WO2021215999A1

WO2021215999A1 - A method of fabricating superamphiphobic films with doubly re-entrant microstructures

Info

Publication number: WO2021215999A1
Application number: PCT/SG2021/050198
Authority: WO
Inventors: Chee Leng Lay; Shu Mei Man; Chuan Hui Lionel MOH
Original assignee: Agency For Science, Technology And Research
Priority date: 2020-04-22
Filing date: 2021-04-08
Publication date: 2021-10-28

Abstract

There is provided a process of fabricating a polymeric negative mould comprising microstructures selected from multiple overhangs, hierarchical and a combination thereof, comprising the steps of (a) fabricating a positive master mould by two-photon lithography (TPL); and (b) fabricating a polymeric negative mould which is a polymeric negative replica of said positive master mould. There is further provided a process of fabricating a polymeric film, comprising further steps of (c) imprinting the polymeric negative replica; and (d) removing the polymeric negative replica from the imprinted polymeric film. There is further provided a process of generating multiple generations of a polymeric film by repeating steps (b)-(d) using said imprinted polymeric film of step (d) as the positive master mould.

Description

A Method of Fabricating Superamphiphobic Films With Doubly Re-Entrant Microstructures

References to Related Applications

This application claims priority to Singapore application number 10202003673T filed on 22 April 2020, the disclosure of which is hereby incorporated by reference.

Technical Field

The present invention generally relates to a process of fabricating a polymeric negative mould. The present invention also relates to a process of fabricating a polymeric film. The present invention further relates to a process of generating multiple generations of a polymeric film. The present invention further relates to an array of microstructures and a polymeric film comprising said array.

Background Art

Surface properties of materials, particularly surface morphology and topography have great impact on liquid wettability. For example, insects such as springtails have cuticles decorated with mushroom or serif-T microstructures, allowing them to survive against suffocation by water and alkane -based solvents. Manipulating wettability of liquids on surfaces is vital in various industrial and academic research areas, such as anti-fouling and anti-smudge coatings, liquid flow manipulation in microfluidic devices, and shielding off harmful chemical and biological contaminants.

A wide range of natural surfaces inspire the creation of synthetic liquid repellent surfaces which exhibit high contact angle (CA). To achieve superamphiphobic surface that repel water and oils, only surfaces decorated with doubly re-entrant microstructures can retain water and oils on top of the microstructures, preserving the liquid droplets in the Cassie-Baxter (or non-wetting) state.

Doubly re-entrant microstructures can be fabricated via conventional subtractive techniques (e.g. chemical and physical etching) and masked-based UV photolithography. Both subtractive techniques and masked-based UV photolithography involve a series of tedious and time-consuming alignment, coating, rinsing and caustic reactions. Additionally, subtractive techniques are normally conducted in protective environment due to the use of reactive and toxic chemicals. Microstructures are normally patterned on hard substrates when subtractive techniques are employed. Other techniques of making re-entrant or doubly re-entrant microstructures involve a series of conventional top-down approaches. Conventional top-down approaches involved time-consuming nanoimprint, UV curing, chemical and physical vapor deposition, as well as chemical and physical etchings. Besides tedious processes, there are drawbacks on conventional top-down approaches, as follow: (1) only one structural design and orientation is printed, (2) time consuming alignment, (3) limited choices of photoresists, (4) toxic and corrosive chemicals are used and (5) microstructures are normally patterned on hard substrates.

One known method uses a process for the replication of complex 3D microstructures with acrylic polymer by multiphoton absorption lithography and microtransfer moulding. Microstructures that have high aspect ratios or re-entrant features, such as truncated pyramids, single re-entrants, cantilevers and towers, are replicated rapidly and with high fidelity. However, this method was found to be inefficient as it has only been used for the replication of a single vertical column instead of an array.

Another known method uses master microstructures initially structured by two- photon lithography in nanoimprint lithography (NIL) for scalable parallel stamp fabrication of complex geometries. However, this method was found not to be effective because the microstructures fabricated were simple two-dimensional microstructures, which cannot be applied in surface modification, rather than doubly re-entrant microstructures.

Conventional methods of NIL have the following issues:

(1) Conventional methods of NIL use hard moulds such as nickel moulds to pattern polymeric films. The hard moulds are not flexible, resulting tearing off of any hanging/suspending features from the imprinted microstructures.

(2) Conventional methods of NIL involve tedious and complicated steps of generating moulds for trapezoidal, T-shaped and mushroom- shaped microstructures, causing detail features of the microstructures be compromised due to processing errors in any imprinting steps.

Another known method uses vapour deposition of 1H,1H,2H,2H- perfluorodecyltrichlorosilane (FDTS) on a polydimethylsiloxane (PDMS) surface, which resulted in an anti-stiction layer for the improved release after PDMS moulding. However, this method was found not to be effective as it may not be used in the fabrication of doubly re-entrant microstructures or suspending/overhanging features.

Accordingly, there is a need for a method to fabricate superamphiphobic films with doubly re-entrant microstructures that ameliorates one or more disadvantages mentioned above. Summary

In one aspect, there is provided a process of fabricating a polymeric negative mould comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, comprising the steps of: (a) fabricating a positive master mould by two-photon lithography (TPL); and (b) fabricating a polymeric negative mould which is a polymeric negative replica of said positive master mould.

In another aspect, there is provided a process of fabricating a polymeric film comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, comprising the steps of: (a) fabricating a positive master mould by TPL; (b) fabricating a polymeric negative replica from the positive master mould; (c) imprinting the polymeric negative replica on the polymeric film to form the microstructures thereon; and (d) removing the polymeric negative replica from the imprinted polymeric film.

In another aspect, there is provided a process of generating multiple generations of a polymeric film comprising microstructures selected from multiple overhangs, hierarchical microstmctures or a combination thereof, comprising the steps of: (a) fabricating a positive master mould by TPL; (b) fabricating a polymeric negative replica from the positive master mould; (c) imprinting the polymeric negative replica on the polymeric film to form the microstructures thereon; (d) removing the polymeric negative replica from the imprinted polymeric film; and (e) repeating steps (b) to (d) in sequence to generate multiple generations of polymeric negative replicas and polymeric positive replicas using the imprinted polymeric film of step (d) as the positive master mould.

In another aspect, there is provided an array of microstmctures selected from multiple overhangs, hierarchical microstmctures or a combination thereof, wherein at least one of the stmctural dimensions of the microstmctures are as follow: (i) connection point diameter (CPD) of about 8.5 pm to about 17.5 pm, (ii) connection point- to-cap ratio (CCR) > 0.55, (iii) aspect ratio (AR) < 4.5, microstmcture height < 90 pm, and (iv) pitch size > 30 pm, wherein

Connection point diameter connection point — to — cap ratio (CCR) Cap diameter ’

Microstructure height aspect ratio (AR) = Diameter of column base’ and pitch size = length from the centre of a micro structure to the centre of an adjacent micro structure.

In another aspect, there is provided a polymeric film comprising an array of microstmctures selected from multiple overhangs, hierarchical microstmctures or a combination thereof, wherein at least one of the structural dimensions of the microstmctures are as follow: (i) connection point diameter (CPD) of about 8.5 pm to about 17.5 pm, (ii) connection point-to-cap ratio (CCR) > 0.55, (iii) aspect ratio (AR) < 4.5, microstructure height < 90 pm, and (iv) pitch size > 30 pm, wherein

Two-photon lithography (TPL) is a three-dimensional (3D) printing process based on layer-by-layer photopolymerization. TPL is capable in generating micro-/nano- structures of arbitrary shapes and orientation. Notably, TPL has excellent spatiotemporal resolution in the nanometer scale, allowing fabrication of 2D/3D micro-/nano-structures at pre-designated locations. While the fabrication speed of TPL is fast, TPL is not able to repeatedly write large area of doubly re-entrant microstructures for industry applications and is hence disadvantageous for large-scale applications.

Nanoimprint lithography involves the use of moulds to transfer a negative replica of the pattern on the mould to a polymeric substrate to be imprinted. NIL is able to create imprints on the nano-scale or micro-scale and has been found to be of low cost, high throughput and high resolution. NIL can be used to replicate columnar and highland like trapezoidal 2.5D/3D microstructures. However, use of NIL to replicate inverted trapezoidal, T-shaped and mushroom-shaped micro structures has not been reported. It then follows that replication of doubly re-entrant microstructures using NIL has not been done before as such doubly re-entrant microstructures would prevent the release of the mould from the imprinted substrate.

The inventors have thus found that a combination of TPL to fabricate the mould for later use in NIL is a robust, efficient and reproducible way of generating replicates of various structural designs. This combination of TPL and NIL allows for the reproduction of replicates of microstructures such as doubly re-entrant microstructures, hierarchical doubly re-entrant microstructures and hierarchical mushroom microstructures, even in the sub-micron resolution. All the replicates are able to retain pre-designated structural designs and features. The replicates are also found to be superamphiphobic, which can be improved further via surface modification.

In addition, the combination of TPL and NIL allows for on-demand structural designs and customizable surface patterning with flexible material selections via versatile green chemistry. By controlling the right parameters, the re-entrant microstructure does not have a large overhang and is strong enough to withstand a demoulding process from the soft nanoimprint mould. The soft mould works mainly because it is soft but flexible enough to hold its shape. As TPL is capable in creating microstructures with specific geometrical designs in micrometer scale, this capability of TPL facilitates NIL with more freedom on generating targeted designs at pre designated locations. Chemicals used during the TPL and NIL are eco-friendly chemicals, which are thus more environmental friendly compared to techniques used in the art. The fabricated moulds with 3D complex microstructures can also be reusable which impart low carbon footprint.

Definitions

The following words and terms used herein shall have the meaning indicated:

The term “re-entrant” as used herein refers to a microstructure that has an overhang part and a base part connected to and below the overhang part, wherein the overhang part extends in a way that re-enters a plane that is parallel to the transverse plane of the base part.

The term “doubly re-entrant microstructure” as used herein refers to a microstructure that has a shape of its longitudinal cross section (cross sections parallel to the vertical axis) consisting of more than 2 vertices having an angle of more than 180°. The microstructure has an overhang part and a base part connected to and below the overhang part, wherein the overhang part extends and turns in towards the base part. A flat plane exists for the microstructure that intersect the overhang part at least twice.

The term “microstructures” as used herein refers to structures having a dimension of about 0.1 pm to about 100 pm. Therefore, this includes structures that have dimension(s) in the nano-scale or the micro-scale.

The term “hierarchical microstructures” as used herein refers to microstructures comprising structural features which have themselves internal microstructures, forming self-similar repeating patterns at different scales. Therefore, the hierarchical microstructures can be viewed as having different layers of microstructures placed on top of one another, whereby each layer of microstructure has a different dimension as compared to the adjacent layer of microstructure.

The term “superamphiphobic” as used herein refers to a property of a surface that is both superhydrophobic and ultraoleophobic.

The term “superhydrophobic” as used herein refers to a property of a surface having a contact angle for water of greater than 150°, as measured according to static sessile drop method.

The term “ultraoleophobic” as used herein refers to a property of a surface having a contact angle for an organic liquid of greater than 90° and no more than 150°, as measured according to static sessile drop method. Such organic liquids may comprise liquids having a surface energy Y_LA<30 mJ m^-2. Such liquids may be characterized as hydrophobic and may be liquid at ambient temperature and pressure. Such liquids may carbon-based oils (such as soy bean oil), fluorinated oils and combinations thereof.

The term “generations” as used herein refers to a number of repetitions in which a new master mould for polymeric replication is made from a previous master mould or polymeric replication. A polymeric film made according to the process as described herein is of the first generation without any repetition. The second generation is obtained after repeating steps of the process once, and subsequent generation(s) is/are obtained in a similar manner.

The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the invention.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

The term "about" as used herein typically means +/- 5 % of the stated value, more typically +/- 4 % of the stated value, more typically +/- 3 % of the stated value, more typically, +/- 2 % of the stated value, even more typically +/- 1 % of the stated value, and even more typically +/- 0.5 % of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein. Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of a process of fabricating a polymeric negative mould comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof will now be disclosed.

The process may comprise the steps of: (a) fabricating a positive master mould by two- photon lithography (TPL); and (b) fabricating a polymeric negative mould which is a polymeric negative replica of said positive master mould.

After fabricating step (a) but prior to fabricating step (b), the positive master mould of step (a) may undergo a treatment process, wherein said treatment process may comprise the steps of: (al) surface treating said positive master mould with oxygen plasma to obtain a surface-treated positive master mould; and (a2) silanizing said surface-treated positive master mould with a release agent.

The fabricating step (b) may be carried out by (b’) filling the positive master mould with a curable polymer or resin; and (b”) curing the polymer or resin, optionally via nanoimprint lithography, to obtain said polymeric negative mould.

Advantageously, the process may enable the nano-three-dimensional (3D) printing of intricate features of sizes as small as 120 nm on the mould, including multiple overhangs/ suspensions and/or hierarchical microstructures, using a wide variety of photoresist materials.

Exemplary, non-limiting embodiments of a process of fabricating a polymeric film comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof will now be disclosed.

The process may comprise the steps of: (a) fabricating a positive master mould by TPL; (b) fabricating a polymeric negative replica from said positive master mould; (c) imprinting the polymeric negative replica on the polymeric film to form the microstructures thereon; and (d) removing the polymeric negative replica from the imprinted polymeric film.

After fabricating step (b) but prior to imprinting step (c), the polymeric negative replica of step (b) may undergo a treatment process, wherein said treatment process may comprise the steps of: (bl) surface treating said polymeric negative replica with oxygen plasma to obtain a surface-treated polymeric negative replica; and (b2) silanizing said surface-treated polymeric negative replica with a release agent. The fabricating step (b) may be carried out by (b’) filling the positive master mould with a curable polymer or resin; and (b”) curing the polymer or resin, optionally via nanoimprint lithography (NIL), to obtain said polymeric negative replica.

The imprinting step (c) may be carried out using nanoimprint lithography (NIL).

Advantageously, the process may enable the nano-3D printing of intricate features of sizes as small as 120 nm, including multiple overhangs/ suspensions and/or hierarchical microstructures, whilst allowing the parallel replication of such microstructures into a robust, high-throughput and industrially scalable manufacturing process.

Further advantageously, the process may be environmentally-friendly in the synthesis of microstructures as it avoids the production of toxic chemical waste streams often seen in conventional subtractive technique such as chemical etching.

Still further advantageously, the process may be more efficient in the synthesis of microstructures as it avoids time-consuming alignment steps as seen in conventional subtractive technique such as physical etching.

Still further advantageously, the process may be flexible to meet on-demand requirements as the replicated film may have multiple structural designs as well as extensive material selection from various UV-curable or heat-curable photoresists.

Still further advantageously, the process may be independent of replica peeling angle and replica peeling speed, which may in turn improve the time efficiency and cost effectiveness of the process during manufacturing.

Exemplary, non-limiting embodiments of a process of generating multiple generations of a polymeric film comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof will now be disclosed.

The process may comprise the steps of: (a) fabricating a positive master mould by TPL; (b) fabricating a polymeric negative replica from said positive master mould; (c) imprinting the polymeric negative replica on the polymeric film to form the microstructures thereon; (d) removing the polymeric negative replica from the imprinted polymeric film; and (e) repeating steps (b) to (d) in sequence to generate multiple generations of polymeric negative replicas and polymeric positive replicas using said imprinted polymeric film of step (d) as the positive master mould.

After fabricating step (a) but prior to fabricating step (b), the positive master mould of step (a) may undergo a treatment process, wherein said treatment process may comprise the steps of: (al) surface treating said positive master mould with oxygen plasma to obtain a surface-treated positive master mould; and (a2) silanizing said surface-treated positive master mould with a release agent. After fabricating step (b) but prior to imprinting step (c), the polymeric negative replica of step (b) may undergo a treatment process, wherein said treatment process may comprise the steps of: (bl) surface treating said polymeric negative replica with oxygen plasma to obtain a surface-treated polymeric negative replica; and (b2) silanizing said surface-treated polymeric negative replica with a release agent.

After removing step (d) but prior to repeating step (e), the imprinted polymeric film may undergo a treatment process, wherein said treatment process may comprise the steps of: (dl) surface treating said imprinted polymeric film with oxygen plasma to obtain a surface-treated imprinted polymeric film; and (d2) silanizing said surface- treated imprinted polymeric film with a release agent.

In the repeating step (e), following each of steps (b) and (d), the newly generated polymeric negative replica and polymeric positive replica respectively, may each undergo a treatment process, wherein said treatment process may comprise a step of surface treating the polymeric replica with oxygen plasma to obtain a surface-treated polymeric replica, followed by a step of silanizing the surface-treated polymeric replica with a release agent.

The fabricating step (b) may be carried out by (b’) filling the positive master mould with a curable polymer or resin; and (b”) curing the polymer or resin.

The imprinting step (c) may be carried out using nanoimprint lithography (NIL).

Each copy of the polymeric negative replica or polymeric positive replica can be regarded as a generation of the polymeric negative replica or polymeric positive replica.

Advantageously, the first-generation polymeric negative replica that functions as a negative mould may produce at least 12 polymeric positive replicas, thus reducing the frequency of fabricating first generation polymeric negative replica via repeating of the drop-cast and curing requiring the TPL-fabricated positive master mould.

Further advantageously, multiple polymeric replicas may be attached together to form a larger continuous single replica using an adhesive agent or cured into a thin layer, which in turns allows the multiple polymeric replicas to be made into a single mould. This may increase the dimensions of the mould to scale up the area of imprinting and speed up the process of making large areas of microstructures

Still further advantageously, the process may allow multiple generations of polymeric moulds of at least 3 generations. This may in turn result in time- and cost-effective successive replications, eliminating the need of continuous TPL fabrication of master moulds, as each TPL step requires more time than each imprinting step.

Still further advantageously, the process may result in polymer films, polymer films with microstructures and polymeric replicas possessing a light transmission of above 85 %.The high transparency polymeric films and polymeric replicas patterned with microstructures may find potential applications in anti-fingerprint devices/ accessories, endoscopy and fingerprint sensors.

In the process, the multiple overhangs may be selected from the group consisting of cantilever, primary re-entrant, primary mushroom and primary doubly re-entrant microstructures.

In the process, the hierarchical microstructures may be selected from the group consisting of hierarchical cantilever, hierarchical re-entrant, hierarchical doubly re entrant, hierarchical mushroom, and combinations thereof.

In the process, the positive master mould, the polymeric negative replica or the polymeric positive replica has microstructures of at least one of the following dimensions: (i) connection point diameter (CPD) of about 8.5 pm to about 17.5 pm, (ii) connection point-to-cap ratio (CCR) > 0.55, (iii) aspect ratio (AR) < 4.5, (iv) microstructure height < 90 pm, and pitch size > 30 pm, wherein

Connection point diameter connection point — to — cap ratio (CCR) Cap diameter ’ and

Microstructure height aspect ratio (AR) = Diameter of column base^’

With regard to the connection point diameter, with reference to Fig. 3, this is defined as the diameter of the connection point (32).

With regard to the cap diameter, with reference to Fig. 3, this is defined as the diameter of the cap (34).

With regard to the microstructure height, with reference to Fig. 3, this is defined as the combined height (36) of the cap (40) and the column base (42). The height of the cap is termed skirt length.

With regard to the pitch size (38), with reference to Fig. 3, this is defined as length from the centre of a microstructure to the centre of an adjacent microstructure.

With regard to the diameter of column base, with reference to Fig. 3, this is defined as the diameter of the column base (44).

In the process, for the fabricating step (a), the TPL may involve the use of a laser to create the positive master mould on a support substrate. The fabricating step may be undertaken in a layer-by-layer manner using the laser.

The laser may be used in the form of laser pulses. The laser may be used in a scanning manner. The scanning laser may be x-y raster scanning.

In the process, the laser travels in a line-by-line mode in x- and y-directions. The lines may have a fixed distance between adjacent ones, which can be controlled. The laser moves from one layer (or one plane) to another layer (or plane) in z-directions after travelling through all the lines in x- and y-directions. The laser then travels in x- and y-directions again on the new layer (or new plane). The distance between the layers (or planes) can be manipulated. Both controllable line-to-line distance and layer- to- layer distance are termed slicing distance. It can be visualized as a 3D microstructure that is re-constructed by stacking up with sliced pieces of controlled dimensions.

In the process, the laser pulses used in the fabricating step (a) may have an average power rating in the range of about 60 mW to about 80 mW, about 65 mW to about 80 mW, about 70 mW to about 80 mW, about 75 mW to about 80 mW, about 60 mW to about 75 mW, about 60 mW to about 70 mW or about 60 mW to about 65 mW.

In the process, the laser used in the fabricating step (a) may have a laser scanning velocity in the range of about 55 mm/s to about 85 mm/s, about 65 mm/s to about 85 mm/s, about 75 mm/s to about 85 mm/s, about 55 mm/s to about 75 mm/s or about 55 mm/s to about 65 mm/s.

In the process, the fabricating step (a) may have a slicing distance in the x, y, and z Cartesians of about 0.2 pm during an x-y raster- scanning and a layer-by-layer fabrication of TPL.

In the process, the steps (b) and (e) for generating polymeric replicas may be conducted at a temperature in the range of about 25 °C to about 200 °C, about 50 °C to about 200 °C, about 100 °C to about 200 °C, about 150 °C to about 200 °C, about 25 °C to about 150 °C, about 25 °C to about 100 °C or about 25 °C to about 50 °C.

In the process, the steps (b) to (e) for generating polymeric replicas may be conducted for a duration in the range of about 35 seconds to about 12 hours, about 1 hour to about 12 hours, about 4 hours to about 12 hours, about 8 hours to about 12 hours, about 35 seconds to about 8 hours, about 35 seconds to about 4 hours or about 35 seconds to about 1 hour.

In the process, where the negative replica used is of the material polydimethylsiloxane, the steps for generating the polydimethylsiloxane negative replicas may be conducted at a temperature in the range of about 60 °C to about 120 °C, about 80 °C to about 120 °C, about 100 °C to about 120 °C, about 60 °C to about 100 °C or about 60 °C to about 80 °C.

In the process, the steps for generating polydimethylsiloxane negative replicas may be conducted for a duration in the range of about 3 hours to about 12 hours, about 6 hour to about 12 hours, about 9 hours to about 12 hours, about 3 hours to about 9 hours or about 3 hours to about 6 hours.

In the process, the positive master mould and the positive replica are preferably fabricated using liquid-based pre -polymer materials with glass transition temperature near/above usage temperature, high chemical resistance, high gas permeability, high solvent resistance, high elastic recovery and good mechanical strength. The positive master mould and the positive replica may comprise thermal-curable photoresists selected from the group consisting of but not limited to siloxane-based materials, esters, amides, imides, urethanes, phenolic resins, benzoxazines; and light-curable negative photoresists selected from the group consisting of but not limited to acrylates, epoxies, silicones, ceramic -based hybrids, f uoro-based compounds, compounds with vinyl groups, thiolene-based compounds; and mixtures thereof.

In the process, the polymeric negative replica and polymeric positive replica may be selected from thermal- and light-curable polymers consisting of but not limited to poly(dimethylsiloxane) (PDMS), perfluoropolyether (PFPE)-based photoresists, multifunctional hyperbranched polymer-incorporated perfluoropolyether (HPFPE), Teflon AF 2400 (a copolymer of 2,2-bistrifluoromethyl- 4,5-difluoro-l,3-dioxole and tetrafluoroethylene), polyesters, polyimides, polyamides, polybenzoxazines, polyurethanes, penolic resins; UV-curable vinyl polymers including but not limited to poly(urethane acrylate), polystyrene, poly(methyl methacrylate), polyacrylate esters, polyacrylamide, polyethylene glycol, polyallylamine, poly(N- isopropylacrylamide), polyacrylic acid, polymethacrylic acid, polyimides, bisphenol A novolac epoxy, polymer-ceramic hybrids; UV-curable thiolene-based polymers; and combinations thereof.

In the process, the negative mould and the polymeric negative replica are preferably fabricated using liquid-based pre -polymer materials with glass transition temperature below usage temperature, high chemical resistance, extremely low surface energy, high gas permeability, high solvent resistance, high elastic recovery and good mechanical strength. The liquid-based pre-polymer materials include but not limited to poly(dimethylsiloxane) (PDMS)-based materials, UV-curable perfluoropolyether (PFPE)-based photoresists, multifunctional hyperbranched polymer-incorporated perfluoropolyether (HPFPE), Teflon AF 2400 (a copolymer of 2,2- bistrifluoromethyl- 4,5-difluoro-l,3-dioxole and tetrafluoroethylene) and other commercially available UV-/thermal curable mould materials.

In the process, the positive master mould and the polymeric positive replica may be fabricated using a type of material depending on the applications.

In the process, the surface treating step (al), (bl) or (dl) may expose the polymeric replica to oxygen plasma for a duration in the range of about 5 minutes to about 15 minutes, about 10 minutes to about 15 minutes or about 5 minutes to about 10 minutes.

In the process, the surface treating step (al), (bl) or (dl) may expose the polymeric replica to oxygen plasma at a power in the range of about 100 W to about 300 W, about 200 W to about 300 W or about 100 W to about 200 W.

In the process, the release agent may be selected from the group consisting of silane coupling agent, fluorinated alkyl silane coupling agent, polysiloxane, polybenzoxazine, and combinations thereof. The fluorinated alkyl silane coupling agents may be selected from the group consisting of 1H,1H,2H2H- perfluorodecyltrichlorosilane, lH,lH,2H2H-perfluorooctyltriethoxysilane,

(tridecafluoro- 1 , 1 ,2,2,-tetrahydrooctyl)- 1-trichlorosilane, 1H, 1H,2H,2H- perfluorodecyltrimethoxysilane, 1H, 1H,2H,2H- heptadecafluorodecyltrimethoxysilane, ethanolic fluoroalkyl silane and combinations thereof.

The process may be independent of replica peeling angle and replica peeling speed, which may in turn improve the time efficiency and cost effectiveness of the process during manufacturing.

Exemplary, non-limiting embodiments of an array of microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof will now be disclosed.

In the array, the microstructures may have a connection point diameter (CPD) in the range of about 8.5 pm to about 17.5 pm, about 11.5 pm to about 17.5 pm, about 14.5 pm to about 17.5 pm, about 8.5 pm to about 14.5 pm or about 8.5 pm to about 11.5 pm.

In the array, the microstructures may have a connection point-to-cap ratio (CCR) of at least about 0.55, wherein connection point — to — cap ratio (CCR) =

- - .

In the array, the micro structures may have an aspect ratio of no more than about 4.5, wherein

Microstructure height aspect ratio (AR) = Diameter of column base^’

In the array, the microstructures may have a microstructure height of no more than about 90 pm.

In the array, the microstructures may have a pitch size of at least about 30 pm, wherein

With regard to the pitch size (38), with reference to Fig. 3, this is defined as length from the centre of a microstructure to the centre of an adjacent microstructure. With regard to the diameter of column base, with reference to Fig. 3, this is defined as the diameter of the column base (44).

In the array, the multiple overhangs may be selected from the group consisting of cantilever, primary re-entrant, primary mushroom and primary doubly re-entrant microstructures.

In the array, the hierarchical microstructures may be selected from the group consisting of hierarchical cantilever, hierarchical re-entrant, hierarchical doubly re entrant, hierarchical mushroom, and combinations thereof.

Exemplary, non-limiting embodiments of a polymeric film comprising an array of microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof will now be disclosed.

In the polymeric film, the microstructures may have a connection point diameter (CPD) in the range of about 8.5 pm to about 17.5 pm, about 11.5 pm to about 17.5 pm, about 14.5 pm to about 17.5 pm, about 8.5 pm to about 14.5 pm or about 8.5 pm to about 11.5 pm.

In the polymeric film, the microstructures may have a connection point-to-cap ratio (CCR) of at least about 0.55, wherein connection point — to — cap ratio (CCR) =

- - - .

In the polymeric film, the microstructures may have an aspect ratio of no more than about 4.5, wherein

Microstructure height aspect ratio (AR) = Diameter of column base^’

In the polymeric film, the microstructures may have a microstructure height of no more than about 90 pm.

In the polymeric film, the micro structures may have a pitch size of at least about 30 pm, wherein pitch size = length from the centre of a microstructure to the centre of an adjacent microstructure.

In the polymeric film, the multiple overhangs may be selected from the group consisting of cantilever, primary re-entrant, primary mushroom and primary doubly re-entrant microstructures.

In the polymeric film, the hierarchical microstructures may be selected from the group consisting of hierarchical cantilever, hierarchical re-entrant, hierarchical doubly re-entrant, hierarchical mushroom, and combinations thereof.

The polymeric film may have a light transmission of about above 85 %.

The polymeric film may be superhydrophobic, ultraoleophobic or superamphiphobic.

The polymeric film may be liquid-repelling.

The polymeric film may be suitable for a wide range of applications such as anti fingerprint, self-cleaning surface, slippery coating for bottle’s inner wall, feeding tubing and anti-counterfeiting.

The polymeric film may have a static liquid contact angle for water in the range of about 150 ° to about 170 °, about 155 ° to about 170 °, about 160 ° to about 170 °, about 165 ° to about 170 °, about 150 ° to about 165 °, about 150 ° to about 160 ° or about 150 ° to about 155 ° for water as measured according to static sessile drop method.

The polymeric film may have a static liquid contact angle for glycerol in the range of about 150 ° to about 170 °, about 155 ° to about 170 °, about 160 ° to about 170 °, about 165 ° to about 170 °, about 150 ° to about 165 °, about 150 ° to about 160 ° or about 150 ° to about 155 ° as measured according to static sessile drop method.

The polymeric film may have a static liquid contact angle for soybean oil in the range of about 135 ° to about 155 °, about 140 ° to about 155 °, about 145 ° to about 155 °, about 150 ° to about 155 °, about 135 ° to about 150 °, about 135 ° to about 145 ° or about 135 ° to about 140 ° as measured according to static sessile drop method.

The polymeric film may have a surface exhibiting anti-wetting with various liquids.

The polymeric film may have a surface tension for water in the range of about 50 mN/m to about 90 mN/m, about 60 mN/m to about 90 mN/m, about 70 mN/m to about 90 mN/m, about 80 mN/m to about 90 mN/m, about 50 mN/m to about 80 mN/m, about 50 mN/m to about 70 mN/m, about 50 mN/m to about 60 mN/m or about 70 mN/m to about 75 mN/m at 25 °C.

The polymeric film may have a surface tension for glycerol in the range of about 40 mN/m to about 80 mN/m, about 50 mN/m to about 80 mN/m, about 60 mN/m to about 80 mN/m, about 70 mN/m to about 80 mN/m, about 40 mN/m to about 70 mN/m, about 40 mN/m to about 60 mN/m, about 40 mN/m to about 50 mN/m or about 60 mN/m to about 65 mN/m at 25 °C.

The polymeric film may have a surface tension for soybean oil in the range of about 10 mN/m to about 50 mN/m, about 20 mN/m to about 50 mN/m, about 30 mN/m to about 50 mN/m, about 40 mN/m to about 50 mN/m, about 10 mN/m to about 40 mN/m, about 10 mN/m to about 30 mN/m, about 10 mN/m to about 20 mN/m or about 25 mN/m to about 35 mN/m at 25 °C.

Brief Description of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig· 1

[Fig. 1] is a schematic diagram of fabrication procedures using two-photon lithography (TPL) and nanoimprinting lithography (NIL).

Fig. 2

[Fig. 2] shows the robustness of TPL and NIL in generating replicates of (A) doubly re-entrant microstructures, (B) hierarchical doubly re-entrant microstructures, and (C) hierarchical mushroom-shaped microstructures. Schematic illustrations of the respective microstructural designs are shown on the top and scanning electron microscopy (SEM) micrographs of replicated microstructures made of polyurethane acrylates (PUA) as shown at the bottom.

Fig. 3

[Fig. 3] shows the dependency of replication on microstructures ’ pitch size. Fig. 3A shows schematic representatives illustrating structural designs. SEM micrographs of doubly re-entrant replicated microstructures made of polydimethylsiloxanes (PDMS) are provided with a pitch size of (B) 30 pm, (C) 40 pm, (D) 60 pm, (E) 80 pm and (F) 100 pm. All microstructures have a structural height of 15 pm.

Fig. 4

[Fig. 4] shows the effect of connection point diameter (CPD) on replication. Doubly re-entrant microstructures are provided with a CPD of (A) 17.5 pm, (B) 15.0 pm, (C) 11.5 pm and (D) 8.5 pm. Schematic illustrations of the respective microstructural designs are shown on the top and SEM micrographs of replicated microstructures made of PDMS with a height and a pitch size of 15 pm and 80 pm, respectively are shown at the bottom. Connection point to cap ratio (CCR) = CPD / 20 pm. A microstructure having a CCR of at least 0.55 is replicable. Fig. 4C shows microstructures at critical CCR. Microstructures having a CCR lower than that of the microstructures in Fig. 4C will not be replicable.

Fig. 5

[Fig. 5] shows the effect of microstructure height on replication. Fig. 5A is a schematic representative illustrating structural designs. SEM micrographs of doubly re-entrant replicated microstructures made of PDMS are provided with a height of (B) 10 pm, (C) 30 pm, (D) 50 pm, (E) 70 pm and (F) 90 pm. All microstructures have a pitch size of 80 pm, a cap diameter of 20 pm, a skirt length of 1 pm, a diameter of column base of 16 pm and a CPD of 15 pm.

Fig. 6

[Fig. 6] shows replicated microstructures made of (A) PDMS, (B) OrmoCore and (C) PUA. Fig. 6C(ii) is a tilted cross-sectional SEM micrograph of C(i) to showcase capability and delicacy of NIL in replicating overhanging feature of the doubly re entrant microstructures.

Fig. 7

[Fig. 7] shows a combination of TPL and NIL creating arrays constituting multiple microstructures with (A) different pitch sizes while structural height is 15 pm, and (B) different structural heights of (i) 30 pm and (ii) 15 pm, with a constant pitch size of 80 pm.

Fig. 8

[Fig. 8] shows a static contact angle of (A) water, (B) glycerol and (C) soybean oil measured on PDMS replicates with a pitch size of 40 pm. Surface tension (ST) of each liquid is indicated.

Fig. 9

[Fig. 9] shows a combination of TPL and NIL creating arrays constituting multiple microstructures with different structural heights of (A) 30 pm and (B) 15 pm, with constant pitch size of 80 pm and cap diameter of 20 pm. Fig. 9C shows a tilted SEM micrograph of (A) and (B). This is the third PUA replicates obtained from the same PUA negative moulds. Fig. 9D shows a magnified SEM micrograph of (C). Fig. 9 shows large area replications of an array with microstructures of different dimensions while demonstrating feasibility of TPL on creating an array with different structural designs and spatial manipulation. The total area of the array is 8 mm x 1 mm.

Fig. 10

[Fig. 10] shows a comparison on effects of CPD of the doubly re-entrant microstructures on multiple replications. Doubly re-entrant microstructures are provided with a CPD of (A) 17.5 pm, (B) 15.0 pm, (C) 11.5 pm and (D) 8.5 pm. Schematic illustrations of the respective microstructural designs are shown in row (i) and SEM micrographs of replicated microstructures made from the first (row (ii)) and second (row (iii)) generation of PDMS moulds with a height and a pitch size of 15 pm and 80 pm, respectively are also provided.

Fig. 11

[Fig. 11] shows the effect of microstructure height on multiple replications. Fig. 11 A shows schematic representatives illustrating structural designs. SEM micrographs of replicated PUA doubly re-entrant microstructures made from first (row (i)) and second (row ii) generation of polydimethylsiloxanes (PDMS) moulds are provided with a height of (B) 10 pm, (C) 30 pm, (D) 50 pm, (E) 70 pm and (F) 90 pm, respectively. All microstructures have a pitch size of 80 pm, a cap diameter of 20 pm, a skirt length of 1 pm, a base diameter of 16 pm and a CPD of 15 pm.

Fig. 12

[Fig. 12] shows the dependency of multiple replications on microstructures’ pitch sizes. Fig. 12A shows schematic representatives illustrating structural designs. SEM micrographs of replicated PUA doubly re-entrant microstructures made from first (row (i)) and second (row (ii)) generation of polydimethylsiloxanes (PDMS) moulds are provided with a pitch size of (B) 30 pm, (C) 40 pm, (D) 60 pm, (E) 80 pm and (F) 100 pm, respectively. All microstructures have a structural height of 15 pm.

Fig. 13

[Fig. 13] shows the dependency of replication on microstructures’ length of overhangs. The second- generation replicated PUA doubly re-entrant microstructures are replicated from the second-generation polydimethylsiloxanes (PDMS) moulds with a pitch size of 80 pm and a structural height of 15 pm. Cross-sectional SEM micrographs of the second-generation replicated PUA doubly re-entrant microstructures with length of overhangs = 1 pm are shown in row (i). Cross- sectional SEM micrographs of the second-generation replicated PUA doubly re entrant microstructures with length of overhangs = 2 pm are shown in row (ii). Row (i) represents replicates from the first generation mould. Fig. 13A(i) and Fig. 13B(i) demonstrate microstructures from 2 locations; while Fig. 13C(i) is the enlarged version of Fig. 13B(i) to show well-defined overhanging features are reserved. Similarly, row (ii) represents replicates from the second generation mould. Fig. 13A(ii) and Fig. 13B(ii) demonstrate microstructures from 2 locations; while Fig. 13C(ii) is the enlarged version of Fig. 13B(ii) to show well-defined overhanging features are reserved.

Fig. 14

[Fig. 14] shows doubly re-entrant microstructures patterned polymer films. Fig. 14A shows multiple first-generation (1G) PUA positive moulds (1 cm x 1 cm each) fabricated from 1G PDMS mould which were stitched up. Subsequently, second-generation (2G) PDMS negative mould was made from these stitched 1G PUA positive mould. It was followed by formation of 2G PUA positive moulds by casting PUA photoresists over 2G PDMS mould.

Fig. 14B shows SEM micrographs of the 1G PUA positive moulds of Fig. 14A.

Fig. 14C shows a comparison of the visibility of printed words under conditions with and without patterned films to determine relative transparency of the patterned films. Fig. 14C(i) shows printed words “3D printing” displayed without fdm. Fig. 14C(ii) shows printed words “3D printing” covered by the patterned PUA fdm. It was observed that the printed words underneath are highly visible.

Fig. 14D shows transmission measurements of patterned and not patterned PUA and PDMS films using UV-Vis spectrometer. Displayed curves are averages of 3 measurements.

Fig. 15

[Fig. 15] shows replication results of PUA (columns (A) and (B)) hierarchical doubly re-entrant microstructures and (columns (C) and (D)) hierarchical mushroom microstructures after the single (row (i)) and double (row (ii)) replication cycles as described herein. All replicates have well-defined microstructures and possess high design integrity. Distortions observed at base of 2G PUA microstructures might be due to human error during replication or charging during SEM characterization.

Fig. 16

[Fig. 16] shows a potential of a PDMS negative mould to be reused in imprinting of multiple PUA positive replicates.

Fig. 16A shows a digital image of a glass substrate (234) with 12 arrays of PUA positive replicates (232).

Fig. 16B shows an SEM micrograph of one of the 12 PUA positive replicates, demonstrating that well-defined doubly re-entrant microstructures can be repeatedly imprinted on PUA films by using only a PDMS negative mould.

Fig. 17

[Fig. 17] shows a comparison of a static contact angle of liquids on (A) and (B) doubly re-entrant microstructures, and (C) and (D) hierarchical doubly re-entrant microstructures. Both arrays have same structural dimensions and a pitch size of 80 pm. Hierarchical doubly re-entrant microstructures are 5 pm taller due to an addition of miniature microstructures. Detailed Description of Figures

Referring to Fig. 1, there is provided a schematic diagram of a fabricating an imprinted polymeric film comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, which involves the use of two-photon lithography and nanoimprinting lithography.

Fig. 1(A) shows a process of forming a positive master mould by TPL. Laser beam (8) is focused through an objective lens (2) on monomers (6) for TPL, which are placed on a piece of cover glass (4). Inside the monomers, polymers (10) are formed at a focal point of the laser beam to develop microstructures.

Fig. l(B)(i) shows a step of forming a polymeric negative replica (16). Polymers (12) are cured on microstructures formed from TPL (14) to form a polymeric negative replica (16).

Fig. l(B)(ii) shows a step of forming an imprinted polymeric film, which can be carried out using nanoimprinting lithography. A photo resist (20) is placed between the polymeric negative replica (16) and a glass slide (18). Ultraviolet light (22) is then irradiated onto the glass slide to cure the photo resist (20) into an imprinted polymeric film (24). The polymeric negative replica (16) is later removed from the imprinted polymeric film (24).

Examples

Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1 - Overview of Fabricated Superamphiphobic Surfaces

In this example, a replication of superamphiphobic surfaces patterned with doubly re-entrant microstructures via two-photon lithography (TPL) and nanoimprint lithography (NIL) according to the process of Fig. 1 is described.

TPL was used to fabricate arrays of designated three-dimensional (3D) microstructures (Fig. 1(A)). These TPL written 3D microstructures were later coated with polymers (12) such as polydimethylsiloxanes (PDMS) to form a mould for NIL (Fig. (B)(i)). Photo resist (20) such as UV-curable or heat-curable photoresists could be used to create replicates from the PDMS mould (Fig. l(B)(ii)).

Robustness and efficacy of TPL and NIL in generating replicates of various microstructure designs were demonstrated. The versatile approach effectually reproduced replicates of doubly re-entrant microstructures, hierarchical doubly re- entrant microstructures and hierarchical mushroom microstructures (Fig. 2). All the replicates retained pre-designated structural designs and features.

The influence of structural design parameters on final replication was also determined. The effects of pitch size (center-to-center distance) (Fig. 3), connection point diameter (Fig. 4) and structural height on replicating doubly re-entrant microstructures (Fig. 5) were studied. Scanning electron microscopy (SEM) micrographs of the replicates confirmed that the approach was promising in generating replicates of arbitrary designs. Nevertheless, for NIL, it was well-known that microstructures with extreme height and too narrow connection points were unable to withstand peeling and shear forces. These phenomena were also observed on the microstructures with extreme designs. Results obtained from these systematic experiments allowed for better tuning design parameters for scale up purposes.

The approach was flexible for different types of photoresists. Depending on requirements of final products, various UV-curable (polyurethane acrylates, organic- inorganic hybrid resins) or heat-curable (polydimethylsiloxanes, PDMS) photoresists could be used for replicating the well-defined doubly re-entrant microstructures during NIL (Fig. 6). This again illustrated robustness and versatility of the approach.

The capability of the approach in generating an array with microstructures of different designs was showcased by simply using hassle-free TPL-cum-NIL techniques (Fig. 7). The doubly re-entrant microstructures of different pitch sizes shown in Fig. 6 A (I to iii) were actually fabricated on the same substrates. Notably, these doubly re entrant microstructures patterned surfaces exhibited superamphiphobicity (Fig. 8). Static contact angle of water on the array was (152 ± 2)°, those of glycerol and soybean oil were (163 ± 2)° and (138 ± 1)°, respectively. Water, glycerol and oil were commonly used as compositions for artificial fingerprints. Surface properties of substrates patterned with the doubly re-entrant microstructures could be further improved easily via currently established polymer functionalization approaches.

Example 2 - Substrate Cleaning and Functionalization

Glass chips (square shape, 24 mm x 24 mm size, 0.7 mm thickness, fused silica material, purchased from NanoScribe) were cleaned under oxygen plasma in Triple P plasma processor (Duratek) to remove surface contaminants. The glass slides were brought to oxygen plasma for 2 minutes at 100 W, 0.5 torr pressure with oxygen flow. Silanization was performed by solvent base method. Two to four pieces of cleaned glass chips were placed in a centrifuge tube for 10 minutes at room temperature which was then filled with anhydrous ethanol (40 mL, purchased from Sigma Aldrich) and (3-aminopropyl)-trimethoxysilane (APTES, 2 v/v% purchased from Sigma Aldrich) for silanization. Subsequently, the glass chips were submerged in a solution (40 mL) containing equal weight- or volume -ratio of water and ethanol for 5 minutes. It is followed by washing with copious of ethanol and drying in oven at 60 °C to 80 °C for 2 hours to 6 hours, or until solvents are evaporated. Example 3 - Two-photon Lithography

Fabrication of the doubly re-entrant microstructures were performed using the Nanoscribe® Photonic Professional. The system was equipped with a femtosecond laser source with a center wavelength of 780 nm. Micro structures of the doubly re entrant were designed using a CAD software, Solid Works. Parameters of the microstructures were defined by the Nanoslicer and DeScribe softwares. Both the movement of piezo-driven nanopositioning scanning sample stage as well as emitting power of the laser were controlled by NanoWrite software as programmed using the Nanoslicer and Describe softwares. All microstructures were written on square glass substrates with 70mW laser power, 0.2 pm line (x-y) distance, scan speed of 50 mm/s. A resist containing acrylate (0.1 mL to 0.5 mL, IP-Dip and IP-L photoresists purchased from NanoScribe) was deposited on a glass substrate. Photopolymerization started at the interface between the resist and the glass substrate, allowing fabricated microstructures to be firmly attached to the glass substrate. The resist within the confocal volume of the laser underwent polymerization and cross- linking on glass substrate. After fabrication, the substrate was soaked in propylene glycol methyl ether acetate (20 mL to 50 mL, purchased from Sigma Aldrich) for 10 minutes, followed by a 10-minute immersion in isopropanol 20 mL to 50 mL, purchased from Sigma Aldrich) to remove excess unpolymerized resist. Subsequently, the substrate was stored in a nitrogen box. The fabricated microstructures remained fixed on glass substrates after development.

Example 4 - Thermal Nanoimprint Lithography

A negative replica of the doubly re-entrant microstructures was created using polydimethylsiloxane (PDMS). PDMS mould was fabricated using SYLGARD® 184 silicone elastomer kit purchased from Dow Corning. The base and curing agent were mixed in weight ratio 10:1 and the mixture was degassed prior to and after casting over the original doubly re-entrant microstructures on glass. PDMS was cured at 70 °C under ambience atmosphere in an oven for 6 hours before demolding from parent pattern array and was ready to use as a soft mould for subsequent replication.

Example 5 - Functionalization of PDMS Moulds

To reduce adhesion for easy peeling, the PDMS moulds were coated with a layer of release agent. In this example, the negative PDMS moulds were brought to oxygen plasma for 5 minutes at 100 W, followed by surface silanization with 1H,1H,2H,2H- perfluorooctyltriethoxysilane (20 pL, purchased from Alfa Aesar) for subsequent replication of PDMS microstructures. The surface silanization was done via vapor deposition of the silane under vacuum in a glass desiccator at room temperature overnight. Positive replica was created in the same way as mentioned above where PDMS was over the functionalized negative replica/soft PDMS mould. Upon demolding, well- defined doubly re-entrant microstructures were achieved.

Example 6 - Characterisation of Structural Sizes

The following values indicate the structural dimensions that can produce well-defined replicates using the processes as described herein. These values were obtained via a series of thorough systematic studies on structural integrity of replicates against effects of structural dimensions (Fig. 3 to Fig. 5). The below requirements are important as they play vital role in retaining structural integrity of the microstructures against shear stress and pulling force subjected to the microstructures during the mould removal steps in NIL. a. Connection point-to-cap ratio (CCR) > 0.55 (Fig. 4).

Having a connection point-to-cap ratio (CCR) of at least 0.55 was to make sure the overhanging caps had sufficient connection to the main body/support.

When Connection point-to-cap ratio (CCR) was less than 0.55, more overhanging area of the cap (bottom of the cap) was in contact with the negative mould’s material, resulting in greater stiction and friction being experienced by the overhanging area. As a result, imperfect or torn imprinted microstructures were obtained due to high stiction, friction and shear forces impacted on overhanging features of the microstructures.

When connection point-to-cap ratio (CCR) was at least 0.55, contact between imprinted microstructures and the negative mould decreased which implied less stiction and frictional force bring experienced by the microstructures. Importantly, with sufficient connection, the caps could then resist tearing and shearing forces subjected on them (the positive replicas) during removal of negative mould in the NIL step, and render structural details and design integrity in the positive replicas. b. Aspect ratio < 4.5 (Fig. 5).

Shear stress experienced by microstructures increased with height of the microstructures, resulting in possible deformation of the microstructures from the substrates during mould removal as the microstructures were entrapped inside holes of the negative moulds. Therefore, a specific design would have a reasonable maximum aspect ratio value which would make the imprinted microstructures survive from mould removal. c. Pitch size > 30 pm (Fig. 3)

The cap had a radius of 10 pm, the radius of supporting pillar was 8 pm. When pitch size was 30 pm, the gap between edges of 2 caps was 10 pm, while the gap between edges of 2 supporting pillar was about 14 to 20 pm (the size of the features of negative mould that needed to be pulled out from the cured imprinted positive replicates).

Therefore, if the pitch size was 20 pm, the gap between edges of 2 caps was 0 pm, while the gap between edges of 2 supporting pillar was about 2 to 10 pm (the size of the features of negative mould that needed to be pulled out from the cured imprinted positive replicates). In this case, all the imprinted microstructures might be destroyed during mould removal.

The parameters are calculated as follows.

Connection point diameter connection point — to — cap ratio (CCR)

Microstructure height aspect ratio (AR) = Diameter of column base^’

Example 7 - Single Replication Cycle

The single replication cycle involved two-photon lithography of pre-designed doubly re-entrant microstructures (master mould). It was followed by oxygen plasma, surface silanization and casting of PDMS (negative mould). Subsequently, poly(urethane acrylate) (PUA) was dropcasted on PDMS negative mould to obtain well-defined replicated PUA doubly re-entrant microstructures and the outcomes were unexpectedly successful.

Aliphatic urethane acrylate in tripropyleneglycol diacrylate (Ebecryl E265) and trimethylolpropane ethoxy triacrylate (TMPEOTA) were puchased from Cytec. Darocur 1173 and Irgacure 184 were purchased from Sigma Aldrich.

UV-curable PUA was prepared by first mixing Ebecryl E265 with TMPEOTA at a weight ratio of 7:3. Then Darocur 1173 and Irgacure 184 were stirred into the earlier mixture at a final concentration of 1 weight % each. The resulting mixture, known as PUA, was degassed via sonication to remove bubbles prior to usage.

Glass slides were functionalized with 3-(trimethoxysilyl) propyl methacrylate (purchased from Sigma Aldrich). 100 pL of 3-(trimethoxysilyl) propyl methacrylate was added to 5 mL of ethanol and mixed well. Solution was poured over a glass petri- dish and glass slides were immersed in the solution. The petri dish was gently swirled for 3 minutes to allow for uniform reaction on the glass slides. Then glass slides were removed and rinsed with ethanol, followed by drying on a hotplate at 75 °C for 15 minutes.

Approximately 0.05 g of PUA was dropped casted over PDMS negative mold, which was then covered with the functionalized glass slide on top. They were cured for 35 seconds under UV light (Dymax Zip Shutter for B39721 UV Flood Lamp) before demolding PDMS off the PUA replicate on glass. UV light intensity was determined by UV power puck II from EIT at approximately 250 mJ/cm².

By using this process for design optimization to tune the dimensional specification of the microstructures, dimensional specifications were obtained via a series of thorough systematic studies on structural integrity of replicates against effects of structural dimensions (Fig. 3 to Fig. 5, Fig. 7 and Fig. 9). a. Connection point-to-cap ratio (CCR) > 0.55 (Fig. 4). b. Aspect ratio < 4.5 (Fig. 5). c. Pitch size > 30 pm (Fig. 3). d. Ability of proposed strategy in producing an array of microstructures with different pitch size (Fig. 7A). e. Ability of proposed strategy in producing an array of microstructures with different height (Fig. 9).

The parameters are calculated as follows.

.. . . .. Connection point diameter connection point — to — cap ratio (CCR) = - - .

Microstructure height aspect ratio (AR) = Diameter of column base^’ pitch size = length from the centre of a microstructure to the centre of an adjacent microstructure.

With regard to the connection point diameter, with reference to Fig. 4, this is defined as the diameter of the connection point (44).

With regard to the cap diameter, with reference to Fig. 4, this is defined as the diameter of the cap (42). With regard to the microstructure height, with reference to Fig. 4, this is defined as the combined height of the cap (42) and the column base (46).

With regard to the diameter of column base, with reference to Fig. 4, this is defined as the diameter of the column base (46).

Example 8 - Double Replication Cycle

The double replication cycles involved two-photon lithography of pre-designed doubly re-entrant microstructures (master mould). It was followed by oxygen plasma, surface silanization and casting of PDMS over the printed microstructures to obtain the first PDMS negative mould (first-generation (1G) PDMS mould). Subsequently, PUA was dropcasted on the PDMS negative mould to get replicated PUA doubly re entrant microstructures. In the following steps, the replicated PUA doubly re-entrant microstructures acted as the 1G PUA positive mould and was used to generate second-generation (2G) PDMS negative mould. Lastly, 2G PUA positive mould was made out from the 2G PDMS mould.

Multiple PUA positive polymeric replicas could be put together to form a larger mould using an adhesive agent or cured into a thin layer of the polymer solution used to make the replica. This might in turn increase the dimensions of the mould to scale up the area of imprinting and speed up the process of making large areas of microstructures. Furthermore, the process might allow multiple generations of polymeric moulds of at least generations. This might in turn result in time- and cost- effective successive replications, eliminating the need of continuous TPL imprinting of master moulds, as each TPL step requires more time than each NIL step.

The reusable replicating moulds for multiple replications of arrays with doubly re entrant microstructures were verified (Fig. 10 to Fig. 15). Results from the SEM micrographs clearly indicated both suspending and overhanging features of the doubly re-entrant microstructures were successfully reproduced with surprisingly well-defined design integrity, especially in the second replicating cycle (Fig. 13). The replications had high reproducibility and could produce large-area of doubly re entrant microstructures with satisfied structural homogeneity using the second- generation moulds (Fig. 14). Notably, 2G PUA hierarchical microstructures were obtained via this approach with all details intact and high design integrity (Fig. 15). Therefore, as long as surfaces of both first- and second-generation moulds were functionalized with perfluorosilanes to reduce surface stickiness and release force, it might be possible to continuously replicate micro structures from the first- and second-generation moulds. Consequently, it might be possible to generate multiple generations of PDMS moulds (2 times as described herein) for successive replications without the need of continuous two-photon printing of the master moulds. The approach to fabricate reusing imprinting moulds for replications of doubly re-entrant microstructures was time-saving and cost-effective By using this fabrication method, a large area of 2G PUA positive mould (2 cm x 2 cm) was generated with high transparency (Fig. 14). After fabricating 1G PDMS mould from the two-photon written master mould of doubly re-entrant microstructures, multiple 1G PUA positive moulds (1 cm x 1 cm each) produced from 1G PDMS mould were stitched up (Fig. 14A). Subsequently, large 2G PDMS negative mould (2 cm x 2 cm) was made from these stitched 1G PUA positive mould. It was followed by formation of 2G PUA positive mould by casting PUA photoresists over the 2G PDMS mould. The produced 2G PUA positive mould had very high transparency (transmission > 85 %) (Fig. 14C and 14D).

Hitachi UH4150 Spectrophotometer was used to determine the transmission of substrates patterned with doubly re-entrant microstructures having wavelengths in the visible range (from 350 nm to 800 nm). An average was taken from 3 measurements as described herein. Blank polymer films of the same thickness as the substrates patterned with doubly re-entrant microstructures were used as reference. Thickness of samples is kept at 0.5 mm.

Instead, if 2G positive mould was produced using PDMS, transmission of the patterned PDMS film was > 90 % (Fig. 2 ID).

Example 9 - Reusability of the Mould

Experiments were also conducted to test the reusability of the first-generation negative mould made from the TPL-fabricated positive master mould. It was found that the first-generation negative mould could generate multiple positive replicas of at least 12 positive replicas while retaining the structural integrity of the microstructures in the array of the fabricated replicas (Fig. 16A and 16B). Hence, through the results of Fig. 16A and 16B, it was proven that the first-generation polymeric negative replica that functions as a negative mould could produce at least 12 polymeric positive replicas, thereby reducing the frequency of fabricating first generation polymeric negative replica via repeating of the imprinting process using TPL-fabricated positive master mould.

Example 10 - Liquid Repelling Properties of Microstructures

Rame-Hart contact angle goniometer (Model 590-U1), equipped with a high speed GigE camera and DROPimage Advanced program, was used to image and measure the static and dynamic contact angles of liquids on substrates. At least 5 measurements were conducted for each set of data. Static contact angle was measured using sessile drop method. A liquid droplet of 5 pL to 10 pL was carefully dispensed onto the tested surfaces. Measurements were normally conducted at room temperature and with relative humidity of Singapore.

According to Wenzel equation (cos Q* = r cos qg, where Q* is the apparent contact angle and qg is the equilibrium static contact angle from Young’s equation on an ideal flat solid without roughness), introduction of miniature micro structures to flat surface will increase static contact angle of liquids. This statement worked for the doubly re entrant microstructures as described herein. To compare the liquid repelling properties between the primary (Fig. 17A) and hierarchical doubly re-entrant microstructures (Fig. 17B), only solvent with high surface tension was used, which was water, for demonstration. From Fig. 17, static contact angle of water increased almost 10° from about 158° (average of 5 measurements) to 165° (average of 5 measurements). This was due to miniature micro structures on the hierarchical doubly re-entrant microstructures increasing both surface roughness of the patterned films and number of air pockets, leading to lesser contact between liquids and patterned surfaces. This high value can be achieved even without chemical modification, and can be higher if the surface is chemically modified.

Comparative Example 1 - Microstructures and Water Contact Angles of Surfaces Produced by Conventional Methods

Various known microstructures are produced by conventional methods and their contact angles with water measured. The microstructures all have lower water contact angle when compared with the superamphiphobic surface as described herein (as according to Example 10). Table 1 below shows the known microstructures, the conventional methods used in their preparation and the resulting water contact angle.

Table 1 Microstructures and water contact angles of surfaces produced by conventional methods

As can be seen from the above, conventional methods to form microstructures are unable to achieve the level of superhydrophobicity reached by the microstructures of this disclosure, which were fabricated by a combination of TPL and NIL. Industrial Applicability

The polymeric film may be used in a variety of applications such as anti-fingerprint devices/accessories, endoscopy and fingerprint sensors.

Due to the superamphiphobic, superhydrophobic or ultraoleophobic property of the microstructures created on polymeric films, the polymeric films can be formulated into coatings or surfaces where water or oil need to be repelled, such as touch-screens, electronic devices, construction microstructures, blind spot mirrors, fingerprint scanners, contact lenses, etc.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A process of fabricating a polymeric negative mould comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, comprising the steps of:

(a)fabricating a positive master mould by two-photon lithography (TPL); and

(b)fabricating a polymeric negative mould which is a polymeric negative replica of said positive master mould.

2. The process of claim 1, further comprising, after fabricating step (a) but before fabricating step (b), the steps of:

(al) surface treating said positive master mould with oxygen plasma to obtain a surface- treated positive master mould; and

(a2) silanizing said surface-treated positive master mould with a release agent.

3. The process of claim 1 or 2, wherein the fabricating step (b) comprises the steps of: (b’) filling the positive master mould of step (a) with a curable polymer or resin; and (b”) curing the polymer or resin via nanoimprint lithography (NIL), to obtain said polymeric negative mould.

4. A process of fabricating a polymeric film comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, comprising the steps of:

(a)fabricating a positive master mould by TPL;

(b)fabricating a polymeric negative replica from said positive master mould;

(c)imprinting the polymeric negative replica on the polymeric film to form the microstructures thereon; and

(d)removing the polymeric negative replica from the imprinted polymeric film.

5. The process of claim 4, further comprising, after fabricating step (a) but before fabricating step (b), the steps of:

6. The process of claim 4 or 5, further comprising, after fabricating step (b) before imprinting step (c), the steps of:

(bl)surface treating said polymeric negative replica with oxygen plasma to obtain a surface-treated polymeric negative replica; and

(b2) silanizing said surface-treated polymeric negative replica with a release agent.

7. The process of any one of claims 4 to 6, wherein the fabricating step (b) comprises the steps of:

(b’) filling the positive master mould of step (a) with a curable polymer or resin; and (b”) curing the polymer or resin via NIL, to obtain said polymeric negative mould.

8. The process of any one of claims 4 to 7, wherein the imprinting step (c) is undertaken using NIL.

9. A process of generating multiple generations of a polymeric film comprising microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, comprising the steps of:

(a)fabricating a positive master mould by TPL;

(b)fabricating a polymeric negative replica from said positive master mould;

(c)imprinting the polymeric negative replica on the polymeric film to form the microstructures thereon;

(d)removing the polymeric negative replica from the imprinted polymeric film; and

(e)repeating steps (b) to (d) in sequence to generate multiple generations of polymeric negative replicas and polymeric positive replicas using said imprinted polymeric film of step (d) as the positive master mould.

10. The process of claim 9, further comprising, after fabricating step (a) but before fabricating step (b), the steps of:

11. The process of claim 9 or 10, further comprising, after fabricating step (b) but before imprinting step (c), the steps of:

12. The process of any one of claims 9 to 11, further comprising, after removing step (d) but before repeating step (e), the steps of:

(dl) surface treating said imprinted polymeric film with oxygen plasma to obtain a surface-treated imprinted polymeric film; and

(d2) silanizing said surface-treated imprinted polymeric film with a release agent.

13. The process of any one of claims 9 to 12, wherein the repeating step (e) further comprises steps (bl) and (b2) of claim 11 or steps (dl) and (d2) of claim 12.

14. The process of any one of claims 9 to 13, wherein the fabricating step (b) comprises the steps of:

15. The process of any one of claims 9 to 14, wherein the imprinting step (c) is undertaken using NIL.

16. The process of any one of claims 9 to 15, wherein the positive master mould, the polymeric negative replica or the polymeric positive replica has microstructures of at least one of the following dimensions:

(i) connection point diameter (CPD) of about 8.5 pm to about 17.5 pm,

(ii) connection point-to-cap ratio (CCR) > 0.55,

(iii) aspect ratio (AR) < 4.5, micro structure height < 90 pm, and

(iv) pitch size > 30 pm, wherein Connection point diameter connection point — to — cap ratio (CCR) Cap diameter

Microstructure height ^ aspect ratio (AR) = Diameter of column base’ pitch size = length from the centre of a micro structure to the centre of an adjacent mciro structure.

17. An array of microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, wherein at least one of the structural dimensions of the microstmctures are as follow:

(i) connection point diameter (CPD) of about 8.5 pm to about 17.5 pm,

(ii) connection point-to-cap ratio (CCR) > 0.55,

(iii) aspect ratio (AR) < 4.5, micro structure height < 90 pm, and

(iv) pitch size > 30 pm, wherein

. . . Connection point diameter connection point — to — cap ratio (CCR) = - - ;

Mirostructure height aspect ratio (AR) = and Diameter of column base’ pitch size = length from the centre of a micro structure to the centre of an adjacent micro structure.

18. The array of claim 17, wherein the multiple overhangs are selected from the group consisting of cantilever, primary re-entrant, primary mushroom and primary doubly re entrant micro structures; and wherein the hierarchical microstructures are selected from the group consisting of hierarchical cantilever, hierarchical re-entrant, hierarchical doubly re-entrant, hierarchical mushroom, and combinations thereof.

19. A polymeric film comprising an array of microstructures selected from multiple overhangs, hierarchical microstructures or a combination thereof, wherein at least one of the structural dimensions of the microstructures are as follow:

(i) connection point diameter (CPD) of about 8.5 pm to about 17.5 pm,

(ii) connection point-to-cap ratio (CCR) > 0.55,

(iii) aspect ratio (AR) < 4.5, micro structure height < 90 pm, and

(iv) pitch size > 30 pm, wherein

Connection point diameter connection point — to — cap ratio (CCR) Cap diameter

Microstructure height aspect ratio (AR) = and Diameter of column base’ pitch size = length from the centre of a micro structure to the centre of an adjacent micro structure.

20. The polymeric film of claim 19, wherein the multiple overhangs are selected from the group consisting of cantilever, primary re-entrant, primary mushroom and primary doubly re-entrant microstructures; and wherein the hierarchical microstructures are selected from the group consisting of hierarchical cantilever, hierarchical re-entrant, hierarchical doubly re-entrant, hierarchical mushroom, and combinations thereof.