Stimuli-responsive material and method of manufacture

Abstract

A process for manufacturing a hydrogel with optical switching properties for anti-counterfeiting applications or sensor applications, comprising a) adding a prepolymer solution to a receptacle, said prepolymer solution comprising a prepolymer, a photo initiator and a porogen, b) exposing the prepolymer solution to UV light to form a hydrogel, optionally through a first mask to form a hydrogel with a first two dimensional shape. A second prepolymer solution may be exposed to UV light through a second mask to form a hydrogel with a second two dimensional shape. These shapes may be different than each other or complimentary thereto, resulting in the first and second hydrogel having different optical properties.

Description

Stimuli-Responsive Material and Method of Manufacture

[0002] This application claims priority to US provisional Patent Application No. 63 / 581,731 filed September 11, 2023, which is hereby incorporated herein by reference.Field of Invention

[0004] The invention is in the field of hydrogel manufacture.of Invention

[0006] Intensive research efforts have been ongoing over the past three decades to develop stimuli-responsive materials,[14'18]for use in various applications, such as anti-counterfeiting.

[0007] Anti-counterfeiting technologies can be categorized into two types: 1) overt technologies, which allow users to easily identify the difference between authentic and counterfeit products with naked eyes[2], and 2) covert anti-counterfeiting technologies, which do not allow room for counterfeiting due to the invisible and hidden patterns.[3, 4]As compared to the overt technologies, covert technologies provide higher-level security features[5]however, they are often limited by the need for sophisticated and expensive decoding equipment to reveal the invisible encoded patterns made of toxic optical nanomaterials such as organic dyes or carbon dots.[6, 7]

[0008] Optical nanomaterials are known, and include metallic nanoparticles,[8]organic dyes,[9]semiconducting quantum dots,

[0010] and lanthanide-doped nanoparticles.

[0011] These can have stimuli-responsive properties or be used in stimuli-responsive applications.

[0009] Microparticles with colored stripes of up-conversion nanocrystals are known as stimuli- responsive nanomaterials. These up-conversion nanocrystals glow brightly under near-infrared radiation.

[0013]

[0010] Another approach has been to incorporate stimuli-responsive nanomaterials into photonics crystals, which can efficiently diffract light of certain wavelength determined by the crystal dielectric constants and its periodic spatial arrangement. In response to external stimuli, these responsive optical nanomaterials will undergo changes in their physical properties. Theseexternal stimuli can be 1) physical stimuli such as temperature

[0019] , humidity

[0020] , mechanical stress

[0021] , and electric and magnetic fields

[0022] or 2) chemical stimuli such as chemical interactions, ionic strength, and pH.|23‘24]

[0011] A further approach has been the use of millimetric stimuli-responsive hydrogel films with microscopic spatial patterning. An advantage of this approach is that it allows users to authenticate consumer products using smartphones without the need for user training or sophisticated technology, or any post-processing steps. However, one significant limitation of these millimetric stimuli-responsive hydrogel films is that the hydrogels have a high-water content, resulting in low refractive indices.

[0025] To manipulate the refractive index, one approach to improve the millimetric stimuli-responsive hydrogel films has been the use of crystalline arrangements of polymeric microparticles.[26, 27]Another approach has been to embed metallic nanoparticles within the hydrogel framework.

[0018] Due to changes in the crystalline or metallic nanoparticle spacing in response to hydrogel swelling / deswelling, a change in the color signal is typically observed.

[0028] As can be appreciated, this approach typically requires complex and difficult to control multi-step synthesis methods, in which the short order or long order colloidal self-assembly can take several days to complete.

[0012] This long and often unpredictable self-assembly process can be improved through an active assembly process using externally-responsive photonic crystals, such as magnetically- responsive crystals. However, this process requires additional equipment to generate external fields and still can produce defects. Also, because of the toxicity of such optical materials above a critical dosage, 10-15 mg kg-1 (on mice)

[0031] , they may be inapplicable for edible products such as food and pharmaceuticals.

[0013] It would be desirable to manufacture a stimuli-responsive material that can be used at the millimeter or nanometer scale, for use for example in anti-counterfeiting labelling of goods.

[0014] Summary of the Invention

[0015] According to an aspect of the present invention is provided a process for the manufacture of a hydrogel with optical switching properties, comprising: (a) adding a prepolymer solution to a receptacle, said prepolymer solution comprising a prepolymer, aphotoinitiator, and a porogen; and exposing the prepolymer solution to UV light to initiate polymerization of the prepolymer into a hydrogel.

[0016] According to certain embodiments, the porogen is ethanol.

[0017] According to certain embodiments, the porogen is a solvent that creates a photopolymerization-induced phase separation.

[0018] According to certain embodiments, the porogen is one or more of ethanol, toluene, hexane, cyclohexanone, 2-ethylhexanol, p-xylene, n-heptane, poly(ethylene glycol), polymethylmethacrylate in chloroform, low molecular weight PVA, and 1,3 -benzenedib or onic acid.

[0019] According to certain embodiments, the solvent is present at a concentration that induces sub-micron porous structures inducing light scattering.

[0020] According to certain embodiments, the ethanol is present in a concentration of about 50% v / v.

[0021] In certain embodiments, the concentration may vary between 50% v / v and 75 % v / v to yield hydrogels with distinct optical switching properties.

[0022] According to certain embodiments, the prepolymer is poly(ethylene glycol) diacrylate.

[0023] According to certain embodiments, the photoinitiator is selected from one or more of 2- Hydroxy-2 -methylpropiophenone, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and Lithium phenyl-2,4,6- trimethylbenzoylphosphinate.

[0024] According to certain embodiments, the receptacle comprises a PMDS microfluidic channel comprising a PDMS coated glass substrate and a PDMS slit top channel.

[0025] According to certain embodiments, the exposing of the prepolymer solution to UV light is through a mask with a two dimensional shape, resulting in the hydrogel having said two dimensional shape. According to certain embodiments, the mask can be physical or digital photomask.

[0026] According to certain embodiments, the exposing of the prepolymer solution to UV light through said mask is through a magnification lens such that the two dimensional shape of the hydrogel is of a different scale than the two dimensional shape of the mask.

[0027] According to a further aspect of the present invention is provided a process for the manufacture of a hydrogel with optical switching properties, comprising: (a) adding a prepolymer solution to a receptacle, said prepolymer solution comprising a prepolymer, a photoinitiator, and a porogen, said porogen present at a first concentration; (b) exposing the prepolymer solution to UV light through a first mask having a first two dimensional shape to initiate polymerization of the prepolymer into a primary hydrogel having a two dimensional shape corresponding to said first two dimensional shape; (c) changing the prepolymer solution to a second prepolymer solution comprising a prepolymer, a photoinitiator, and a porogen, said porogen present at a second concentration, or alternatively changing the concentration of the porogen from the first concentration to a second concentration, said second concentration being different than the first concentration; (d) exposing the prepolymer solution to UV light through a second mask having a second two dimensional shape to polymerize the prepolymer into a secondary hydrogel, said secondary hydrogel and primary hydrogel forming the hydrogel, said second two dimensional shape being different than the first two dimensional shape and optionally being complimentary thereto, and said secondary hydrogel and primary hydrogel optionally overlap, and wherein said secondary hydrogel and said primary hydrogel have different optical properties.

[0028] In certain embodiments, the first mask and the second mask are physical photomasks.

[0029] In certain embodiments, the first mask and the second mask are different digital photomasks.

[0030] In certain embodiments, the first mask and the second mask are different display configurations of one digital photomask.

[0031] In certain embodiments, the display configuration of the second mask is a simple inversion of the display configuration of the first mask.

[0032] According to certain embodiments, the porogen is ethanol.

[0033] According to certain embodiments, the first concentration is about 0% v / v.

[0034] According to certain embodiments, the second concentration is about 50% v / v, or between about 50% v / v and about 75% v / v.

[0035] According to certain embodiments, the first concentration is about 50% v / v and the second concentration is about 75% v / v.

[0036] According to certain embodiments, the prepolymer is poly(ethylene glycol) diacrylate.

[0037] According to certain embodiments, the photoinitiator is one or more of 2 -Hydroxy-2 - methylpropiophenone, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, or Lithium phenyl- 2,4,6- trimethylbenzoylphosphinate.

[0038] According to certain embodiments, the photoinitiator induces free radical polymerization.

[0039] According to certain embodiments, the receptacle comprises a PMDS microfluidic channel comprising a PDMS coated glass substrate and a PDMS slit top channel.

[0040] According to certain embodiments, the UV dose is at for example a UV intensity of about 11.2 W / cm2, for physical photomask. In other embodiments, for digital photomask, the UV intensity is at for example 7 W / cm2or 20 W / cm2.

[0041] According to certain embodiments, the UV dose is a UV exposure duration of between about 0.3 to about 3 seconds, for example, 0.37, 1.5, or 3 seconds, using the physical photomask, and the UV intensity is maintained at 11.2 W / cm2for a selected photoinitator (2 -Hydroxy -2- methylpropiophenone) According to other embodiments, with digital photomask, the UV intensity can be chosen for example to be 7, or 20 W / cm2. In certain embodiments, , the UV exposure duration is varied between about 0.15 to about 1 seconds, for example 0.15, 0.25, 0.5, or 1 seconds. In certain embodiments, the selected photoinitators can be phenylbis(2,4,6- trimethylbenzoyl)phosphine oxide and Lithium phenyl-2,4,6- trimethylbenzoylphosphinate in combination, for example.

[0042] According to certain embodiments, the exposing of the prepolymer solution to UV light through said first mask, and / or the exposing of the prepolymer solution to UV light through said second mask, is through a magnification lens.

[0043] According to certain embodiments, the exposing of the prepolymer solution to UV light through said first mask and the exposing of the prepolymer solution to UV light through said second mask are performed at different UV exposure durations, to generate a hydrogel with anisotropic porosity and spatially varying optical switching properties.

[0044] According to certain embodiments, the exposing of the prepolymer solution to UV light through said first mask, and the exposing of the prepolymer solution to UV light through said second mask, are through magnification lenses having different magnification.

[0045] In certain embodiments, the exposing to the UV light is at a UV intensity of about 11.2 W / cm2.

[0046] In certain embodiments, the exposing to the UV light is at a UV intensity of about 7 W / cm2.

[0047] In certain embodiments, the exposing to the UV light is at a UV intensity of about 20 W / cm2.

[0048] In certain embodiments, the exposing of the UV light is at a duration of between 0.15 to 1 seconds, preferably between 0.25 and 0.5 seconds.

[0049] According to a further aspect of the present invention is provided a hydrogel made by the hereindescribed process, or a dried variant thereof.

[0050] According to a further aspect of the present invention is provided a hydrogel having optical switching properties, whereby an image is displayed within the hydrogel in the presence a first porogen, and a second image is displayed within the hydrogel in the presence of a second porogen, with the first porogen and second porogen being different porogens or different concentrations of the same porogen, and with the first image and the second image being different images.

[0051] According to a further aspect of the present invention is provided an anticounterfeiting label comprising a hydrogel as hereindescribed, or a dried variant thereof.

[0052] According to a further aspect of the present invention is provided the use of the anticounterfeiting label as hereindescribed on a product, said use comprising the application of the polymerized hydrogel on said product.

[0053] According to certain embodiments, the product is a paper or plastic currency, a coin, eyeglasses, watches, sunglasses, a consumer product, a food, a cosmetic, electronic parts like semi-conductor, luxury goods like gold or precious metals, a gemstone, a diamond, or a pharmaceutical.

[0054] According to a further aspect of the present invention is provided a product comprising an anticounterfeiting label as hereindesribed.

[0055] In certain embodiments, the product is a paper or plastic currency, a coin, eyeglasses, watches, sunglasses, a consumer product, a food, a cosmetic, electronic parts like semiconductor, luxury goods like gold or precious metals, a gemstone, a diamond, or a pharmaceutical.

[0056] According to a further aspect of the present invention is provided a method of determining whether a product as hereindescribed is counterfeit, comprising applying a liquid, for example the porogen, to the product at a location of the anticounterfeiting label and determining whether a color change or shape change occurs in said label.

[0058] Figure 1 A is a schematic diagram of a microfluidic channel used for the polymerization of square membranes. A polymer solution is introduced via channel inlet (top). UV light passing through a photomask is projected onto the microfluidic channel (bottom left). The resulting square hydrogel frame is shown at the bottom right.

[0059] Figure IB shows wet state images of square hydrogel frames synthesized at different UV exposure times shown under different volume percentages of ethanol.

[0060] Figure 1C shows dry state images of square hydrogel frames synthesized at different UV exposure times shown under different volume percentages of alcohol.

[0061] Figure ID shows a quantitative analysis of the light transmittance for both wet and dry state samples taken at different UV exposure times and different ethanol volume percentages.

[0062] Figure 2 shows Scanning Electron Microscopy images of representative hydrogel samples taken at a UV exposure time of 1.5 seconds at UV intensity of 11.2 W / cm2usingphotoinitator (2 -Hydroxy-2 -methylpropiophenone) with (a) 0% ethanol, (b) 25% ethanol,(c) 50% ethanol, and (d) 80% ethanol. Bottom right inserts of (a)-(d) represent the brightfield images of the hydrogel samples in dry state. Scale bar = 1 pm.

[0063] Figure 3 shows brightfield images of miniature logos in representative hydrogel frame samples showing (a) T-shape; (b) maple leaf; (c) bull head, taken under an inverted microscope in the transmissive mode in a semi-dry state. Bottom right inserts of (a)-(c) show the photomasks used for the hydrogel frames. The hydrogel frame is polymerized using a porogen solution containing 50 vol% ethanol, with a UV exposure time through the photomask of 1.5 s.

[0064] Figure 4 shows the experimental setup for sequential and multi-step polymerization. 4(a) shows a schematic of the methodology, for a transparent hydrogel frame with a void feature, with 0 vt% ethanol using a lOx objective in a PDMS channel designed for the multistep polymerization. Step 1: A PEGDA / PI solution containing 0 vol% ethanol is injected in a first inlet port and polymerized using UV exposure of Is through a mask and a lOx objective. A brightfield image of the frame with the void feature polymerized is shown to the right. Step 2: A porogen solution with 50 vt% ethanol is injected into a second inlet port using a syringe pump at 25pl / min for about 10 minutes to gradually replace the void space with the porogen solution for the second polymerization. The translational movement of the frame, induced by the advection, was controlled by the three rectangular bars, allowing 50% ethanol monomer stream to flow through these bars and exit via the first inlet port, but stop the transparent hydrogel frame from sliding. Step 3: a second polymerization, in the 50 vt% ethanol, is performed, at a UV exposure of 3 s, through an inverse mask and a lOx objective. A brightfield image of the void feature is shown on the right of 4(a) Step 3. The photomask containing the T feature is made larger by 20% to ensure that the feature is polymerized and remains intact within the frame’s void space. Step 4: Ethanol solution containing 5 vol% Tween 20 is used to flush the hydrogel out of the PDMS channel. 4(b) shows the special transmittance profile of a hydrogel, with two patterns of different light transmittance, as made using the multi-step polymerization method of 4(a).

[0065] Figure 5 shows the dual optical display properties of representative encoded hydrogels in reflective and transmitted modes, and their application on representative objects. Miniature logos are polymerized using the two-step method of Figure 4. (a) shows the encoded hydrogel in reflected mode - when the miniature logo is viewed at an angle less than 180 degrees from the light source, the T-shaped features on the square logo appear white in color, (b) shows the sameencoded hydrogel in transmissive mode - when the miniature logo is viewed 180 degrees directly from the light source, the T-shaped features on the square logo appear to be brown in color. When we refer to “reflective mode”, we refer to instances where the light source is placed on the same side of the object as the person viewing - i.e. the light reflects off the object and into the user’s eye. When we refer to “transmissive mode”, the light source is placed beneath or behind the object, and through the object and into the user’s eye. Provided that the light source is placed beneath or behind the object, the viewing angle will differentiate if the object is captured in reflective (viewing angle is less than 180 degrees from the light source) or transmissive modes (viewing angle is directly 180 degrees from the light source) (c) shows the application of the encoded hydrogel onto a lens of eyeglasses; the top right inset shows an enlargement of the encoded hydrogel in reflected mode and the bottom right inset shows an enlargement of the encoded hydrogel in transmissive mode, (d) shows the application of an encoded hydrogel (with a “thunder” logo on a heart shaped hydrogel) applied on a vitamin pill (scale bar is 1.4 mm). The upper right inset shows an enlargement of the encoded hydrogel in reflected mode and the bottom right inset shows an enlargement of the encoded hydrogel in transmissive mode. Note that if the patterned hydrogel is applied to or on an opaque solid object, only the reflective mode is available, (e) shows the application of an encoded hydrogel (with a heart shaped logo on a “Buddha” shaped hydrogel) on the clear portion of a Canadian 20 dollar bill. The upper right insert shows an enlargement of the encoded hydrogel in reflected mode and the bottom right insert shows an enlargement of the encoded hydrogel in transmissive mode.

[0066] Figure 6 shows examples of representative encoded hydrogels of the present invention, and shows the stimuli response over time, (a) a covert printed QR code was revealed with a drop of ethanol. Using a one-step method, the covert QR code was polymerized using a porogen solution containing 50 vol% ethanol at a UV exposure time of 3 seconds. Show, the thin film of the QR pattern appears black (0s) and turns brown revealing the QR pattern over time, with the QR pattern completely revealed 10s after addition of a drop of ethanol, (b) a covert “Buddha” feature was revealed within 15 seconds. The Buddha feature was polymerized within the void Buddha feature of the hydrogel frame utilizing a two-step method. A porogen solution containing 50 vol% ethanol was used for the polymerization of both the frame (UV exposure time 0.37s) and the feature (UV exposure time: 3s). As shown, the hidden Buddha feature (0s) isrevealed with an opacity that changes over time, from white (Is) to light brown (15s) following ethanol stimuli.

[0067] Figure 7 shows the swelling-induced optical switching behavior of a hydrogel of the present invention, utilizing a drop of ethanol over time. The reversible optical switching behavior can be used to enhancing the security level in anti-counterfeiting applications. Fast reversible optical switching is achieved with a drop of ethanol using a hydrogel membrane containing highly porous microstructures.

[0068] Figure 8 shows examples of representative quick response (QR) hydrogels of the present invention with contrasting optical features using two-step polymerization, (a) display of first digital photomask used to polymerize the prepolymer solution into primary hydrogel that takes shape of the two dimensional shape of the first digital photomask, (b)the second digital mask is an inversion of the first digital mask. Accordingly, the transparent and opaque regions of the first digital mask become opaque and transparent regions in the second digital mask, respectively, (c) Exposing the prepolymer solution to UV light through the first digital photomask at 0.15 s at 20 W / cm2yielded a transparent primary hydrogel with void spaces (i.e. unpolymerized region). The porogen concentration was 70 vol% ethanol with saturated amount of photoinitator, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (d) The void spaces in the same primary hydrogel was exposed to UV light through the second digital mask for 0.5 s at 20 W / cm2., and the unpolymerized prepolymer solution in the void spaces of the primary hydrogel yielded a much contrasting secondary hydrogel (dark brown), (e) When the UV light is exposed through first digital photomask at 0.25 seconds at 7 W / cm2to the prepolymer solution containing polymer at 50 vol%, porogen at 50 vol% , with saturated amount of photoinitator, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, it yielded a transparent, primary hydrogel with void spaces (i.e unpolymerized region), (f) the void spaces in the primary hydrogel is exposed to UV through second digital mask at 1 second at 20 W / cm2.

[0070] Described is an optically-tuned, microscopic motif embedded in a millimetric hydrogel frame which displays optical shifting. In one exemplified embodiment, the motif displaysoptical shifting from white (reflective mode) to light brown (transmissive mode) induced by purely light scattering through the hydrogel porous microstructure.

[0071] To generate such hydrogels exhibiting porosity-dependent scattering, we use our previously developed method, slit channel lithography in combination with UV-induced phase separation

[0032] (incorporated herein by reference) to eliminate any self-assembly process of colloidal particles.[26, 27]By exploiting the porosity-dependent scattering property, we create motifs / patterns that can be identified via simply optical contrast. We also demonstrate a locally controlled anisotropic swelling behavior to reveal a covert pattern through shape morphing, with a drop of liquid. The hereindescribed fabrication technique has the ability to embed a complex discrete optical pattern in a one-step fabrication process by controlling the hydrogel thin film development. The resultant hydrogels with advantageous optical features are made simply with a biocompatible hydrogel precursor, poly(ethylene glycol) diacrylate and a common solvent, for example, ethanol. These synthetic responsive materials have application in various fields including anti-counterfeiting applications, sensors, displays, and smart devices.

[0072] The hydrogels can be made from any known hydrogel precursor, for example, acrylate polymers such as poly(ethylene) glycol diacrylate, 1,6-hexandiol-diacrylate, lauryl acrylate, trimethylopropate ethoxylate triacrylate (TMPeTA), trimethylopropane triacrylate, and sodium polyacrylate.

[0073] Photoinitiators used in the present invention can be any known photoinitiator which facilitates or catalyzes polymerization upon activation by light, such as UV light, and may include phenylbis(2,4,6-trimethylbenzoyl)phosphine lithium, phenyl-2,4,6- trimethylbenzoylphosphinate, 2 -hydroxy -2 -methylpropiophenone, phenylbis(2,4,6- trimethylbenzoyl)phosphine oxide, etc.

[0074] Porogens used in the present invention can include any solvent which facilitates the creation of pores during polymerization, and are also known as pore-forming agents in the art. These create a photopolymerization-induced phase separation. Examples of solvent porogens include ethanol, toluene, hexane, cyclohexanone, 2-ethylhexanol, p-xylene, n-heptane, poly(ethylene glycol), polymethylmethacrylate in chloroform, low molecular weight PVA and 1,3 -benzenedib or onic acid, and others.

[0075] Square hydrogel frames, made for example from poly(ethylene) glycol diacrylate, were fabricated using slit channel lithography in a polydimethylsiloxane (PDMS) microfluidic channel, an example of which is seen in Figure la. The microfluidic channel as shown comprises a PDMS-coated glass substrate, and a PDMS slit top channel. The slit top channel was filled with a prepolymer solution consisting of poly(ethylene glycol) diacrylate (PEG-DA; 250 Da), a photoinitiator, and a porogen (for example, ethanol) at various concentrations. UV light (for example, at a wavelength of 365 nm) is projected through a patterned photomask to the polymer solution, preferably through a magnifier such as a lOx objective. The shape of the hydrogels polymerized in this manner depend on the photomask’s shape, and the size is dependent on the objective scaling factor in which the photomask’s shape is reduced by approximately 3.9, for example, by using a lOx objective. Thickness of the resulting polymerized hydrogel frame is defined by the height of the slit channel. For example, because of an oxygen inhibition layer of about 2.5pm at the PDMS coated glass substrate and the slit top channel,

[0033] a 70pm channel height yielded a hydrogel approximately 65pm in thickness, of a shape defined by the photomask’ s shape (for a square photomask, the hydrogel will have a square shape), shown in Figure la, left bottom. The hydrogel exhibited a brownish color because of a porous microstructure developed during the photopolymerization induced separation.

[0034] This porous microstructure caused light scattering, resulting in a reduced transmitted light.

[0032]

[0076] We found that light transmittance of different hydrogel samples differed (optical switching behavior) at different ethanol concentrations and UV exposure times. A minor optical switching behavior between wet and dry hydrogel samples was observed with 0% (control) and 25% ethanol for all UV exposure times (0.37, 0.75, 1.50, and 3.00s) (Figures lb and 1c). As the ethanol volume percent increases from 25% to 50%, the intensity of the brownish color was clearly enhanced in wet samples at increasing UV exposure times (Figure lb), and its color becomes either transparent or dark brown in the dry states (Figure 1c). Similarly, for 80 vol% ethanol, the intensity of the brown color was further enhanced as the UV exposure time was increased from 0.37 s to 3.00s for wet hydrogel samples (Figure lb), and the color intensified in the dry states (Figure 1c). Upon quantifying the degree of optical switching between wet and dry states for the control sample and the 25 vol% ethanol, the light transmittance was found to be slightly decreased by less than 10% between the wet and dry states (Figure Id). For 50 vol%ethanol, the hydrogel samples polymerized at 0.37s and 0.75s show a slight transmittance increase from 75% to 90% between the wet and dry states, respectively (Figure Id).

[0077] For exposure times of 1.50s and 3.00s using 50 vol% ethanol, the light transmittance dropped significantly from 75% (wet state) to 25% (dry state). With 80 vol% ethanol, no optical- switching between wet and dry was observed for the hydrogel sample for exposure times greater than 0.75s. Without being constrained by any particular theory, this optical-switching behavior between wet and dry hydrogel samples was believed to be because of their unique physical characteristics, attributed to the combination of porous microstructures within the swellable hydrogel, and the light scattering within these microporous structures of hydrogel material. The presence of ethanol, as the porogen, during photopolymerization was believed to induce phase separation that creates highly porous microstructures in the resulting hydrogels. [32, 34] The polymerization-induced phase separation lead to polymer-rich and solvent-rich regions.

[0035] The polymer-rich phase lead to solid material, while the solvent-rich phase leads to porosity. The porosity profiles of the resultant hydrogels are believed to be dependent on 1) UV dose which controls the degree of polymerization in polymer-rich phase and 2) the ethanol concentration which determines the porosity by controlling the spatial distribution of the solvent-rich phase. Thus, the resulting highly porous hydrogels undergo rapid changes in porous structures during swelling / de-swelling of solvent which leads to opacity changes. On the other hand, at lower concentrations of ethanol, the resulting hydrogels are non-porous (Figure 2a and 2b), which causes no observable light scattering and deswelling / swelling because of no significant porous structure change. Thus, they are optically transparent both in dry and wet states. However, at higher ethanol concentrations and UV dose, highly porous microstructures (Figure 2c and 2d) were developed which evoke light scattering. Also, the de-swelling / swelling of hydrogels, depending on porosity profile, induced pore size shifting which enabled the optical-switching to be observed between dry and wet states. This de-swelling / swelling property of hydrogel can be utilized in stimuli-responsive smart materials.

[0036] Since hydrogel network is crosslinked, its physical structure remained intact during repeated swelling / de-swelling. Thus, utilizing the swelling / deswelling mechanisms of the hydrogel, the reversible optical switching could be exploited for anti-counterfeiting applications. The hydrogel membrane (figure 1) displayed reversible optical switching with a drop of ethanol (see Fig. 7). At higher UV exposures, greater than 1.50s with 80% ethanol, the transmittance reaches a threshold of 20% transmittance in bothdry and wet states. According to the scanning electron microscope (SEM) images of the hydrogel frames taken at exposure time of 1.50s with different ethanol concentrations (Figure 2), the microporous structures were clearly well-developed at ethanol concentration above 50%. The dried hydrogel samples with 80% ethanol concentration at exposure time of 1.50s became a powdered sample.

[0078] One-step process

[0079] Utilizing these tunable optical properties, we first fabricated hydrogel frames with various shapes encoding features of photomasks (Figure 3). We used a prepolymer composition of 50% ethanol with 1.50 s UV exposure. The hydrogel frames, where photopolymerization and photoinduced phase separation simultaneously occur, were opaque, with void shape features depending upon the UV projection defined by the photomasks. However, with a longer UV exposure time, for example, 1.50 s, free radicals diffused from the UV projected zone to the non- UV projected zone (the void shape feature) and initiated free radical polymerization. This free- radical polymerization outside the UV projected zone formed a thin hydrogel film, shorter than the hydrogel frame thickness.

[0037] This hydrogel film outside the UV projected zone was seen in Figure 3c, where bull eyes and noses are intact. Without the thin hydrogel film developed by the free-radicals diffusion-initiated polymerization, the eyes and noses should be disintegrated from the hydrogel frame. Thus, the disclosed method can be used to embed complex discrete patterns in a one step and high throughput fabrication process, by controlling the hydrogel thin film development. These shape-encoded hydrogels can be further miniaturized by using higher magnification objectives and photomasks with smaller feature sizes. With the same photomask dimensions (Length (mm) / Width (mm): 1.5 / 1.5) designed for the 10X objective, we can make unlimited 2D-extruded shaped hydrogel frame size down to -380 microns with microscopic motifs, which can be reduced by a scaling factor of -7.8 and - 15.6 for 20X and 40X objectives, respectively. The uniformity in the hydrogel thickness is only limited by the optical resolution and the microscope objective’s depth of field and focal depth. The depth of field can be visualized as the length over which the light emanating from the objective will have a uniform path direction.

[0033] The focal depth is the distance the image plane can be displayed from the objective plane with acceptable tolerance.

[0038] In general, the higher the magnification, the shorter the depth of field. By adopting stop flow lithography,|39‘40]this fabrication process can berepeated in an automated flow cycle: flow the prepolymer solution, stop the flow, project UV, and resume the flow for a high throughput production. Using photomasks with multiple features and at higher magnification objectives, the UV exposure times can be reduced by 1 / 4 and 1 / 16 for 20X and 40X objectives, respectively, while facilitating a high throughput production of miniaturized hydrogel frames with microscopic shape-encoded features.

[0080] In one embodiment, a smart-phone can be used to scan shape-encoded hydrogels and identify unique products using an image-processing algorithm. By tracking the optical transmittance, the optical technology coupled with an image processing algorithm can be utilized to decode the microscale shape-encoded hydrogels. The light beam can easily pass through the void features in the shape-encoded hydrogels without any hindrance. Accordingly, this contrasting light-transmitting property allows the user to decode the shape-encoded hydrogels.

[0081] Two-step process

[0082] To develop hydrogel materials with multi-layer protection, we embed the motif within a transparent and swelling-controlled hydrogel frame using a two-step polymerization method (Figure 4a). The swelling of the hydrogel frame is controlled by the hydrogel frame porosity, which is a function of porogen (ethanol) concentration and UV exposure time. Firstly, we fabricate a transparent and rigid hydrogel frame with a void feature using a pure PEGDA 250 prepolymer solution. This is followed by a porogen wash for 10-15 min, the designed motif is polymerized with different optical properties by controlling the ethanol concentration or / and UV exposure times. For instance, we fabricate a square hydrogel frame with a void T-shaped feature in the center area using the pure PEGDA 250 prepolymer solution at a UV exposure time of 1 s at 80% intensity (equivalent to 11.2 W / cm2) (Figure 4a). Then, we introduce the second prepolymer solution of 50 vol% ethanol and form the opaque motif at a UV exposure time of 3 s with 80% UV light intensity (11.2 W / cm2) after aligning the second photomask and matching its feature to the void region of the initially polymerized square frame (Figure 4a). The photomask containing the void feature was larger by 20% to ensure that the feature was polymerized and remained intact within the frame’s void space. This multistep approach can enable geometrically selective motifs that show different transmittance via sequential polymerization with the prepolymer solutions of various ethanol concentrations and UV exposure times. This multistep approach can be used for encryption. The hydrogel is displayed white in reflective mode andbrown in transmissive mode. The same hydrogel will display different intensity of brownish hue depending upon the light transmittance of the hydrogel synthesized at various UV exposure times and ethanol concentration. It is noted that it is commercially desirable to show opacity change under both reflective and transmissive modes, as has been done here, for “enhanced” use as an anti-counterfeiting label on certain types of products. The spatial transmittance profile of the hydrogel frame can be tracked using an image processing algorithm for optical transmittance analysis. Using a 50% ethanol solution, the square and triangular features were polymerized sequentially at different exposure times (Square: 3s and triangular: Is) by using the appropriate photomasks. The spatial transmittance profiles vary as a function of UV exposure time for the same ethanol concentration (Fig 4b) as previously observed (Fig 1). The time required to wash the PDMS channel with sufficient pure PEGDA oligomers or porogen solution between each polymerization step determines the throughput of motifs-embedded millimetric frames. For instance, it takes at least 10 min to wash away the trapped PEGDA oligomers within the void region of the square frame before the second polymerization step. A parallelized, multi-layer design with multiple inlets would facilitate a high throughput production with minimal washing time between the introduction of each fluid.

[0083] We also noted the dual optical properties of the encoded motif based on light scattering effect (Figure 5a). The T-shaped motif within the transparent square frame appears to be white color in the reflected mode (Figure 5a), light brownish color in the transmittance mode (Figure 5b), as presented on glass substrates. In the transmissive mode, the T-shaped motif should be viewed exactly 180 degree away from the light source while, in reflective mode, the motif can be viewed at any angle other than 180 degree. We also fabricate uniquely shaped motifs within various 2D-shaped hydrogel frames: T-shaped motif in a square frame (Figure 5c), a thunder motif in a heart frame (Figure 5d) and heart motif in a buddha frame (Figure 5e), presented on various objects: eye glasses (Figure 5c), pharmaceuticals products (Figure 5d) and currency (Figure 5e). Since the encoded hydrogels were based on biocompatible polymers, they can be more safely used on edible and pharmaceutical products compared to the up conversion crystals.

[0084] To facilitate high throughput production, it would be desirable to shorten or minimize the relatively lengthy porogen washing period. Accordingly, we tuned the optical properties of the frame and the motif using the same porogen solution at different UV exposure times after properalignment with appropriate photomasks. Using this fabrication method, we were able to exploit the shape-shifting and opacity-changing effects of the optically tuned motifs under the hydrodynamic stimuli. We use our one-step fabrication in combination with the thin film development to reveal a QR pattern (Figure 6a) in a square hydrogel frame. In the dry state at time t = 0 s, the whole hydrogel frame appears in black. Once a drop of ethanol is introduced, t > 05, while the hydrogel swells, the QR pattern reveals and at t = 10 s when the swelling stops, the QR code displays full. The thin film, formed in the non UV-projected area, was more transparent than the frame because of the thickness difference. Thus we have created a simple, facile, and swelling-controlled opacity-shifting behavior which does not involve the use of photonics crystals or any nanoparticle materials. With the UV exposure time of 3s, the free radical polymerization outside the UV-projected area enabled the formation of thin film between the QR micropatterns. The thin film appeared black and light brown in dry and wet state, respectively. Due to the contrasting refractive index of the polymerized feature and thin film at wet state, the QR pattern was distinctly visible upon a drop of liquid. Using this technology, any vivid micro shapes and micro patterns can be revealed with a drop of liquid. The color-shifting behavior is completed within 10s.

[0085] We then used our two-step method which evokes a shift in light transmission and shape (Figure 6b). To enable this optical-shifting and shape-transformation, we fabricated a square porous hydrogel with 50% ethanol at a UV exposure time of 0.37s. Inside the void spaces (i.e. buddha feature) of the porous hydrogel frame, the buddha pattern was later polymerized at a UV exposure time of 3 s. At dry state, the hydrogel frame and buddha feature lost their original shapes due to shrinkage and appear transparent and opaque. With a drop of ethanol, the hydrogel starts swelling, and the frame and the buddha feature undergo a shape transformation while the frame remains transparent, and the Buddha displays several opacity changes at different time periods (0 < t < 15s). For better shape transformation and revelation, we found that use of an over-sized feature photo mask was advantageous. At time t=0, an oversized, black feature is visible. This shape is then transformed into a buddha image at time t=ls while undergoing colorshifting at t=3s.

[0086] The shape morphing motifs embedded in hydrogel can be stamped onto currency, security documents, and customer products safely, and provide anti-counterfeiting verification.They will give the opportunity for the customers to validate the genuine products with a drop of liquid. The hereindescribed hydrogel can also be used in a dried form, and activated by liquid for anti-counterfeiting verification.

[0087] We also used maskless lithography or digital photomask techniques to improve the throughput time of manufacturing of optically-tuned motifs within a hydrogel frame using the two-step polymerization process . For example, we found that using a digital photomask allowed us to display different photomasks (similarly to the first and second photomasks, described above) without needing to remove the first photomask and replace it with the second photomask, which made photomask alignment much more accurate. In addition, the use of digital photomasks allowed us to very precisely control the UV exposure times, rapidly change the shape of the photomask, and allowed for more flexibility and rapidity in photomask design.

[0088] For the two-step polymerization process, without changing the porogen second concentration, we used a digital photomask to achieve optically contrasting QR hydrogel tags by exposing the UV through the first digital mask (Figure 8a) and second digital mask (Figure 8b) at different UV durations and UV intensities, as per the manufacturing requirements in terms of transparency and brownish hue of the primary and secondary hydrogels, respectively. Using the same prepolymer solution with porogen concentration at 70 vol% ethanol, the UV was exposed through a first digital mask for 0.15s while the second mask was displayed immediately thereafter for 0.5s to yield a transparent primary hydrogel (Figure 8c) with a much darker brownish hued secondary hydrogel (Figure 8d). Similarly, while keeping the porogen concentration of the prepolymer solution the same at 50% (v / v), the UV was projected through the first digital and second digital masks for 0.25s at 7 W / cm2and Is at 20 W / cm2, respectively, to yield a transparent primary hydrogel (Figure 8e) with less brownish hued or optically contrasting secondary hydrogel (Figure 8f).

[0089] In summary, we have developed a simple and facile method to tune the optical properties of the stimuli-responsive hydrogel by eliminating the need for a complex photonics fabrication or a long-lasting self-assembly of polymeric particles. By exploiting a common solvent, ethanol as a porogen, biocompatible hydrogel, Poly (ethylene glycol) diacrylate, and UV-induced free radical polymerization using microfluidics, we have produced swellable microporous structures within millimetric hydrogels that undergo optical shifting, which is dependent on the ethanolconcentrations (0 to 80%) of the prepolymer mixture and UV exposure times (0.37s to 3.00s). The herein disclosure provides a controllable, customizable, and a user-friendly approach for the fabrication of biocompatible, next-generation anti-counterfeiting technology in which the motifs embedded within the hydrogel frame undergo swelling-dependent optical switching and shape morphing with a drop of ethanol. The hereindisclosure also provides, using a one-step polymerization, a technique that exploits the diffusion of free radicals outside the photomask transparency region at high UV exposure times (3.00s) to fabricate a covert label, such as a QR pattern, which can be revealed with hydrodynamic stimuli. We disclose a one-step fabrication of shape-encoded hydrogel in which the discrete features remain intact due to the diffusion of free radicals outside the photomask transparency region. Finally, we demonstrate the use of maskless lithography in the two-step polymerization of QR hydrogel with a transparent primary hydrogel and brownish hued secondary hydrogel. By simply varying the porogen concentration of the prepolymer solution, UV duration, and UV intensities, we are to achieve QR hydrogel taggants with contrasting optical switching properties.

[0090] The herendescribed process provides an innovative method to fabricate next-generation anti-counterfeiting technology by utilizing the optical tuning parameters such as ethanol concentration and UV exposure times.

[0091] Example 1 : Microfluidic device fabrication

[0092] All chemicals were of analytical grade and used without further purification: PEGDA (Aldrich, Mn= 250 Da), 2 -hydroxy-2 -methylpropiophenone (photo-initiator, Aldrich), ethanol (Greenfield Global).

[0093] The master mold for the microfluidic devices was fabricated using SU-8 2050 photoresist (Kayaku Advanced Materials) spin-coated for a single layer thickness of 70 / im onto a 10 cm diameter silicon wafer (ID 452, University Wafer Inc.). The spin-coated wafer was pre-baked at 65 °C and 95 °C prior to exposure to unfiltered UV light through the custom photomask, designed in AutoCAD and printed at a resolution of ~1 pixel per micron (25 000 dpi, 8 / <m minimum resolution limit) by CAD / Art Services (OR, USA), using NxQ 4006 mask aligner ( Neutronix Quintel). The master mold was then inspected using a Contour GT-K profilemeter (Bruker) if desired mold thickness was achieved. The mold was then salinized with a few drops oftrichloro(l,l,2,2-perfluoocytl) silane (Aldrich) placed in an aluminum cup inside a desiccator chamber for about 2 hours. Poly dimethylsiloxane (PDMS, Sylgard 184, Dow Corning) precursor with an elastomer to base ratio of 10: 1 was poured onto the surface of the silicon wafer containing the master mold and baked at 85 °C for 30 minutes for the first curing of PDMS precursor. The partially cured PDMS coated glass slides and PDMS channels were baked in a 65 °C oven for 8 min and 18 min, respectively. After bonding the partially cured PDMS coated glass and the PDMS channel, it was left in 65 °C oven to cure overnight. The methods were further described elsewehere.

[0041]

[0094] Example 2: Two-step synthesis of motif-encoded hydrogel film.

[0095] Using a photomask, the hydrogel frame was synthesized using a prepolymer solution (95% (v / v) PEGDA (Mn = 250), 5% (v / v) photo-initiator). The porogen solution (45% (v / v) PEGDA (Mn = 250), 50% (v / v) ethanol, 5% (v / v) photo-initiator) was injected into the PDMS channel at 25 pL / min for about 10 minutes to ensure the void features within the hydrogel film are filled with porogen solution. By using another photomask in the field-stop of the microscope, the motif feature was polymerized at UV-exposure times of 3.00s to induce the opacity change.

[0096] Example 3: One-step synthesis of shape-encoded hydrogel film.

[0097] Using a photomask, the hydrogel frame was synthesized using porogen solution (45% (v / v) PEGDA Mn = 250), 50% (v / v) ethanol, 5% (v / v) photo-initiator) at UV-exposure times of 1.50s to induce the opacity change in the transmittance mode.

[0098] Example 4: Shape morphing hydrogel for revealing hidden motifs.

[0099] Using a photomask, the hydrogel frame was synthesized using porogen solution (45% (v / v) PEGDA (Mn = 250), 50% (v / v) ethanol, 5% (v / v) photo-initiator) with UV exposure time of 0.37 s. By using another photomask in the field-stop of the microscope, the motif feature was polymerized at UV-exposure times of 3.00s to induce the opacity change using the same porogen solution.

[0100] Example 5: Two-step synthesis of quick response (QR) code hydrogel taggants using digital photomask (maskless lithography)

[0101] A prepolymer solution containing 30 % (v / v) PEGDA Mn = 250), 70% (v / v) ethanol and 0.6% (w / w) photo-initiator was exposed to UV light for 0.15s at 20W / cm2using a first digital photomask to create the primary hydrogel with void spaces (i,e, spaces with unpolymerized prepolymer solution). Using a second digital photomask (in this case, an inversion of the first digital photomask), the same porogen solution was exposed to UV light for 0.5s at 20W / cm2to create a more contrasting optical features at the void regions of the primary hydrogel (See Fig. 8).

[0102] Experimental set-up for Examples 1-4

[0103] Using a 365-nm wavelength LED collimator source (Type-H, Mightex, Toronto, ON, Canada) with an LED controller (BLS-13000-1E, Mightex, Toronto, ON, Canada), the UV exposure time and intensity were carefully adjusted to control the propagation of UV light through an Axio Observer inverted microscope (Carl Zeiss, Jena, Germany) with a lOx objective (Ec plan-Neofluar, Carl Zeiss, Jena, Germany). When a UV light was projected onto a 70 jtm a microfluidic channel containing the prepolymer solution (95% (v / v) PEGDA (Mn = 250), 5% (v / v) photo-initiator), a hydrogel frame of certain opacity was created based on the dimensions of the photomask. To generate motif features of different opacity, a porogen solution (45% (v / v) PEGDA Mn = 250), 50% (v / v) ethanol, 5% (v / v) photo-initiator, 2 -Hydroxy-2 - methylpropiophenone was injected using single-drive syringe pump (Pump 33 DDS and Pump 11 Elite, Harvard Apparatus, Holliston, MA, USA) via tubing (Tygon S3 E-3603, Saint-Gobain, Malvern, PA, USA) at 25 tL / mij? for about 10 minutes to ensure the void features within the hydrogel frame were filled with porogen solution. To avoid the hydrogel frame from sliding due to the injection of porogen solution, the microfluidic channels contained three rectangular bars which allowed 50% ethanol monomer stream to flow through these bars and exit via inlet port, but stop the hydrogel frame from sliding. The photomasks containing the features that were used were larger by 20% to ensure that the feature was polymerized and remained intact within the frame’s void space. We utilized the Image J software to calculate the transmittance, which is the ratio of the mean gray scale values of the hydrogel to its background mean gray scale values, for different ethanol concentration and UV exposure times. Scanning Electron Microscopy (SEM, Model JSM-6380 LV) was used to characterize the microporous structure of the hydrogel. Upon changing the ethanol concentration in the prepolymer composition from 0 to 80% with variousexposure times (0.37, 0.75, 1.50, and 3.00s), we were able to tune the light transmittance of hydrogel frames (Figure la). We measured the light transmittance by taking the ratio of the mean greyscale value of images of the hydrogel frames in wet (Figure lb) and dry (Figure 1c) states to the mean greyscale value of the background. All images are taken using our inverted microscope with an Axio camera and 10X objective. The wet state images of the polymer samples were taken within the PDMS slit channel in the presence of the prepolymer solution. In contrast, the dry samples weere taken out of the channel after washing with pure ethanol (-99%) and left in the oven at 65°C for 12 hours.

[0104] Experimental set-up for Example 5

[0105] Using a 365-nm wavelength LED collimator source (Type-H, Mightex, Toronto, ON, Canada) with an LED controller (BLS-18000-1E, Mightex, Toronto, ON, Canada) and Pattern Illuminator (Polygon400-G, Mightex, Toronto, ON, Canada), the UV exposure time and intensity were carefully adjusted to control the propagation of UV light through an Axio Observer inverted microscope (Carl Zeiss, Jena, Germany) with a lOx objective (Ec plan- Neofluar, Carl Zeiss, Jena, Germany). After careful calibration of the UV projection area through a Polygon400-G using a lOx objective, the duration of first and second digital photomasks, UV exposure times through first and second digital photomasks, and its UV intensities were programmed using Poly Scan software (Version 4). When a UV light was projected onto a 70 pm microfluidic channel containing the prepolymer solution (30% (v / v) PEGDA (Mn = 250), 70% (v / v) ethanol 0.6% (w / w) photo-initiator, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide), a primary hydrogel frame of certain opacity was created based on the dimensions of the first digital photomask. Accordingly, the porogen solution was exposed to UV light through the first digital photomask at 0.15s at 20 W / cm2. To generate motif features of different opacity, the same porogen solution was exposed to UV light at 0.5s at 20 W / cm2using the second digital photomask. Similar contrasting QR hydrogel taggants were also synthesized using different porogen concentration (i.e 50% (v / v) PEGDA (Mn = 250), 50% (v / v) ethanol 0.6% (w / w) photoinitiator, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide) by exposing the first digital photomask and second digital photomask for 0.25s at 7 W / cm2, and for Is at 20 W / cm2. All images are taken using an inverted microscope with an Axio camera and 10X objective. Thedisplayed image is the wet state images of the polymer samples taken within the PDMS slit channel in the presence of the prepolymer solution.

[0106] References:

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Claims

Claims:

1. A process for the manufacture of a hydrogel with optical switching properties, comprising:(a) adding a prepolymer solution to a receptacle, said prepolymer solution comprising a prepolymer, a photoinitiator, and a porogen;(b) exposing the prepolymer solution to UV light to initiate polymerization of the prepolymer into a hydrogel.

2. The process of claim 1 wherein the porogen is ethanol.

3. The process of claim 1 wherein the porogen is a solvent that creates a photopolymerization- induced phase separation.

4. The process of claim 3 wherein the solvent is present at a concentration that induces submicron porous structures inducing light scattering.

5. The process of claim 1 wherein the porogen is one or more of ethanol, toluene, hexane, cyclohexanone, 2-ethylhexanol, p-xylene, n-heptane, poly(ethylene glycol), polymethylmethacrylate in chloroform, low molecular weight PVA, and 1,3 -benzenedib or onic acid.

6. The process of claim 2 wherein the ethanol is present in a concentration of 50% v / v.

7. The process of claim 2 wherein the ethanol is present in a concentration between 50% v / v and 75% v / v.

8. The process of any one of the preceding claims wherein the prepolymer is poly(ethylene glycol) diacrylate.

9. The process of any one of the preceding claims wherein the photoinitiator is selected from one or more of 2 -Hydroxy-2 -methylpropiophenone, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and lithium phenyl-2,4,6-trimethylbenzoylphosphinate.

10. The process of any one of the preceding claims wherein the receptacle comprises a PMDS microfluidic channel comprising a PDMS coated glass substrate and a PDMS slit top channel.

11. The process of any one of the preceding claims wherein the exposing of the prepolymer solution to UV light is through a mask with a two dimensional shape, resulting in the hydrogel having said two dimensional shape.

12. The process of claim 11 wherein the mask is a physical photomask or a digital photomask.

13. The process of claim 11 or 12 wherein the exposing of the prepolymer solution to UV light through said mask is through a magnification lens such that the two dimensional shape of the hydrogel is of a different scale than the two dimensional shape of the mask.

14. A process for the manufacture of a hydrogel with optical switching properties, comprising:(a) adding a prepolymer solution to a receptacle, said prepolymer solution comprising a prepolymer, a photoinitiator, and a porogen, said porogen present at a first concentration;(b) exposing the prepolymer solution to UV light through a first mask having a first two dimensional shape to initiate polymerization of the prepolymer into a primary hydrogel having a two dimensional shape corresponding to said first two dimensional shape;(c) changing the prepolymer solution to a second prepolymer solution comprising a prepolymer, a photoinitiator, and a porogen, said porogen present at a second concentration, or alternatively changing the concentration of the porogen from the first concentration to a second concentration, said second concentration being different than the first concentration;(d) exposing the prepolymer solution to UV light through a second mask having a second two dimensional shape to polymerize the prepolymer into a secondary hydrogel, said second two dimensional shape being different than the first two-dimensional shape and optionally being complimentary thereto, said secondary hydrogel and primary hydrogel forming the hydrogel, wherein said secondary hydrogel and said primary hydrogel optionally overlap, and wherein said secondary hydrogel and said primary hydrogel have different optical properties.

15. The process of claim 14 wherein the first mask and the second mask are each a separate physical photomask.

16. The process of claim 14 wherein the first mask and the second mask are different display configurations of one digital photomask.

17. The process of claim 14 wherein the display configuration of the second mask is a simple inversion of the display configuration of the first mask.

18. The process of any one of claims 14 to 17 wherein the porogen is ethanol.

19. The process of claim 18 wherein the first concentration is about 0% v / v.

20. The process of claim 19 wherein the second concentration is between about 50% v / v and about 75% v / v.

21. The process of claim 20 wherein the second concentration is about 50% v / v.

22. The process of claim 18 wherein the first concentration is about 50% v / v and the second concentration is about 75% v / v.

23. The process of any one of claims 14 to 22 wherein the prepolymer is poly(ethylene glycol) di acrylate.

24. The process of any one of claims 14 to 23 wherein the photoinitiator is 2 -Hydroxy-2 - methylpropiophenone, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, or lithium phenyl- 2,4,6-trimethylbenzoylphosphinate.

25. The process of any one of claims 14 to 24 wherein the photoinitiator induces free radical polymerization.

26. The process of any one of claims 14 to 25 wherein the receptacle comprises a PMDS microfluidic channel comprising a PDMS coated glass substrate and a PDMS slit top channel.

27. The process of any one of claims 14 to 26 wherein the exposing of the prepolymer solution to UV light through said first mask, and / or the exposing of the prepolymer solution to UV light through said second mask, is through a magnification lens.

28. The process of claim 27 wherein the exposing of the prepolymer solution to UV light through said first mask is through a magnification lens of a first magnification, and the exposing of the prepolymer solution to UV light through said second mask is through a magnification lensof a second magnification, wherein the first magnification is different from the second magnification.

29. The process of any one of the preceding claims wherein the exposing to the UV light is at a UV intensity of about 11.2 W / cm2.

30. The process of any one of the preceding claims wherein the exposing of the UV light is at a UV intensity of about 7 W / cm2.

31. The process of any one of the preceding claims wherein the exposing of the UV light is at a UV intensity of about 20 W / cm2.

32. The process of any one of the preceding claims wherein the exposing of the UV light is at a duration of between 0.15 to 1 seconds, preferably between 0.25 and 0.5 seconds.

33. A hydrogel made by the process of any one of the preceding claims, or a dried variant thereof.

34. A hydrogel having optical switching properties, whereby an image is displayed within the hydrogel in the presence a first porogen, and a second image is displayed within the hydrogel in the presence of a second porogen, with the first porogen and second porogen being different porogens or different concentrations of the same porogen, and with the first image and the second image being different images.

35. An anticounterfeiting label comprising a hydrogel of any one of the preceding claims or a dried variant thereof.

36. Use of the anticounterfeiting label of claim 33 on a product, said use comprising the application of the polymerized hydrogel on said product.

37. Use of claim 36 wherein the product is a paper or plastic currency, a coin, eyeglasses, sunglasses, watches, a consumer product, a food, a cosmetic, electronic parts such as a semiconductor, luxury goods such as gold bullion or precious metals, a gemstone, a diamond, or a pharmaceutical.

38. A product comprising an anticounterfeiting label of claim 35.

39. The product of claim 38 wherein the product is paper or plastic currency, a coin, eyeglasses, a watch, sunglasses, a consumer product, a food, a cosmetic, electronic parts such as a semiconductor, luxury goods such as gold bullion or precious metals, a gemstone, a diamond, or a pharmaceutical.

40. A method of determining whether the product of claim 38 is counterfeit, comprising applying a porogen to the product at a location of the anticounterfeiting label and determining whether a color change or shape change occurs in said label.

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