EA045047B1 - MICRONEEDLE, MICROCONE AND MANUFACTURING METHODS USING PHOTOLITHOGRAPHY - Google Patents

Description

Cross reference to related applications

The invention claims the benefit of U.S. Provisional Patent Application No. 62/961,931, filed January 16, 2020, entitled Microneedle, microcone, and photolithography fabrication methods, incorporated by reference in its entirety herein.

Prerequisites for creating the invention

Field of invention

The present invention relates to new photolithography techniques for the production of microstructures, in particular microcones and/or microneedles.

Description of the Prior Art

The development of microneedles has been long established since 1990, and many studies have shown that microneedles have significant advantages in drug delivery compared to oral and injection needle administration. Microneedles typically have a sharper tip than injection needles and are only 10-2000 µm in height, providing the least invasive method of drug delivery. Oral administration is convenient but has the disadvantage of low drug delivery efficiency due to drug degradation and ineffective absorption in the human body. The subcutaneous drug injection method also faces the problem that many drugs cannot penetrate the outer layer of the skin, resulting in low delivery efficiency. However, recent reports have shown that microneedles can painlessly penetrate the skin and deliver drugs to the epidermis and/or dermis layer.

The geometric configuration of the microneedles plays an important role in the insertion pattern and mechanical stability of the microneedles. A sharp microneedle tip with a small tip angle and diameter reduces the insertion force, but increases the possibility of crack formation and loss of stability. A recent study outlined some common types of microneedles based on the tip angle and body of the needle, and their corresponding insertion forces into chicken breast. The study concluded that a needle tip with a 30° isosceles triangle geometric configuration is the optimal needle tip shape as it showed the highest critical force resistance among the four types of geometric configurations, as well as moderate insertion force without crack failure. . However, the 3D formation of indirect geometric configurations, such as curved or conical shapes, requires a layer-by-layer forming process or multiple photomask registration processes, which can potentially increase fabrication time as well as production cost.

Brief description of the invention

The present invention utilizes light diffraction associated with cross-linking of liquids and solids, which forms a light waveguide due to the different refractive indices of uncrosslinked and crosslinked resin, allowing the formation of various types of microconical structures in a fast and straightforward process within 30 minutes, including impact processes UV and extensions.

The proposed microneedle fabrication method is extremely advantageous as different photomask patterns generate different microneedle shapes, including round, star, hexagonal and triangular bases, as well as advanced functional microneedles such as hollow and inclined microneedles. Since the traditional UV lithography approach to fabricate the mentioned microneedles requires multiple UV exposure and alignment processes, the flexible and straightforward fabrication process should definitely be considered as a low-cost, well-designed drug delivery product.

The present invention is broadly concerned with new fabrication methods for creating microneedles and other micron-sized structures with sharp tips. The fabrication process involves upward exposure of a liquid photosensitive resin through a photomask template containing a plurality of holes, such as 200 μm diameter holes, or other shapes. UV light is applied through a photomask template. The irradiated photosensitive resin polymerizes and forms microscopic structures with sharp tips. The refractive index of the material becomes higher compared to the surrounding photosensitive resin in the liquid state. The later refractive index contrast between the solid and liquid resin causes UV light to be reflected at the interface like a light waveguide and sends light to the top of the cone. That is, after the liquid resin becomes solid, the solidified portion acts as a light waveguide to focus the additional transmitted light and form a conical shape (first harmonic microcone). Additional UV exposure causes UV light to be emitted through the apex of the cone to form a small sharp tip. The light is again diffracted through the tip as a second harmonic beam with even more intense impact to form a second conical shape (second harmonic shape). The third cone is created using the same principle with a smaller size due to the lower light intensity. We observed up to the fourth cone during experiments.

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Unlike the previous solid resin technique, which aimed to form vertical sidewalls, the present liquid resin technique provides converging or tapering sidewalls that form microneedle-type structures or structures with inclined sidewall angles. The fabricated structures are rinsed with a solvent to remove unreacted composition. The fabricated structures can be used as microneedles and microprobes.

In one aspect, there is provided a method of making a plurality of microscopic structures with converging tips, the method comprising: providing a substrate having a top surface and a back surface, said substrate comprising a template having open areas configured to allow radiation to pass through the substrate, and continuous areas configured to prevent radiation from passing through the substrate; forming a layer of photosensitive resin in a liquid state on said top surface; exposing said photosensitive resin in a liquid state to radiation through said substrate from a rear surface for a first period of time to produce light-irradiated portions of said photosensitive resin in a liquid state, wherein said light-irradiated portions are cross-linked and/or polymerized to form corresponding resin structures in solid state on said top surface in alignment with said open areas, wherein said solid state resin structures have an increased refractive index compared to said liquid state photosensitive resin structure, whereby each solid state resin structure acts as a waveguide guiding said radiation passing through said open regions of said substrate to a convergence point, thereby forming solid state resin structures with tapering side walls and converging tips; and contacting the coating layer with the solvo system to remove non-light irradiated portions of said photosensitive resin in a liquid state to form a plurality of microscopic solid state resin structures with converging tips on said top surface of said substrate.

Brief description of graphic materials

The publication of a patent or application contains at least one graphic material in color. Copies of this patent publication or patent application with color graphics will be provided by the Office upon request and upon payment of the required fee.

In fig. 1 is a schematic illustration (not to scale) of an exemplary microstructure formation process;

in fig. 2 is an additional illustration showing the conical light profile of UV light propagating through microscopic apertures for the polymerization of a liquid photosensitive resin and the growth of additional harmonic structures over time as more and more energy is propagated through the resin;

in fig. 3 is an image from an experimental test of UV diffraction showing the propagation of light inside a liquid photosensitive resin using a photomask with a pattern size of 200 μm;

in fig. 4 shows several photographs of the manufacture of a microcone using the conditions from the table of Example 1;

in fig. Figure 5 shows SEM images of the second harmonic microcone using a photomask base of 120 µm, 884 µm in height, 50 µm in girth (left image) and a close-up view of the second harmonic cone (right image);

in fig. 6A illustrates the light intensity distribution of collimated UV light with a Gaussian profile after passing through a circular hole photomask;

in fig. 6B illustrates the light intensity distribution with a Gaussian profile after passing through a circular hole photomask with an opaque core to form a hollow shape;

in fig. 7 is a plot of the measured UV intensity (375 nm) at 0.5 inches above the light source, not diffused by glass (dots), diffused by one glass (triangles), and diffused by two glasses (squares);

in fig. Figure 8 shows SEM images of (a) a single microneedle unit and (b) an enlarged view of the tip;

in fig. 9 shows images of a microstructure with several harmonic components: (a) fabricated microneedle, (b) UV light propagation, microstructure array with (c) secondary harmonic component and (d) tertiary harmonic component;

in fig. 10 is an image showing an array of microstructures with different geometric base configurations and heights produced by one simultaneous impact, and the corresponding structured photomask (insert) that was used;

in fig. 11 is a graph of the relationship between the height of the fabricated microneedles and the applied energy, the secondary y-axis refers to the aspect ratio of the corresponding height and time

- 2 045047 impact of the corresponding applied energy;

in fig. 12 shows photographs of microstructures at various applied energies corresponding to (a) 2 seconds, (b) 3 seconds, (c) 5 seconds and (d) 20 seconds of exposure time and the resulting exposure doses;

in fig. 13 shows photographs of (a) a PLA microneedle array fabricated using PDMS micromolding from a micropattern matrix for diffraction lithography; and (b) pigskin with insertion marks;

in fig. Figure 14 shows the results of testing microstructures for displacement by force: the tip of the needle is deformed, while the main part remains strong;

in fig. 15 shows images of a circular array of 3x3 microstructures: (a) conical light profile of UV light propagation, (b) corresponding microneedles (SEM);

in fig. 16 is a graph showing the ratio of the height of microstructures at different applied energies; (secondary x-axis) exposure time; (secondary y-axis) aspect ratio;

in fig. Figure 17 shows SEM images of a microneedle array with different geometric base configurations (inset): (a) round, (b) hexagonal, (c) triangular, and (d) star;

in fig. 18 shows SEM images of (a) a hollow microneedle array and (b) an inclined circular microneedle array;

in fig. 19 shows photographs from the result of the insertion test: a 3x3 PLA round microneedle array of insertion marks on porcine skin, including inserted images of the microneedles before and after insertion;

in fig. 20A shows SEM images from a force displacement test result of a 3x3 PLA round microneedle array: before insertion (top view), broken tip (middle view), and broken body (bottom view);

in fig. 20B is a graph of data from a force displacement test;

in fig. 21 is a graph of the minimum cross-linking energy for surgical guide resin at various light intensities with a 405 nm UV LED;

in fig. 22 is a graph of the measured transmittance of 405 nm UV light through various thicknesses of surgical guide resin;

in fig. 23 is a graph of the measured height of cross-linked resin at different energies;

in fig. 24 is an illustration of an experimental setup for manufacturing a solid microneedle according to Example 4;

in fig. 25 is a graph of the measured height of microneedles at different exposure energies and times;

in fig. 26 shows photographs of microcones and microneedles, with the first, second and third harmonic components corresponding to the indicated impact energy in the previous figure;

in fig. 27A is an SEM image of an array of 20x20 solid perpendicular microneedles;

in fig. 27B is an enlarged SEM image of a single microneedle shown in FIG. 27A;

in fig. 27C is an additional enlarged SEM image of the tip of the individual microneedle shown in FIG. 27B;

in fig. 28 is an illustration of an experimental setup for manufacturing a hollow microneedle according to Example 4;

in fig. 29 is a photograph of an array of 271 units of hollow microneedles with a base diameter of 280 µm and a height of 550 µm - inserted as an image of the photomask ring template used;

in fig. 30 is a photograph of a close-up image of three different hollow microneedles shown in FIG. 29, showing high uniformity of shape profiles throughout the array;

in fig. 31 shows the result of the injection test and the inserted SEM image of the used microneedle array;

in fig. 32A shows data from the force displacement test of perpendicular microneedles in Example 4;

in fig. 32B shows images of perpendicular microneedles shown in FIG. 32A showing (a) to; (b) needle tip breakage; and (c) failure of a major part;

in fig. 33A shows data from a force displacement test on carbon microneedles in Example 4;

in fig. 33B shows images of the carbon microneedles shown in FIG. 33A showing (a) to; (b) needle tip breakage; and (c) detachment from the substrate;

in fig. 34A shows data from applying in-phase and out-of-phase force to carbon microneedles in Example 4; and in fig. 34B shows images showing the direction of application of force for the out-of-phase test and the in-phase test.

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Detailed description

In more detail, referring to FIG. 1, the process involves providing a generally flat substrate 10 having a top surface 12 and a back surface 14. The substrate 10 is typically transparent, substantially transparent to allow activating radiation to pass through the substrate 10. Suitable substrates include glass, fused silica, polymers, or plastics ( acrylic compounds, organic glass, etc.) etc. The substrate 10 further includes a template (eg, a photomask) having open areas 16 configured to allow radiation to pass through and solid (opaque) areas 18 configured to prevent or block radiation from passing through. The template may be formed as a whole as part of the substrate itself, as shown in FIG. 1, or the template may be a separate structured layer adjacent to the top surface and/or back surface of the substrate. In one or more embodiments, the pattern may comprise an array of spaced holes (windows) distributed over the surface. It should be understood that the geometric shape and size (eg, width or diameter) of the holes can be selected as desired to generate microstructures having the desired geometric shape, as will be discussed in more detail below. Typically, in the context of the present invention, the holes have a microscopic size, which means that they have a maximum size of up to 1000 μm, where the size refers to the maximum dimension from edge to edge (for example, diameter in the case of round holes, maximum width in the case of rectangular holes, or tip to tip in the case of stars).

As shown in FIG. 1(B), the photosensitive resin 20 in a liquid state is then applied to the top surface 12 of the substrate 10 to form a coating layer thereon. The photosensitive resin 20 is preferably applied at a thickness (measured from the top surface 12 of the substrate 10) that is greater than the desired height of the structures to be formed. Typically, the thickness of the coating layer is in the range from about 50 μm to about 9 mm. As used herein, resin refers to various monomer, oligomer and/or polymer compositions that typically contain monomers, oligomers and/or polymers dispersed in a solvo system along with optional photoinitiators. Such photosensitive resins are known in the art, including compositions traditionally used for negative tone photoresists in microelectronics fabrication, as well as resins for 3D printing. Examples of resins include various epoxy resins, acrylates, polyurethanes, methacrylate oligomers, monomers, or polymers, urethane methacrylates, diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide, bisphenol A glycidyl ether novolac (trade name SU-8), etc. P. The viscosity of the composition can be adjusted using solvents such as y-butyrolactone (GBL) or propylene glycol methyl ether acetate (PGMEA), isopropyl alcohol (IPA), and the like. For example, the viscosity of the resin can be adjusted at will to change the tip shape of the structure. For example, less viscous liquid resin tends to form a smaller base angle and a larger tip angle, while those with higher viscosity tend to form a larger base angle and a smaller tip angle. However, virtually any light-colored liquid photoreactive resins can be used, including those to which photoinitiators have been added, such as acrylic and/or methacrylic acid esters with photoinitiator (e.g., phenones such as benzophenones, acetophenones, as well as phosphine oxides, phosphinates, and etc.). Photoinitiators are commercially available, including the trade name Irgacure, as well as triarylsulfonium salts (eg, Cyracure UVI, Union Carbide Corp.). Photosensitive resins of plant origin are also commercially available. In some embodiments, a clear resin is used. In some embodiments, a translucent resin may be used. In some embodiments, the resin may be opaque and come in any number of available colors.

The resin is then exposed to activating radiation of a suitable wavelength and energy intensity. As shown in FIG. 1(C), the substrate 10 and the resin 20 are affected from the rear side of the substrate in a bottom-up direction. That is, the substrate 10 with the resin layer 20 is placed above the radiation source (light) so that the radiation source acts on the back side of the substrate and is transmitted through the substrate from the back side to the top surface and then into the photosensitive resin. It should be understood that the entire structure can be inverted and still be considered from the bottom up, as long as the radiation source is applied to the structure from the rear side of the substrate, as described.

Preferably, the radiation source includes a collimating lens that directs the radiation such that the direction of propagation of the energy (light) flow is parallel and enters the substrate at an angle of incidence perpendicular to the back surface of the substrate, and accordingly, the photosensitive resin. As radiation passes through the open areas of the template, it then penetrates and propagates through the photosensitive resin layer generally away from the top surface 12 of the substrate 10, creating light-irradiated portions and non-irradiable portions of the resin layer. In particular, diffraction occurs with a spread of radiation intensity such that the central region of radiation in each hole has a higher intensity, which gradually decreases near the edge of the hole. It should be understood that most resin may receive some dose

- 4 045047 exposure to radiation, so the non-irradiated parts simply refers to those parts of the resin layer that have received an insufficient dose to induce cross-linking and/or photopolymerization. Diffracted radiation polymerizes the liquid photosensitive resin after the energy of the propagated radiation in the light-irradiated parts accumulates above the threshold polymerization energy of the photosensitive resin at the time of exposure to a constant radiation intensity. The light-irradiated parts are cross-linked and/or polymerized so that the liquid state resin is converted into a solid state resin structure 22 in these parts. This transition is accompanied by a change in the refractive index of the resin as it crosslinks and/or photopolymerizes. This change in refractive index similarly changes the propagation path of the radiation (i.e., controls the beam profile) as it travels deeper into the resin layer. In particular, the difference in refractive index between the solid-state resin structures 22 and the surrounding liquid-state resin creates a barrier to interfacial diffraction, thereby limiting and directing the radiation propagation path. In view of this, the resulting solid state resin structures 22 have a generally tapered structure in which the base of the cross-linked and/or photopolymerized structure is larger than the top of the structure (also called the tip of the structure). Such structures are initially defined by a first height (h1), measured from the top surface of the substrate to the tip.

As radiation induces further cross-linking and/or photopolymerization in adjacent areas of the resin layer, these cross-linked and/or photopolymerized areas then act as a waveguide or lens to limit and further focus the radiation, with the cross-section of the radiation path decreasing with propagation distance from the light source, converging at a point (for example, the radiation beam becomes self-focusing). The radiation exhibits centric radiation intensity, where the intensity is higher through the center of the hole than at the edges, allowing it to propagate further into the resin layer. As illustrated in FIG. 1(D), self-focusing of radiation and subsequent cross-linking and/or photopolymerization in adjacent areas leads to further elongation of the tip of the structure. With additional exposure time, the resulting structures 22' can be defined by a second height (h ₂ ) that is greater than the first height (h1).

As illustrated in the worked examples, the height and shape of the cone can be changed by the applied energy in terms of radiation intensity and exposure time. Greater impact creates secondary and tertiary harmonic structures as the first solidified cone shape acts as light channels or metastate lenses to create a secondary ellipsoidal shape at the apex of the initially formed cone, as illustrated in FIG. 2. As further illustrated in the worked examples, the microstructures may have side walls with different taper angles. That is, diffraction of radiation as it propagates through the resin can result in sidewalls with variable tilting and deflecting angles, resulting in more diamond-shaped tips instead of a stable point. The term taper as used herein covers all structures that are relatively tapered from base to tip (highest point or apex), and preferably a tip where the base is wider than the tip, and is not limited to conical or pyramidal shapes having a stable lateral taper. walls. Advantageously, these complex shapes and microneedles can be formed by simply extending the exposure time and an appropriate dose of energy applied to the resin using a single exposure, meaning that the exposure step does not need to be started and stopped to change the position of the substrate or photomask or apply radiation from a different angle etc. Instead, the exposure step in the proposed method is continuously applied until the desired shape is formed.

It should be understood that different photosensitive resins have different crosslinking energy requirements. In addition, radiation exposure instruments have different capabilities related to light intensity. Typically, the applied energy or dose (mJ/cm ² ) represents the most important parameter for crosslinking calculations. Dose = intensity (mW/cm ² ) x time (seconds). Thus, at higher light intensities, the exposure time can be reduced to achieve the same applied energy (dose). Likewise, at lower intensities, longer exposure times can be used to achieve the required energy dose. Typically, the exposure wavelength may be in the range of 300 to about 450 nm for a period of time from about 1 second to about 1 hour, preferably from about 10 seconds to about 30 minutes. Typically, the energy dose applied will be in the range of 5 to approximately 100,000 mJ/cm ² . It should also be understood that dose information may be publicly available or may be determined experimentally to calibrate the manufacturing process for a particular selected resin without departing from the spirit of the invention.

Once the desired structure 22' has been formed, the structures can be obtained by washing the substrate with a suitable solvo system to remove uncrosslinked or unpolymerized resin remaining on the substrate. Suitable solvents include isopropanol (IPA), acetone and aqueous compositions (eg, deionized water, etc.), and the like. Mechanical agitation (for example, planetary shaking) can be used to help dissolve unreacted resin. The substrate can then be dried to obtain a substrate with a plurality of microstructures 22' formed thereon (FIG. 1(E)).

It should be understood that this process promotes the formation of precise microstructures using a single exposure step and/or a single photomask. Moreover, the sharpened shape of the microstructure can be obtained without complex equipment. For example, in the process the substrate is preferably a flat substrate. Preferably, the substrate remains flat or at a fixed angle during lithography. Moreover, the substrate preferably remains stationary during microstructure lithography. That is, in preferred embodiments, it is not necessary to tilt, rotate, or otherwise move the substrate during the exposure step to form pointed microstructures.

The resulting microstructures are typically characterized by a pointed shaft. More preferably, the width or diameter of the microstructures is greatest at the end at the base of the microstructure adjacent to the substrate and tapers to a point at the end distal from the base. Depending on the shape of the template used to form the structures, microstructures can be formed with rods having circular geometric cross-sectional configurations (cone-shaped), or any other desired shape, including square base (pyramidal), star-shaped, triangular, oblong etc. The angle of the taper may also vary. As mentioned in the compression test below, the steeper the angle, the sharper the tip. Depending on the intended purpose, the sharpness of the tip can be balanced by the strength of the structure. Typically, if the apex angle is too small (eg <30°), the microstructure can easily break when force is applied. However, it should also be understood that the tensile strength is also determined by the specific material used to make the microstructures, as well as the overall size/width of the tip. In one or more embodiments, the process can be used to produce structures with a size (edge-to-edge size, i.e., diameter), as measured at the base, from about 5 μm to about 1000 μm, more preferably from about 50 μm to approximately 300 µm. The height of the microstructures, measured from the surface of the substrate to the tip, can range from about 30 μm to about 9 mm, more preferably from about 300 μm to about 1000 μm. The exposure time is typically less than 1 hour, even more preferably less than 45 minutes, and even more preferably less than 30 minutes.

Moreover, as also presented in the working examples, this process can be further modified to obtain the desired microstructure shape. For example, hollow microstructures can be formed using a template in which the holes have a solid core to prevent the transmission of light, and therefore prevent cross-linking and/or photopolymerization in this central region. In view of this, radiation is passed through the annular gap in the mask and cures the corresponding part of the resin adjacent to the mask template. When the uncrosslinked or unpolymerized resin is removed, the resulting structures are hollow with a substantially annular hole or channel extending from the base of the structure to the tip. The microstructures may be fabricated to extend away from the top surface 12 of the substrate in a substantially perpendicular direction. Alternatively, as also demonstrated in the examples, they can be manufactured at an angle to the surface 12 of the substrate. Depending on the template used, an array of microstructures can be produced on the entire substrate with a single exposure process having a mixture of different geometric configurations and/or sizes and/or angles using a template having holes with different sizes and/or shapes.

Additional modifications include the use of one or more intermediate layers adjacent to the top surface 12 of the substrate prior to application of the photosensitive resin. Such intermediate layers can promote the detachment of microstructures. Intermediate layers can also be used to further refine the direction of propagation of the activating radiation into the resin layer or template used to prevent transmission. Intermediate layers can be rigid or flexible. The shadow mask is presented in the examples solely as one example of such intermediate layers. For example, the intermediate layer may be applied to a substrate, wherein the intermediate layer has (eg, preformed, or may be structured in situ to have) a corresponding aperture that is flush with the array of apertures on the substrate. In the case of hollow microstructures, the intermediate layer can provide or stabilize the formation of an inner sidewall and hollow drill hole through the microstructure during diffraction lithography and ensure that the hollow drill hole extends from the base of the structure through the tip. The intermediate layer in this embodiment also facilitates the lifting of the structured array of microstructures after lithography and overgrowth, since it stabilizes the formed structures. In a further embodiment, the intermediate layer can be applied to the substrate as a planarization layer. That is, although planar substrates are presented herein by way of example, the surface of the substrate may be non-planar, with one or more height changes on the surface of the substrate. Moreover, the photomask may itself be an intermediate layer with a non-flat surface (eg open holes and solid parts). It should be understood that certain changes in the structures or masks of the substrate surface can be used to change the characteristics of the microstructures to be formed by changing the path of light during the exposure process. Thus, an intermediate layer can be applied to the surface of the substrate (or photomask) to pre-planarize the layer before applying the photosensitive resin.

It should also be understood that the resulting microstructures can be used as a template for conventional micromolding to further fabricate additional microarray structures using techniques other than lithography. For example, the substrate and microstructures can be used to create polydimethylsiloxane (PDMS) molds (negatives), which can then be used to fabricate microstructures using a variety of light-insensitive polymer compositions using stamping. In this embodiment, PDMS is applied to microstructures formed by diffraction lithography and cured to form a negative shape. This negative mold can then be used to form corresponding arrays of microstructures using various polymers (e.g., light-insensitive resins) by applying liquid resin to the negative mold, curing the resin, and peeling off the PDMS mold. It should be understood that the subsequent micromolding process option expands the possible resin systems from which microstructures can be made so that the resulting structures are not limited to photosensitive resins. For example, microneedles can then be manufactured using micromolding from a variety of biodegradable materials (eg, coated microneedles and/or dissolving microneedles) as well as various hydrogels.

The resulting microstructures formed through diffraction lithography (or subsequent micromolding) have a variety of potential applications, including as microneedles for both medical/clinical and cosmetic applications, microprobes for stimulation or detection of electrical signals, and or light detection. Using the same principles used to make them, the microstructures can be used as light waveguides, and light passed through the microcone will hit the tip of the microcone.

Additional advantages of various embodiments of the present invention will become apparent to those skilled in the art upon consideration of the description provided herein and the working examples below. It should be understood that the various embodiments described herein are not necessarily mutually exclusive unless otherwise indicated herein. For example, a feature described or displayed in one embodiment may also be included in other embodiments, but inclusion is not required. Thus, the present invention covers various combinations and/or integrations of the specific embodiments described herein.

As used herein, the phrase and/or, when used in a list of two or more items, means that any of the items listed may be applied by itself, or that any combination of two or more of the items listed may be applied. For example, if a composition is described as including or excluding components A, B, and/or C, the composition may contain or exclude only A; B only; C only; A and B combined; A and C combined; B and C combined; or A, B, and C in combination.

The present description also uses ranges of numerical values to quantify certain parameters related to various embodiments of the present invention. It should be understood that where ranges of numerical values are provided, such ranges should be understood to provide verbatim support for claim limitations that set forth only the lowest value of the range, as well as claims limitations that set forth only the highest value of the range. For example, the disclosed numerical range from about 10 to about 100 provides verbatim support for claims that claim more than about 10 (no upper limits) and claims that claim less than about 100 (no lower limits).

Examples

The following examples present methods in accordance with the present invention. However, it should be understood that these examples are provided by way of illustration and nothing therein should be construed as limiting the general scope of the present invention.

Introduction.

This technique describes a lithography method using self-focusing diffractive ultraviolet (UV) to fabricate various conical-shaped microneedle structures. The general process is illustrated in FIG. 1. Direct exposure of the UV photosensitive resin in liquid state through a photomask generates a unique diffraction pattern of light in which

- 7 045047 the exposed area of the photosensitive resin becomes needle-like structures. Changing the state of the resin from liquid to solid by photopolymerization and/or cross-linking changes the refractive index of the light-sensitive resin such that the photopolymerized resin acts as a light waveguide to guide and focus the light as it propagates through the resin to create a new sharp tip at the top of the cone. Removing uncured resin leaves behind micro-cone shapes. In more detail, collimated light is used, which travels as a plane wave and is diffracted when it reaches holes in the photomask. The intensity distribution of diffracted light on the opposite side of the holes functions as a Gaussian profile, also known as a diffraction circle, which has high intensity at the center of the hole and low intensity at the periphery. The light-sensitive resin in its liquid state is cross-linked/cured by diffracted light to form a small cone with a Gaussian profile at the hole. The cured resin has a higher refractive index than the surrounding uncrosslinked liquid resin, causing light that travels through the solid resin to refract at an angle that is wider than the angle of incidence as it passes through the solid-liquid interface (which forms the side wall of the microconical structure), and may even be reflected back to the interface in a conical light profile. This sidewall of the microconical structure acts as a waveguide and focuses all the light at one point, resulting in centric light intensity, which increases the steepness of the cone angle and ultimately forms a needle-shaped tip of the cone (first harmonic component).

As illustrated in FIG. 2, additional exposure to UV energy can lead to elongation of the conical tip and the formation of secondary and tertiary harmonic cone shapes with different geometric profiles. Specifically, after the first harmonic component is formed, the sharp tip of the first harmonic component acts as a secondary light-focusing hole, whereby a secondary conical structure can be formed by a higher light intensity at the center/tip of the generating structure. The microneedle shape at this point is essentially the optimal shape, which has a slightly pointed main body and an isosceles triangular tip with a pointed angle of approximately 30°. With further application of the impact energy, a secondary harmonic structure will be formed, followed by the formation of a third core and a tertiary harmonic component.

The proposed method is unique and flexible, as it allows the formation of various cone-shaped microstructures with straight, angular or curved side walls, such as a built-in tip cone and several harmonic cones having different heights and base shapes, as well as microneedles with optimal shape, standard pointed needle-like structures, or even microconical structures with rounded tips through a single backlight exposure on a single photomask. The height and shape of the microneedle can be modulated by using different impact energies and resin materials. A photograph of the resulting experimental UV diffraction test using a 4x4 200 µm photomask pattern is shown in FIG. 3, where the formation of microcones on the surface of the substrate is clearly visible.

Example 1.

During the initial setup of the experiment, a glass slide with a photomask containing a photomask of several holes was coated with a photosensitive resin. A UV LED covered with a collimated lens was placed under a photomask. Liquid photosensitive resin was cast onto the top of the photomask until it covered the surface of the photomask but was held in place by surface tension. Selectable UV LED wavelengths ranging from 300 to 450 nm are applicable for this fabrication. LEDs with peak wavelengths of 365, 375, 385, 395 and 405 nm have been tested and verified to form microcones. Each wavelength has different optical characteristics relative to the photosensitive resin, including transparency and scattering patterns.

A light resin from Anycubic POT016 LCD UV 405 nm Rapid Resin was used for one fabrication with a photomask array with 200 µm holes. The thickness of the photosensitive resin on the photomask ranged from approximately 2 mm, which is thicker than the target height of the microcone. Light exposure time varied from 10 s to 30 min at a UV intensity of 10 mW/ ^cm2 , depending on the target cone profile. After exposure, the sample was soaked in isopropyl alcohol (IPA) for 10 min in a planetary shaker at 20 rpm to remove uncured resin. After the IPA build-up process, the sample was dried to complete the process.

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Optical dose * for different microcone profiles

Time in s Height (mm) Tip Tip length Tip Width 5 0.407 absent 10 0.594 absent thirty 0.739 absent absent absent 65 0.808 almost educated absent absent 120 0.736 present 0.135 150 0.87 present 0.212 0.067 180 0.9 present 0.286 210 0.893 present 0.259 0.076 240 0.852 present 0.264 300 1.047 present 0.346 0.072 360 0.98 present 0.327 0.091 450 0.989 present 0.322 0.092

*Each dose at a wavelength of 365 nm through a 200 µm circular mask, intensity 10 mW/ ^cm2 .

Photographs of various microstructures formed at different exposure times are shown in FIG. 4. As shown in the images in FIGS. 5(A)-(B), initial experiments were also able to produce secondary harmonic cones with additional exposure time. These experiments were performed using a 120 μm photomask.

Example 2.

Diffraction lithography for 3D microneedle fabrication

Strategies for the formation of solid and hollow microstructures and the subsequent construction of different geometric configurations are illustrated in Fig. 6A and 6B and are governed by the principles of light diffraction and intensity distribution, as well as the variability of the refractive index of the photosensitive resin as it is converted from a liquid state to a photopolymerized and/or cross-linked/cured solid state. As illustrated in FIG. 6A, a bottom-up exposure process is used with collimated light, wherein (1) an initial micro-conical structure is formed as a base-like structure adjacent to the substrate when exposed to a light source through the hole of the photomask. In (2), the sidewall of the microcone, which was formed as the liquid resin transitioned to a solid state, now acts as a waveguide to guide light to produce the first harmonic component. In (3), the first conical tip is formed as the light that propagates through the resin self-focuses into a conical light profile such that the light intensity converges at the tip. In (4), as more energy is applied and focused on the tip to photopolymerize and/or cross-link/cure the resin in that area, a secondary harmonic structure is thus formed. Again, a secondary harmonic structure also focuses the light intensity to form a second sharp tip (5), and a tertiary harmonic component can also be formed as even more energy is applied (6).

As illustrated in FIG. 6B, a similar technique can be used to form the hollow microstructure, however the photomask is further structured such that the central portion of the hole is blocked to prevent light from passing through that portion of the hole, thereby creating a shadowed area in the central portion of the forming structures where the resin remains uncured. In view of this, the central core of the structure can be made hollow by removing the uncured resin after structuring.

Several different microstructures have been formed using these principles. In one experiment, a glass slide was used as a transparent substrate on which a structured photomask was placed. The photomask was coated with a photosensitive resin. In these experiments, UV LEDs having different wavelengths (365, 375 and 405 nm), integral with a collimating waveguide, were installed as light sources. Different wavelengths will have different scattering/absorption coefficients within the liquid photosensitive resin. Longer wavelengths will have less scattering and this can therefore lead to taller structures. Generally, the thickness of the liquid photosensitive resin should be greater than the desired height of the structures to be manufactured.

A photomask with an array of circular holes was coated with a light-colored photosensitive resin (Clear Resin, Formlabs Inc.) and placed on a microscope slide positioned 13 mm above the light source. Considering that in this setup the light must pass through two slides (the microscope slide and the photomask glass) before reaching the resin, the optical energy was measured with a spectrometer (BLUE-Wave Miniature Spectrometer, StellarNet Inc.) at a constant distance of 13 mm above the light source . The results in Fig. 7 show that the measured peak optical energy was 1.7271 mW/cm ² with no glass in the optical path, 1.7149 mW/cm ² with one glass and 1.6932 mW/cm ² with two glasses in the optical path. Optical energy was dissipated by 0.0339 mW/cm ² when passing through two glass slides, which was also equivalent to 2% dissipation, which is negligible for this experiment.

The microstructures were patterned by exposing the photosensitive resin to direct backlight from a 375 nm UV light source. The slide was then transferred to isopropanol (IPA) to remove uncrosslinked resin using gentle planetary shaking (20 rpm). Once the growth was completed, the sample was carefully dried with compressed air and the microneedles with a pointed core were completed. SEM images (Fig. 8(A)-(B)) show that the microneedle array was successfully fabricated on a photomask structured with circular holes by a single exposure to UV backlight. The optimal shape of microneedle tips with a sharpening angle of approximately 30° was obtained. The microneedles had a measured base diameter of 180 μm and a height of 550 μm, with an aspect ratio of 3.06. The tip diameter was 3 μm with a tip angle of 25.7°, which is sharp enough to penetrate the skin without breaking from a crack.

In another experiment, more impact energy was applied on a different photomask with a smaller circular hole, and 800 μm microneedles with a second harmonic structure were successfully fabricated, as shown in FIG. 9A. The base diameter and height of the microneedles were 160 μm and 800 μm, respectively, with an aspect ratio of 5. In FIG. 9B, an optical image of UV light taken during the fabrication process shows the propagation of light within the liquid photosensitive resin and experimentally confirms that the second harmonic component could be obtained by applying a sufficient amount of energy. An array of microneedles with secondary and tertiary harmonic structures is shown in FIG. 9(c) and 9(d), demonstrating the manufacturing capability of the proposed method as well as its uniqueness in fabricating a complex microneedle structure using only UV exposure.

To further demonstrate this flexible fabrication method, different geometric photomask hole configurations (circular, triangular, star-shaped, and triangular with curved base) were used with different hole sizes to fabricate microneedles. In fig. Figure 10 shows different hole geometries (inset) used to generate different optical diffractions that produce different microneedle shapes on the same substrate using a single UV exposure.

To study the minimum amount of energy required to initiate cross-linking of a liquid photosensitive resin, a photomask with a 150 µm circular aperture (inset) and an optical energy of 1.6932 mW/ ^cm2 under a 375 nm UV light source was set to constant while exposure time was set to variable (from 1 to 90 s). The data (Fig. 11) shows the relationship between the height of the fabricated microneedle and the energy applied to cross-link the resin. The secondary y-axis shows the aspect ratio of the corresponding height and the secondary x-axis shows the exposure time of the corresponding applied energy. The minimum amount of energy to crosslink the photosensitive resin was 3.39 mJ/cm ² and the measured height was 8.4 μm. The microconical structure grows relatively quickly during the first 20 seconds and thereafter a constant growth rate of 2.44 μm was observed for every 1 mJ/cm ² of additional energy applied.

Optical images of microconical structures at 2, 3, 5 and 20 seconds of exposure time are shown in FIG. 12, where the sharp tip characteristic of the microneedle begins to appear at 20 seconds of exposure time.

Insertion tests and force displacement tests were conducted to demonstrate the functionality of the fabricated microneedle, and the results showed that the tip of each tested microneedle could withstand up to 0.15 N before failure. The fabricated cone-shaped microneedle with a sharp tip has good potential to be used as a microneedle for subcutaneous drug delivery. To prove the functionality of the fabricated polylactic acid (PLA) microneedles, a 4x4 microneedle array with essentially the same geometric configuration as shown in FIG. 8. This geometric configuration was chosen for mechanical testing because it is similar to other microneedle geometries that have been reported to be able to penetrate the skin without breaking. The PLA microneedle array shown in FIG. 13(a) was injected into the skin of a pig cadaver using thumb pressure, and the injection area was painted with blue tissue marking paint for visualization.

In fig. 13(b) shows 4x4 blue injection marks on pig skin. Force displacement testing was also performed using a 4x4 microneedle array. Microneedles were made based on Fig. 11 with a diameter of 150 μm and a height of 500 μm, while the tip had a length of 80 μm with a sharpening angle of 27.6°. A dynamometer (FC200, Torbal Inc.) was mounted on a stepper motor integral with a threaded rod along the z-axis and controlled by an Arduino (Arduino UNO Rev 3, Arduino). A 4x4 microneedle array was placed directly below the dynamometer and the dynamometer was directed downward at a speed of 1.2 mm/min while recording the force every 1 millisecond. The total test time was converted into displacement in micrometers and the result was plotted (Fig. 14). As the microneedle was compressed, the first tip began to deform at 2.38 N, indicating that each needle tip could withstand at least 0.15 N without any mechanical failure. The tips were then bent, causing a sudden drop in force. After all the tips were completely deformed, the detected force increased linearly, indicating that the main parts of the microneedles still remained attached to the substrate without deformation, which is also shown in the corresponding image. This characteristic shows good accessibility for drug transport method such as coating and piercing and piercing and releasing. In other words, the tips of the microneedles can be pre-coated with the drug or made from the drug itself and can be designed to break during administration, while the core parts of the needles serve as delivery supports that can be disposed of after use.

The manufacturing method demonstrated its uniqueness in the formation of structures in the form of microneedles in the form of microcones using light diffraction and the corresponding intensity distribution. With a simple setup of a light-collimating LED system, microneedle arrays with complex geometric configurations can be fabricated with just one exposure within 30 minutes. An insertion test and a force displacement test were conducted and the results demonstrated that the fabricated microneedle tip could withstand up to 0.15 N before deformation occurred, making it strong enough to penetrate the skin. The fabricated cone-shaped microneedle with a sharp tip has good potential to be used as a microneedle for subcutaneous drug delivery.

Example 3.

Fabrication and characterization of nipple tip, hollow and tilted microneedles by UV LED lithography.

The proposed fabrication method is improved over the previous technique as it introduces functional microneedles such as hollow and inclined microneedles with a single UV exposure pattern. Hollow needles are useful for delivering drug in a liquid state, and an inclined microneedle can be applied to a non-flat skin surface in a hook-like manner. And these microneedle manufacturing processes can be completed within 30 minutes, including sample preparation, UV exposure, extension and cleaning, which will greatly reduce the production cost. Tests were performed with different UV LED wavelengths (365, 375, 385, 395, and 405 nm) to generate different microneedle shapes.

Since smaller wavelengths scatter more than larger ones, prediction of the shape of microneedles can be made from the experimental setup. In fig. 15 shows a test of UV light propagation within a photosensitive resin. In fig. 15(a) shows the light propagation shape, and FIG. 15(b) shows the SEM image of the corresponding 3x3 circular microneedle array. Data in Fig. 16 shows the relationship between the heights of microneedles at different applied energies with a photomask of 150 μm, and the figure shows the overall shape of the needle at the corresponding applied energy. Using this data and different photomask geometries, different microneedles can be formed, as shown in FIG. 17. The fabrication method has also been used to create hollow microneedles and inclined circular microneedles, as shown in FIG. 18.

Insertion and force displacement experiments were also performed using a circular 3x3 PLA microneedle array. The porcine skin insertion test with blue markings visualizes the needle insertion points on the porcine skin (FIG. 19). Force displacement data is shown in FIG. 20. SEM images are shown in FIG. 20A for a circular 3x3 PLA microneedle array with a height of 750 μm fabricated based on the data from FIG. 16. Microneedles were placed under the force transducer and the compression rate of the force transducer was set to 1.2 mm/min. The top image shows the microneedles before any displacement. The middle image shows the shape of the microneedles after 168 μm of displacement. Only the tip of the needle began to deform, while the main part of the needle remained unchanged after compression. The bottom image shows the shape of the microneedle after 624 µm of displacement, where the main

- 11 045047 part of the needle is predominantly deformed or bent, while the base remains relatively in place. As shown in FIG. 20B, the slope of the force displacement curve also increased significantly at this point, implying that the strength of the microneedle body is suitable for use in microneedle application.

Example 4.

Improved fabrication of microneedles.

In this technique, microneedles were fabricated using surgical guide resin from Formlabs (Somerville, MA). This resin is a commercially available, autoclavable, biocompatible resin that is commonly used for 3D printing of dental surgical guides for implant placement. The resin is a proprietary formula consisting of methacrylate monomer (2545 wt.%), urethane dimethacrylate (55-75 wt.%) and photoinitiator (1-2 wt.%) according to PBB.

I. Minimum cross-linking energy.

Initial testing looked at the minimum amount of energy required to crosslink the surgical guide resin. Understanding crosslinking energy allows crosslinking height to be predicted more accurately. A 405 nm UV LED (UV 405 nm LED, Shenzhen Chanzon Technology Co., Ltd.) was used as the UV source. A 3D printed waveguide was used to collimate light with a UV LED. A clear glass slide was placed over the waveguide at a constant distance of 1 mm to minimize the loss of UV intensity in space. A thin layer of surgical guide resin was spin-coated onto clear glass at a thickness of 50 µm. Finally, a UV intensity sensor was installed 1 mm above the resin surface to monitor the intensity of the UV source. Very low UV light intensities were used with gradual increases (0.1, 0.22, 0.3, 0.4, 0.49, and 0.6 mW/cm ² ) to accurately measure cross-linking time intervals. The results are presented in Fig. 21 and show the resulting height of the cross-linked resin at different light intensities and different exposure energies with a gradual increase. Regardless of the light intensity, for this resin, when the exposure energy reaches 6.8 mJ/cm ² , the height of the cross-linked resin increases, indicating that the minimum cross-linking energy for this surgical guide resin is 6.8 mJ/cm ² .

II. UV light transmittance of 405 nm through various resin thicknesses for surgical guides.

The transmittance of 405 nm UV light through different thicknesses of surgical guide resin was examined. Understanding the transmittance of UV light at 405 nm also helps to better predict the cross-linking behavior of resin for surgical guides. A 405 nm UV LED (UV 405 nm LED, Shenzhen Chanzon Technology Co., Ltd.) was used as the UV source. A 3D printed waveguide was used to collimate light with a UV LED. A clear glass slide was placed over the waveguide at a constant distance of 1 mm to minimize the loss of UV intensity in space. Unlike the previous section, different thicknesses of surgical guide resin were applied to clear glass, ranging from 0 to 3000 µm. Finally, a UV intensity sensor was installed at a constant distance of 11 mm above the clear glass to monitor the intensity of the UV source for different surgical guide resin thicknesses. Before applying the surgical guide resin, the UV light intensity was measured and recorded as the initial UV intensity, I0. The UV light intensity was again measured after the resin was applied and called parameter I. Using the two recorded intensities, the transmittance was calculated using the equation below:

I

Transmission = T = —

The results are presented in Fig. 22. A curve was generated from the experimental points as shown in the equation below, giving R ² 0.99565824. The dispersion coefficient, a ₃ , for surgical guide resin is 0.00287837.

Transmission - T - - =1 - (a ₀ + a _x z - a ₂ e ^-a3Z ) (1)

Ιο where a0 = 0.96265990, a1 = 0.00000366, a2 = 0.96265990, a ₃ = 0.00287837 (dispersion coefficient).

To predict cross-linked resin height, we start with a basic formula for calculating energy based on UV light intensity and exposure time:

Energy = E = I t (2) where I is the intensity of the 405 nm UV light in mW/cm ² and t is the exposure time in seconds. Based on equation (1), we know that I is a function of z, where

- 12 045047 z represents the thickness of the resin for surgical guides, therefore we can rewrite equation (1) as:

I = I _o -I ₀ (a ₀ Na^-age^ ² )

Then replace with equation (2):

E = I ₀ tI ₀ t(a ₀ + a ₁ za ₂ e ^a 3 ^z ) (3) where I ₀ represents the intensity of UV light when z = 0. Based on equation (3), if we set I ₀ and t, we can see that E is inversely proportional to z, the higher the resin thickness, z, the lower the energy received, E. Knowing this relationship we can say that for every constant value of I ₀ and t there must be a vertical resin thickness z, which corresponds to the minimum resin cross-linking energy for surgical guides, which is 6.8 mJ/ ^cm2 , as we measured and described above.

To test the equation, a real data set was generated using the conditions shown below:

Options Values Units Voltage V 2.88 V Current strength I 14 mA Nutrition R 40.32 mW UV wavelength λ 405 nm radiation Intensity Ιο 5.75 mW/cm ² optical radiation at z = 0 Exposure time t 0-900 Seconds Energy E 0-5175 mJ/cm ²

Using the same experimental system setup and experimental conditions given above, the heights of the cross-linked resins at various amounts of energy were measured and recorded as shown in FIG. 23.

III. Microneedle height characteristics.

The characteristics of the microneedle and its height were then studied at different impact energies. The conditions of this experiment are listed in the table below:

Options Values Units Voltage V 3 V Current strength I 60 mA Nutrition R 180 mW UV wavelength λ 405 nm radiation Intensity Ιο 19.65 mW/cm ² optical radiation at z = 0 Exposure time t 0-120 Seconds Energy E 0-2358 mJ/cm ²

- 13 045047

In fig. 24 illustrates the setup of the microneedle fabrication system for these experiments. Starting from the bottom, a 405 nm UV LED integral with the waveguide was used as the UV light source, with a collimated lens converting the LED light into parallel light. A structured photomask with a 150 μm hole pattern was installed at a distance of 25.4 mm from the UV light source, which acts as a substrate for the surgical guide resin. A layer of surgical guide resin was applied to the photomask and then irradiated with 405 nm UV light. The exposure process was stopped when a certain exposure energy was reached, and the corresponding microneedle heights were fixed at these designated exposure energy points. After completion of the exposure process, the sample was washed with isopropanol and the fabrication of microneedles was completed. In fig. Figure 25 shows the measured heights of microneedles at different impact energies. In this particular example, the graph is divided into four sections based on the shape of the microneedle structure, including the microconical structure, the first harmonic microneedle, the second harmonic microneedle, and the third harmonic microneedle (FIG. 25). In fig. 26 shows photographs of microconical structures and microneedles that represent the shape at each stage of the impact energy. This discovery shows that microneedles with different sizes and shapes can be produced by using a single photomask and simply varying the irradiation energy.

IV. Fabrication of microneedle array.

Using the above setup (FIG. 24), various microneedle arrays were fabricated using surgical guide resin. A 20x20 hole array photomask having 150 μm diameter holes was used to fabricate solid, straight microneedles. In fig. 27A shows an SEM image of the resulting array after substrate growth and removal of uncured resin. As can be seen, the result of the process is a 20x20 array of microneedles with constant size and shape, each having a base diameter of 133 µm with an average height of 385 µm. Also provided are enlarged SEM images of individual microneedles (Fig. 27B) and individual tips (Fig. 27C). The microneedle tip had an approximate width of 2.5 μm and a pointed angle of 28.5°.

Using the modified setup shown in FIG. 28, hollow microneedles were produced.

As in other experiments, a 405 nm UV LED, integral with the waveguide, was used as a source of UV light. The structured photomask was installed at a distance of 25.4 mm from the UV light source. The photomask array contained 271 holes, each of which had a solid core to prevent light from passing through the center of the holes (see inserted image in Fig. 29). The holes had an outer diameter of 300 μm, with a solid core having a diameter of 200 μm, thereby leaving a 100 μm wide annular gap open for light transmission. To further enhance the effect, a shadow mask (made with resin) with full through holes was prepared using 3D printing and aligned with the photomask holes using a mask leveler. The shadow mask, which was placed above the photomask, contains 271 units of fully through holes with a diameter of 300 μm (open core), corresponding to the holes of the photomask. After completion of the UV exposure process, the sample was washed with isopropanol and the shadow mask was detached from the photomask along with the hollow microneedles. A photograph of the 271 units obtained is shown in Fig. 29. Since the material that forms the hollow microneedles is the same as that in the shadow mask, the shadow mask can be easily detached from the photomask along with the hollow microneedles, since the connection with the shadow mask is stronger than the photomask. Measurement of the hollow microneedles showed that they have a base diameter of 280 μm and a height of 550 μm (Fig. 30, enlarged view). As can be seen in FIG. 30, the shapes and sizes of individual needles are the same for several needles.

V. Introduction test.

The pig skin was first cleaned with isopropyl alcohol to remove any possible contamination. A 20x20 solid straight microneedle array was fabricated using diffraction lithography and then used as a template to create a PLA array using a PDMS micromolding process. Shortly after the microneedle array is fabricated using diffraction lithography on a glass substrate, the PDMS process occurs. SYLGARD™ 184 silicone elastomer was mixed with curing agent in a 10:1 ratio. The bubbles formed during mixing were degassed in a vacuum oven. A light, clear elastomer solution was carefully poured onto the diffraction lithography-based microneedle array and cured at 80°C for an hour. After cooling to room temperature, the diffraction lithography-based microneedle array was detached from the cured PDMS to obtain a PDMS mold as a negative matrix for the microneedle array (an array of grooves/holes corresponding to the microneedles). PLA molding process involving coating a PDMS mold with PLA beads (1-2mm). The sample was heated to 180°C in an oven to melt the PLA pellets to fill the grooves of the PDMS mold. After cooling to room temperature, the PLA microneedle array was detached from the PDMS mold to complete the PLA microneedle creation process.

The PLA microneedle array was inserted into the pig skin by pressing the back of the PLA substrate with the thumb. The injection area was stained with blue tissue marking dye (CDI's Tissue Marking Dye, Cancer Diagnostics, Inc.) for visualization. In fig. 31 shows the result of an injection test on porcine skin.

VI. Force displacement test.

To further understand the mechanical strength of the microneedles, a force displacement test was performed. A 3x3 microneedle array was fabricated and used as a micromolding template to create a PLA microneedle array using a two-step molding process including PDMS molding and PLA molding. The characteristics of the solid straight microneedle array are shown below:

diameter = 133 µm, height = 526 µm, tip diameter = 40 µm, tip height = 134 µm, material = PLA, number of needles = 9 (3x3 array).

A 3x3 microneedle array was positioned straight up while the dynamometer was slowly moved downward at a speed of 1.2 mm/min. The tip of the microneedle was first compressed with a dynamometer and fully bent with a measured force of 0.552 N and 0.0613 N on the needle. The dynamometer continued to compress the bulk of the microneedles until it reached the maximum programmed time frame. The maximum compressive force measured by the dynamometer on the main parts of the microneedles was 9.284 N and 1.0316 N per needle, the measured total compression displacement was 436 μm, which also corresponds to the compressed height measured after using a microscope. The data is shown in Fig. 32A with corresponding images of the microneedles at each stage in FIG. 32B._____________________

Dynamometer Reset Pressure The needle tip is broken Offset = 436 µm Offset = 52.5 µm Force = 9.284 N Force = 0.552 N Needle force = 1.0316 N Needle force = 0.0613 N

A 2x2 solid tilted microneedle was fabricated using a diffraction lithography method. The fabrication process was very similar to that of solid straight microneedles, except for the introduction of an additional inclined angle during the UV exposure process. The measured tilt angle for the microneedles was 14°. In this test, microneedles were not converted to PLA. The conditions of solid tilted microneedles are given below:

diameter = 300 µm, height = 900 µm, tip diameter = 90 µm, tip height = 266 µm, material = surgical guide resin, number of needles = 4 (2x2 array).

A 2x2 tilted microneedle array was placed straight up while the dynamometer was slowly moved downward at a speed of 1.2 mm/min. The tip of the microneedle was first compressed with a dynamometer and fully bent with a measured force of 0.106 N and 0.0265 N on the needle. The dynamometer then continued to compress the bulk of the microneedles until the microneedles were released from the substrate. Since the microneedles were not converted to PLA, the adhesive force between the needles and the substrate is weaker than the solid straight needle test described above. The measured detachment force from the substrate was 4.06 N and 1.015 N per needle. The dynamometer continued compression away from the main parts of the microneedles until the maximum programmed time frame was reached, with the measured force at this point being 5.276 N and 1.319 N per needle. The data is shown in Fig. 33A with corresponding images of the microneedles at each stage in FIG. 33B.

-

Claims (21)

Relief pressure dynamometer The needle has become detached from the substrate from The needle tip is broken Offset = 642 µm Offset = 577 µm Offset =147 µm Force = 5.276 N Force = 4.06 N Force = 0.106 N Needle force = 1.319 N Needle force = 1.015 N Needle force = 0.0265 N Finally, the detachment force required to detach tilted microneedles from two directions was investigated. In this case, if the dynamometer is moved against the inclined direction of the microneedle, it is called out-of-phase compression, while if the dynamometer is moved towards the inclined direction of the microneedle, then it is called in-phase compression. The conditions of solid tilted microneedles are given below: diameter = 300 µm, height = 900 µm, tip diameter = 90 µm, tip height = 266 µm, material = surgical guide resin, number of needles = 1. Although the dynamometer continued to move downward at a speed of 1.2 mm/min, the microneedle was positioned as illustrated in FIG. 34B, depending on whether in-phase or out-of-phase compression was experienced. The out-of-phase detachment force measurement showed 0.224 N, while the in-phase detachment force measurement showed 0.574 N. The data is shown in FIG. 34A, with corresponding images of the microneedles showing the compression direction in FIG. 34B.____________ Out of phase disconnection Common Mode Disconnect Offset = 34 µm Offset = 45 µm Force = 0.224 N Force = 0.574 N CLAIM 1. A lithography method for producing a plurality of microscopic structures with converging tips, the method comprising: providing a substrate having a first surface and a second surface, said substrate comprising a pattern having open regions configured to transmit radiation and solid regions configured to prevent transmission of radiation; forming a layer of photosensitive resin in a liquid state on said first surface; exposing said photosensitive resin in a liquid state to radiation through said substrate from a second surface for a first period of time to produce light-irradiated portions of said photosensitive resin in a liquid state, wherein said light-irradiated portions are cross-linked and/or polymerized to form corresponding initial resin structures in a solid state on said first surface in alignment with said exposed regions, wherein said initial solid state resin structures have an increased refractive index compared to said photosensitive resin in a liquid state, such that each initial solid state resin structure acts as a waveguide directing said radiation passing through said open regions of said template to a convergence point, thereby forming solid state resin structures with tapering side walls and converging tips; and bringing the coating layer into contact with the solvo system to remove non-light-irradiated portions of said photosensitive resin in a liquid state to produce a plurality of said microscopic resin structures in a solid state with tapering side walls and converging tips on said first surface of said substrate, - 16 045047 wherein the height of said microscopic resin structures in the solid state is controlled by changing at least one of the exposure time, the light intensity or the wavelength of said radiation. 2. The method according to claim 1, characterized in that said open areas are holes having a geometric shape selected from the group consisting of a circle, a rectangle, a polygon and a star. 3. The method of claim 2, wherein said holes have a size of from about 1 to about 1000 microns. 4. The method of claim 2, wherein said open areas have central portions that are opaque to prevent radiation from passing through the central portion of each opening. 5. The method according to claim 4, characterized in that said microscopic structures with converging tips have a hollow rod. 6. The method according to claim 1, characterized in that said template is a photomask located adjacent to said first surface and/or said second surface of said substrate. 7. The method according to claim 1, characterized in that said template is formed as a single unit with said substrate. 8. The method according to claim 6 or 7, characterized in that the template contains an array of multiple spaced holes distributed over the substrate. 9. The method according to claim 1, characterized in that said layer of photosensitive resin in the liquid state has a thickness that is greater than the height of said microscopic resin structures in the solid state. 10. The method of claim 1, wherein said layer of photosensitive resin in the liquid state has a thickness ranging from about 50 μm to about 9 mm. 11. The method of claim 1, wherein said radiation is light with a wavelength of from about 300 nm to about 450 nm. 12. The method according to claim 1, characterized in that said radiation passes through a collimating lens, thus the direction of propagation of the energy flow from the radiation source is parallel and enters the substrate at an angle of incidence perpendicular to the second surface of the substrate. 13. The method of claim 1, wherein said exposure step is performed for a period of time from about 1 second to about 1 hour. 14. The method according to claim 1, characterized in that said microstructures are formed in one stage of exposure, and this method does not include more than one stage of exposure. 15. The method of claim 1, wherein said exposure step includes said first period of time and further includes at least a second period of time extending said first period of time, said microscopic resin structures in a solid state with tapering side walls and converging tips have a first height after said first period of time, and wherein said microscopic solid state resin structures with tapered side walls and converging tips have a second height after said second period of time that is greater than said first height. 16. The method according to claim 15, characterized in that exposure to radiation during said second period of time induces additional cross-linking and/or photopolymerization in areas of the resin layer adjacent to the converging tips of said initial microscopic solid state resin structures of the first height, thereby forming one or more additional harmonic structures on said initial microscopic resin structures in the solid state. 17. The method of claim 16, wherein said one or more additional harmonic structures have side walls with alternating inclined and deflecting angles, ultimately converging at respective tips. 18. The method of claim 1, wherein the microscopic solid state resin structures comprise respective rods having geometric cross-sectional configurations selected from the group consisting of round, rectangular, polygonal and oblong, and a combination of any of the above geometric configurations can be provided in one array of microstructures throughout the specified substrate. 19. The method of claim 1, wherein each of the microscopic solid state resin structures has a base size ranging from about 5 µm to about 1000 µm, and a height ranging from about 30 µm to about 9 mm. 20. The method of claim 1, wherein said substrate is substantially flat and wherein said substrate remains stationary during said exposure. 21. The method of claim 1, further comprising applying one or more intermediate layers to said substrate prior to applying said photosensitive resin layer. -