Cork Substrate Research Articles

Responsible Humidity Sensor by Direct Laser Writing on Cork Substrate

AbstractA novel laser‐induced graphene (LIG) resistive relative humidity (RH) sensor is successfully fabricated by direct laser writing on a common natural cork sheet. In this work, a straightforward fabrication is presented where LIG, porous multilayer graphene, is generated by a laser photothermal process on a carbon‐based substrate, in this case, an agglomerate cork sheet. The formation of LIG material is revealed by structural and morphological characterization using various analyses, including Raman spectroscopy, X‐ray Photoelectron spectroscopy (XPS), and Scanning Electron Microscopy (SEM). The electrical analyses are conducted to measure the resulting sheet resistance, getting results as low as 31 Ω sq−1. With the laser parameter optimized, a LIG resistive humidity transducer is manufactured and characterized in a climate chamber. This biodegradable sensor shows good linearity with a sensitivity of 0.015%/%RH from 40 to 80% RH. Additionally, the influence of temperature is studied and demonstrated a low impact on the sensor's response toward RH. Furthermore, a proof of concept is successfully implemented by integrating the transducer onto a cork stopper. Considering all, the prospect of creating an easy, fast, scalable, and cost‐effective transducer combined with the use of natural and abundant materials, leads the way for future large‐scale production of sustainable sensors.

Safeguarding Cork’s Beauty and Longevity: Innovations in Deposition of Protective Thin Films

Cork is a sustainable natural material widely used as a wine stopper. However, some other uses, such as wall coverings, flooring, bags and shoes, face UV damage. To mitigate this issue, we explored the deposition of TiO2 and ZnO thin films via magnetron sputtering on glass and cork substrates. Both films displayed uniformity and the lack of any discernible cracks or voids, remained transparent in the visible region, and offered UV protection. Thus, TiO2 and ZnO blocked UV light with a wavelength of up to 310 nm (Eg = 4 eV) and 370 nm (Eg = 3.3 eV), respectively. Exposure tests, under a sun simulator lamp, revealed that the uncoated cork showed noticeable color changes, even when located under a glass substrate. The TiO2 coating did not prevent cork discoloration, while ZnO prevented it. This study highlights ZnO thin films as a durable solution to safeguard cork materials from UV damage and extend their usability.

Cork derived laser-induced graphene for sustainable green electronics

The demand for smart, wearable devices has been dictating our daily life with the evolution of integrated miniaturized electronics. With technological innovations, comes the impactful human footprint left on the planet’s ecosystems. Therefore, it is necessary to explore renewable materials and sustainable methodologies for industrial processes. Here, an eco-friendly approach to producing flexible electrodes based on a single-step direct laser writing is reported. A 1.06 µm wavelength fiber laser was used for the first time to produce porous three-dimensional laser-induced graphene (LIG) on an agglomerated cork substrates. The obtained material exhibits the typical Raman spectra, along with an exceptionally low sheet resistance between 7.5 and 10 ohm sq−1. LIG on cork high electrical conductivity and the friendliness of the used production method, makes it an interesting material for future technological applications. To show its applicability, the production of planar micro-supercapacitors was demonstrated, as a proof of concept. Electrochemical performance studies demonstrate that LIG interdigitated electrodes, using PVA-H2SO4 electrolyte, achieve an area capacitance of 1.35 mF cm−2 (103.63 mF cm−3) at 5 mV s−1 and 1.43 mF cm−2 (109.62 mF cm−3) at 0.1 mA cm−2. In addition, devices tested under bending conditions exhibit a capacitance of 2.20 mF cm−2 (169.22 mF cm−3) at 0.1 mA cm−2. Here, showing that these electrodes can be implemented in energy storage devices, also successfully demonstrating LIG promising application on innovative, green, and self-sustaining platforms.

Floating TiO2-Cork Nano-Photocatalysts for Water Purification Using Sunlight

In the present study, titanium dioxide (TiO2) nano-photocatalysts were synthesized through microwave irradiation. In a typical microwave synthesis, TiO2 nanomaterials were simultaneously produced in powder form and also directly covering cork substrates. The TiO2 nanopowder was analyzed by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM), revealing that the solvothermal microwave synthesis resulted only in the TiO2 anatase phase. From Fourier-transform infrared spectroscopy (FTIR), cork’s organic species, along with bands of TiO2, were detected. UV–VIS absorption spectrum revealed an absorption extension to the visible region, since a brown powdered TiO2 product was obtained. Very fine nanoparticles were observed displaying a nearly spherical shape that agglomerates in larger particles. These larger particles fully covered the surface of the honeycomb cork cells, originating TiO2 functionalized cork platforms. The TiO2 functionalized substrates were further tested as floating photocatalysts and their photocatalytic activity was assessed from rhodamine B degradation under solar simulating light and natural sunlight. Reusability tests were also performed under natural sunlight. The strategy applied in this research work allowed the production of green and low-cost cork platforms based on TiO2 photoactive materials with the ability to purify polluted water under natural sunlight.

Sustainable Fully Printed UV Sensors on Cork Using Zinc Oxide/Ethylcellulose Inks.

Low-cost and large-scale production techniques for flexible electronics have evolved greatly in recent years, having great impact in applications such as wearable technology and the internet of things. In this work, we demonstrate fully screen-printed UV photodetectors, successfully fabricated at a low temperature on a cork substrate, using as the active layer a mixture of zinc oxide nanoparticles and ethylcellulose. The photoresponse under irradiation with a UV lamp with peak emission at 302 nm exhibited a quasi-quadratic behavior directly proportional to the applied voltage, with a photocurrent of about 5.5 and 20 μA when applying 1.5 V and 5 V, respectively. The dark current stayed below 150 nA, while the rise and falling times were, respectively, below 5 and 2 s for both applied voltages. The performance was stable over continuous operation and showed a degradation of only 9% after 100 bending cycles in a 45 mm radius test cylinder. These are promising results regarding the use of this type of sensor in wearable applications such as cork hats, bracelets, or bags.

Biodegradable Dual Semicircular Patch Antenna Tile for Smart Floors

A dual semicircular microstrip patch antenna implemented on a biodegradable substrate is presented for operation in the [863–873] MHz and [2.4–2.5] GHz frequency bands. To cover these frequency bands, two semicircular patches are compactly integrated onto a biodegradable cork tile, commonly found as support in laminate flooring, serving as a substrate. Thereby, the antenna tile may be seamlessly embedded as a sublayer of the floor structure. A higher-order mode is generated by applying via pins in the antenna topology to produce a conical radiation pattern with a null at broadside and sectoral coverage in the vertical plane. As such, the concealed floor antenna covers all azimuth angles of arrival in smart houses. The antenna performance is fully validated, also when the tile is covered by different polyvinyl chloride sheets. Owing to the supplementary design margins, the antenna impedance bandwidth remains covered. Moreover, the radiation patterns are measured in various elevation planes. Under stand-alone conditions, a radiation efficiency and a maximum gain of 74.3 $\%$ and 5.8 dBi at 2.45 GHz and 48.1 $\%$ and 2 dBi at 868 MHz are, respectively, obtained. Its omnidirectional coverage in the horizontal plane, stable performance on the inhomogeneous and biocompatible cork substrate and for various inhomogeneous superstrates, and its low-profile integration make the proposed antenna an excellent candidate for smart floors and smart houses.

RFID-Based Wireless Passive Sensors Utilizing Cork Materials

This paper presents the design of low-cost, conformal UHF antennas and RFID tags on two types of cork substrates: 1) natural cork and 2) agglomerate cork. Such RFID tags find an application in wine bottle and barrel identification, and in addition, they are suitable for numerous antenna-based sensing applications. This paper includes the high-frequency characterization of the selected cork substrates considering the anisotropic behavior of such materials. In addition, the variation of their permittivity values as a function of the humidity is also verified. As a proof-of-concept demonstration, three conformal RFID tags have been implemented on cork, and their performance has been evaluated using both a commercial Alien ALR8800 reader and an in-house measurement setup. The reading of all tags has been checked, and a satisfactory performance has been verified, with reading ranges spanning from 0.3 to 6 m. In addition, this paper discusses how inkjet printing can be applied to cork surfaces, and an RFID tag printed on cork is used as a humidity sensor. Its performance is tested under different humidity conditions, and a good range in excess of 3 m has been achieved, allied to a good sensitivity obtained with a shift of >5 dB in threshold power of the tag for different humid conditions.

Fruit colour conceals endocarp dimorphism from avian seed dispersers in a tropical beach plant, Scaevola taccada (Goodeniaceae), found in Okinawa

Abstract:Theory predicts that honest signalling strategies will not always be evolutionarily stable in interspecific communication, yet to demonstrate such a transition of signalling modality between honesty and dishonesty in the wild would be difficult. An endocarp dimorphism has been found in Scaevola taccada fruits: a morph with a cork substrate that facilitates ocean current seed dispersal and a morph without the cork. Both types of fruit are covered with sugar-containing flesh, and are similar in size and colour to one another (at least from a human perspective). The apparent lack of external differences between morphotypes could potentially degrade mutualistic relations between the plant and seed-dispersing birds because the presence of a cork could lower the fruit's nutritional value. Thus, unless seed dispersers can discriminate between the different types of fruit, this system may provide an example of a transition between honest and dishonest signalling. We examined S. taccada fruit and leaf colours from an avian visual perspective. Even though the fruits and leaves were different in colour from one another to birds, there was no perceivable difference in the colours between fruit morphotypes. Therefore, fruit colour is not an honest indicator of reward to seed dispersers. Further, we propose an adoption of a statistical method in avian visual modelling studies that avoids the common statistical errors, such as violation of the congruence principle.

Frequency Tunable Metamaterial Absorber Using Hygroscopicity of Nature Cork

In this letter, a novel frequency tunable metamaterial absorber is proposed for wireless humidity sensor applications made with natural cork. Natural cork has excellent hygroscopicity and its electrical properties can be changed depending on the moisture content. In this work, the absorption ratio of the proposed absorber are simulated and measured in different moisture content ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm M} = 0\% $</tex> </formula> , 1.44%, 5.88%, 13.32%, 18.8%) of the cork substrate. The resonant frequency changed from 4.92 GHz in the dry ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm M} = 0\% $</tex> </formula> ) state to 3.49 GHz in the wet ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm M} = 18.8\% $</tex> </formula> ) state; the measured absorptivity is higher than 90% in all moisture content values. To the best of our knowledge, it is the first metamaterial absorber using natural cork.

Characteristics of Internal Porosity of Cork Container Media

The structure of cork (Quercus suber L.) bark presents a series of characteristics, suggesting that internal porosity is partly occluded. This study determined the porosity in the waste cork industry (C) and when such waste product had been composted during 4 (CC-4), 7 (CC-7), and 10.5 months (CC-10.5). The particle density of the intact and finely ground material differed significantly in all particle size ranges larger than 0.5 mm. The porosity of the cork substrates ranged from 80% to 94% of the total volume, according to granulometry and the degree of decomposition. However, large particles and less decomposed material with a high porosity had up to 10% of the total volume as occluded pores. The material's effective porosity varied slightly between the various particle sizes and degrees of decomposition, which ranged between 80% and 89%, having an average value of 85%. The ash content was highly correlated with the particle density of the finely ground material. Nevertheless, and due to occluded porosity, we cannot estimate the “effective porosity” from the ashes; therefore, we must resort to techniques that involve the displacement of a fluid, such as liquids or gas pycnometry or submersion.

Influence of the wetting properties of polymeric adhesives on the mechanical behaviour of cork agglomerates

A series of adhesives, polyurethane prepolymers with alkane chains of different functionalities, were used in the production of cork agglomerates. The polymeric structure varied from long, linear chains in adhesive M1 (14% isocyanate groups) to short, branched chains in adhesive M4 (30% isocyanate groups). The wetting properties of the adhesives were studied through surface tension and contact angle measurements of the polymeric liquids deposited on cork substrates. The mechanical behaviour of the cork agglomerates was studied through compression and three-point bending tests, and the agglomerate structure was analysed by scanning electron microscopy (SEM). The relationship between the structural and wetting characteristics of the adhesives and the mechanical properties of the agglomerates was investigated. The experimental results indicate that it is the work of adhesion for the adhesive/cork system, and not only the contact angle, which determines the mechanical resistance of the cork agglomerate.

Wetting of cork by polymeric adhesives

The cohesion of cork agglomerates is determined by the strength of the adhesive joint established between the polymeric adhesive and the cork particles. The ability of adhesives to form good joints depends, among other factors, on the wetting of cork by the adhesives. The main objective of this research was to study the behaviour of adhesive drops deposited on cork substrates through measurements of contact angles and their time dependence. Several polyurethane prepolymers were tested to establish a correlation between the wetting characteristics and the chemical structure and physical properties of the adhesives. The effect of the morphology of cork on the interfacial properties was also investigated. The initial contact angles were related to the chemical nature of the adhesive. The kinetics of the wetting process were found to depend mainly on the viscosity of the adhesive.

Laboratory colonization of the human onchocerciasis vector Simulium damnosum complex (Diptera: Simuliidae), using an enclosed, gravity-trough rearing system.

The onchocerciasis vector Simulium damnosum s. lat. was colonized for almost 6 generations using an inexpensive, enclosed, gravity-trough rearing system designed to flood itself during power failure. Egg-to-adult survival for F1–F5 was 74.3, 85.5, 25.8, 17.6, and 0%, respectively. Maximum egg-to-adult survival and developmental synchrony were obtained with F2's reared at 25–26 °C. Insemination of females was estimated at 0.4, 6, 7, 6, and 2% for the parental–F4 generations, respectively. Behavioral observations suggested female refractoriness prevented higher insemination rates. The male: female ratio was 1:1, 1.1:1, 0.7:1, 0.5:1, and 0.07:1 for the parental–F4 generations, respectively. The altered sex ratios and decreased larval survival of later generations are unexplained but may indicate inbreeding depression. Sixty-six percent of emerged females (total females emerged = 4768) survived to day 2 and an average of 92.5% of these blood-fed on humans. Egg development required 72 h at 27–28 °C. An average of 65.2% of engorged females survived to day 4 and 94.6% of surviving females oviposited on cork substrates under simulated twilight conditions. Fecundity of F2 and F4 females was 450.50 ± 145 ( n = 40, range = 200–811) and 502.79 ± 180.02 ( n = 30, range = 80–836), respectively. The difference in fecundity between F2 and F4 females was not significant. Eleven percent of fertile eggs stored 18–20 days at 0.5–1 °C hatched.

Sustained colonization of the black fly Simulium decorum Walker (Diptera: Simuliidae)

The black fly Simulium decorum Walker has been colonized for 16 generations (18 months) using an inexpensive larval rearing system. More than 90% of the females are induced with simulated twilight to oviposit on floating cork substrates. Eggs on corks have been successfully stored for up to 2 months at 0.5–1.5 °C. Female survival averages 87% for 4 days at 22 °C and 90–95% relative humidity. Mating in aspirator tubes results in nearly 100% insemination, whereas a cage mating trial resulted in only 26% insemination. Under crowded conditions where larvae are underfed, emerging males are smaller and mature fewer eggs. Though autogenous, nulliparous females readily blood feed on humans but do not mature additional eggs.