, and increases the pore size uniformity 425%p compared to the conventional porous PDMS layer fabrication method. The present porous PDMS layer, fabricated at t e = 150 °C and WMR = 0.5, forms the membrane in the thickness of 21 ~ 101 µm by spin-coating for the skin attachable application. The present porous PDMS layer, fabricated at t e = 150 °C and WMR = 0.5, has the 2.2 times higher water vapor transmission rate (947 ± 10.8 g/day m 2 ) compared to the water vapor evaporation rate of the human skin, thereby enabling the effective water vapor evaporation on human skin for the skin trouble reduction. The present porous PDMS layer is applicable to the human daily-life skin attachable devices. Table 4. Human skin attachment test of the conventional and present porous PDMS layer. *Redness level is measured on 6 spots of each specimen with HSL color code (0: red, 25: yellow). **Skin redness index = Redness level of the specimen attached skin / Redness level of surround skin. ***Itchiness level survey (0: normal state, 6: mosquito biting. All results are measured followed by 7 days of the specimens attachment on 1 subject.

Materials and methods
The amount of the material amount used in the present porous PDMS layer fabrication. The amounts of the citric acid and the PDMS are decided by WMR. The volume of the ethanol is decided as the volume saturated by the citric acid. The volume of the toluene volume is decided as same as the volume of the ethanol. t e range. t e range of the present porous PDMS layer is limited as below 70 °C or between 120-153 °C. t e in the range of 80 and 110 °C crystalize the citric acid not in micrometer-dimensions but in centimeter-dimensions due to the vapor pressure differences between the ethanol 18 and the toluene 19 (Fig. s8). t e over 153 °C does not form pores by melting the citric acid crystals 20 . Therefore, the present research controls t e on 60, 70, 120,130, 140, and 150 °C for the present porous PDMS layer fabrication (Fig. s9).
Volumetric contraction measurement method. The volumetric contraction is measured 3 times for the 5 specimens of the present porous PDMS layer for the fabrication conditions of varying t e and WMR. The volumetric contraction is calculated using the equation of where C is volumetric contraction, V b is specimen volume before the citric acid removal, and V a is specimen volume after the citric acid removal. where y i is deviation of the assessed profile, p is maximum peak height, v is maximum valley depth, and R p−v is roughness.
Young's modulus measurement method. Young's modulus is measured with 4 specimens for the present porous PDMS layer fabricated at t e = 150 °C and WMR = 0.5. Young's modulus measurement specimens are fabricated by laser cutting of the present porous PDMS layers. The stress-strain curves are plotted with a tensile testing machine (instron 8818, instron, UK) in the condition described in Table s3. Figure s12 shows the specimen designs for Young's modulus test. Figure s13 shows the experimental setup for Young's modulus test.
Water vapor transmission rate measurement method. The water vapor transmission rate is measured 4 times. The water vapor permeability is measured by wet cup method of ASTM E 95-96 (Fig. s14) in the constant temperature and humidity chamber. The chamber temperature and humidity are controlled as 30 °C and 11%RH, respectively. The water vapor transmission rate is calculated with the equation 22 of The theoretical water vapor transmission rate is calculated considering the water vapor transmission rate of the conventional PDMS, the law of mixture 23 and the pore distortion level. Thermal conductivity. The thermal conductivity is measured 3 times. The thermal conductivity is calculated with the equations of The thermal diffusivity is measured by LFA447 (Netzsch, German). The specific heat is measured by DSC204F1 Phoenix (Netzsch, German).
Human skin test. The human skin attachment test is performed with a single subject who had no disease, including skin trouble, and all experimental protocols were approved by the KAIST Institutional Review Board (approval ID: KH2011-18) guidelines. Relevant guidelines are used in the study (Declaration of Helsinki). Before the experiments, all subjects receive detailed explanation of the human experiments and sign the informed consents. The present porous PDMS layer fabricated with the optimal condition and the conventional PDMS layer are attached on the subject dorsal lower arm for 7 days. Skin redness level and itchiness level are measured and surveyed respectively. The skin redness level is measured by photo taken of both PDMS layer attachment spots after the detachment in the conditions of 460 lx illuminance, F/5.6 F-stop, 1/250 s exposure time, and 3200 ISO. The skin redness is measured based on hue index of the hue saturation light (HSL) color code. The hue index 0 and 25 means the red and the yellow, respectively. The skin redness is measured 6 times on the skin of the specimen attached and the skin near the specimen attached, respectively. The skin redness index is calculated by dividing the skin redness of the specimen attached by the skin redness of the skin near the specimen attached. The itchiness level of the subject is surveyed in the range of 0 to 6. 0 means the normal state and 6 means the itchiness level same with the mosquito biting. The reason of the skin redness and the itchiness are analyzed by diagnose from a dermatologist.