**2. Fabrication techniques**

common particle geometries and their respective surface area-to-volume ratios. For the nanofiber and layered material, the surface area to volume ratio is dominated by the first term in the equation, especially for nanomaterials. The second term (2/l and 4/l ) has a very small influence and is often omitted compared to the first term. Therefore, a changefrom the micrometer to nanometer in particle diameter, layer thickness, or fibrous material diameter

range, will affect the surface area to volume ratio by three orders of magnitude [8].

166 Advances in Nanofibers

**Figure 1.** Common particle reinforcements/geometries and their respective surface areatovolume ratios. [8].

classified as a layered nanomaterial such as an organosilicate [10].

Typical nanomaterials currently under investigation include nanoparticles, nanotubes, nanofibers, fullerenes, and nanowires. In general, these materials are broadly classified by their geometries [9]: particle, layered, and fibrous nanomaterials [8], [9]. Carbon black, silica nanoparticle, polyhedral oligomericsislesquioxanes, can be classified as nanoparticle reinforc‐ ing agents, while nanofibers and carbon nanotubes are examples of fibrous materials [9]. When the filler has a nanometer thickness and a high aspect ratio (30–1000) plate-like structure, it is

In general, the high aspect ratio of nanomaterials provides the necessary reinforcement properties. The properties of reinforced polymers are greatly influenced by the size of its nanomaterial and the quality of the interfacing between the matrix material and the filler material. Significant differences in composite properties may be obtained depending on the nature of the filler material used whether its layered silicate or nanofiber, cation exchange capacity, or polymer matrix and the method of preparation [11]. As an example, when the polymer is unable to intercalate (or penetrate) between the silicate sheets, a phase-separated composite is obtained, and the properties stay in the same range as those for traditional microcomposites [10]. In an intercalated structure, where a single extended polymer chain can penetrate between the silicate layers, a well-ordered multilayer morphology results with alternating polymeric and inorganic layers. An exfoliated or delaminated structure is obtained The advancement in the technology of MicroElectroMechanical Systems (MEMS) has demand for the fabrication of 3D micro/nanostructures and devices. The excitement surrounding the nanoscale science and technology gives us unique opportunities to develop and examine revolutionary processes and materials. Nanofibers reinforced polymer embedded with 2D and 3D micro/nano materials find many applications in the field of medicine, tissue engineering, drug delivery, antibacterial implants or catheters, modification of textiles, and modification of polymers. Many optical, electrical and magnetic applications, have opened up new areas of research for manufacturing nanofibers reinforced polymer with engineered nanoparticles materials.

Researchers have been working on different micro/nano manufacturing techniques. LIGA (German acronym for Lithographie, Galvanoformung, Abformung) [7], [8], Photolithography (6), Electrochemical Fabrication (EFAB) [9], localize electrochemical deposition [10] and laser sintering [13] are some of the techniques used for micro/nano fabrication. Some of these techniques have been used for the fabrication of nanofibers reinforced polymer by dispersing nanofibers in polymer resins.

The LIGA technique was invented approximately 20 years ago. It is a powerful method that facilitates the high volume production of nanofibers reinforced polymer components for many fields of applications [14]. Researchers presented designs, fabrication and experimental results of high power electrostatic microactuators, using LIGA process. This process is capable of producing high aspect ratio microstructures of nanofibers reinforced polymer, as shown in Figure 3 [15].

The LIGA process uses the simple shadow printing process onto a resist on an electrically conducting substrate. After development of the irradiated resist, an electroforming step fills the holes of the relief with metal. This more stable body is used as a mold insert for further molding or embossing steps [8]. The deep lithography step, performed by the use of highly parallel and collimated synchrotron radiation, is the basic step, thus not only defining the shape but also the structural accuracy of the final product. [16].

ceptor based on the reinforced polymer film as a charge generation layer (CGL) was designed

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EFAB is a solid free-form fabrication technology that creates complex, miniature threedimensional shapes based on 3-D computer aided design CAD data [8]. Inspired by rapid prototyping methods, EFAB can fabricate complex shapes by stacking multiple patterned layers. Unlike rapid prototyping,EFAB is a batch process that is suitable for volume production of fully functional devices in engineering materials, not only models and prototypes [9]. EFAB has some limitations and shortcomings. EFAB is a technique that requires the use of masks to build two-dimensional (2D) planar structures. High aspect ratio microstructures are thus a result of multiple steps of material deposition or removal through the use of several masks,

A conducting microelectrode is untilizedin a localized electrochemical deposition (LECD) technique to fabricate high aspect ratio structures. During fabrication, a localized deposition is produced by placing an electrode tip that has micrometre-scale dimensions, near a substrate in an electrolyte and applying an electric potential between them. Confined deposition is produced due to the highly localized electric field in the region between themicroelectrode and the substrate. High aspect ratio microstructures result from the displacement of the end of the electrode along the trajectory of the desired geometry while maintaining continuity with the deposited materials. However, nanocompositemicrostructures fabricated by this method are usually porous and have feature sizes in the tens of micrometers due to the limitation in fabricating and maintaining a sharp conductive probe, and in confining the electric field down

Laser sintering is another technique that has been widely used for the microfabrication of nanofibers reinforced polymers. In this method, a high intensity laser is used to ablate the material and form nanoparticles. Chen et al, reported a new technique for conductive nano‐ fibers reinforced polymer microfabrication and they were able to lower the percolation threshold of the nanofibers reinforced polymer [21]. This process takes place inside a liquid polymer resin which is then polymerized using UV light to form nanofibers reinforced polymers layers. The main advantage of this technique is that metals, non-metals, glass, polymers can be utilized to fabricate nanofibers reinforced polymer s with various properties. The main drawback of this technique is its limitation to planar geometries. Stacking of different layers has been tried, but the alignment and handling of the layers contributes to the limitation

The nonlinear optical process of multiphoton absorption (MPA) was first predicted in 1931 by the Nobel laureate physicist Marie Goeppert-Mayer in her doctoral dissertation [22]. The technique was not verified experimentally until the advent of laser. An intuitive explanation of MPA is the transition from the ground electronic state to an excited electronic state which is usually achieved by the absorption of a high-energy photon or instead reached by simulta‐ neous absorption of multiple low energy photons. The most common implementation of this method is degenerate two-photon absorption (TPA) where both photons have the same energy. The spatial confinement of MPA excitation is used to induce a chemical reaction at the laser focal point. This polymerization reaction occurs by radical reaction mechanisms that

and fabricated [20].

that requires increased fabrication time and cost.

to the nanoscale dimensions [10].

of higher aspect ratio of the micro device [13].

Typically LIGA structures allow for the free choice of the lateral 2D pattern that is projected into the third dimension to form prismatic or cylindrical geometries. Generally, this technique has been used to produce structures with straight walls. However, for all major LIGA process steps, variations have been developed to increase the fabrication flexibility. Geometrical variations in the third dimension (vertical) are possible and can be obtained in different ways by modifying or combining process steps, in particular for producing shapes with increased dimensionality. The consecutive process steps of deep UV lithography, electroforming, and plastic molding can be used to fabricate three dimensional (3D) microstructures with almost no restrictions in their lateral shape in a large variety of snanofibers reinforced polymer.

Photolithography is one of the widely used techniques for fabrication of nanofibers reinforced polymers. Two photolithography-based approaches has been presented to directly microma‐ chinephotopatternablesuperhydrophobicmicropatterns with excellent adaptability and flexibility to a wide variety of substrates, employing the nanomorphology and hydrophobicity of polytetrafluoroethylene (PTFE) nanoparticles and the photopatternability and transparency of an SU-8 photoresist [17].

A light source such as UV light is used in photolithography to polymerize the photoresponsive resin with suspended nanoparticles through a mask [6]. Depending upon the requirement of the feature, a mask is prepared and the feature is transferred using a UV source light. The removal of the unaffected part by the light source is performed using the secondary process of chemical etching. UV light has been used primarily for transferring the mask, but various other alternatives such as X-ray lithography,lithography, nanoimprint and ion projection lithography are being explored [6], [18]. For several decades, this technique has been researched and developed, but the resolution of the fabricated feature using photolithography is limited due to the optical diffraction limitation. Photolithography also requires the use of expensive masks and molds for the fabrication process. These techni‐ ques are effective for the mass fabrication of high resolution microfeatures, but they are limited to 2D geometries [18]. Nanofibers reinforced polymer microstructures can be fabricated by dispersing the photosensitive resin with naomateirals [14]. Figure 2 shows SEM photographs of the photolithographic patterns for photosensitive polyimide with montmor‐ illonitenanofibers reinforced polymers [19].

Electrochemical Fabrication (EFAB) was originally invited to addresses the long develop‐ ment time for Optical MEMS, which can go up to few weeks. This method has also been used for the fabrication of nanofibers reinforced polymer microstrucutres. The fabrication of nanofibers reinforced polymer film of polypyrrole (PPY) and TiO2 nanotube (TNT) arrays via electrochemical methods was reported in several articles. A novel dual-layered photore‐ ceptor based on the reinforced polymer film as a charge generation layer (CGL) was designed and fabricated [20].

The LIGA process uses the simple shadow printing process onto a resist on an electrically conducting substrate. After development of the irradiated resist, an electroforming step fills the holes of the relief with metal. This more stable body is used as a mold insert for further molding or embossing steps [8]. The deep lithography step, performed by the use of highly parallel and collimated synchrotron radiation, is the basic step, thus not only defining the shape

Typically LIGA structures allow for the free choice of the lateral 2D pattern that is projected into the third dimension to form prismatic or cylindrical geometries. Generally, this technique has been used to produce structures with straight walls. However, for all major LIGA process steps, variations have been developed to increase the fabrication flexibility. Geometrical variations in the third dimension (vertical) are possible and can be obtained in different ways by modifying or combining process steps, in particular for producing shapes with increased dimensionality. The consecutive process steps of deep UV lithography, electroforming, and plastic molding can be used to fabricate three dimensional (3D) microstructures with almost no restrictions in their lateral shape in a large variety of snanofibers reinforced polymer.

Photolithography is one of the widely used techniques for fabrication of nanofibers reinforced polymers. Two photolithography-based approaches has been presented to directly microma‐ chinephotopatternablesuperhydrophobicmicropatterns with excellent adaptability and flexibility to a wide variety of substrates, employing the nanomorphology and hydrophobicity of polytetrafluoroethylene (PTFE) nanoparticles and the photopatternability and transparency

A light source such as UV light is used in photolithography to polymerize the photoresponsive resin with suspended nanoparticles through a mask [6]. Depending upon the requirement of the feature, a mask is prepared and the feature is transferred using a UV source light. The removal of the unaffected part by the light source is performed using the secondary process of chemical etching. UV light has been used primarily for transferring the mask, but various other alternatives such as X-ray lithography,lithography, nanoimprint and ion projection lithography are being explored [6], [18]. For several decades, this technique has been researched and developed, but the resolution of the fabricated feature using photolithography is limited due to the optical diffraction limitation. Photolithography also requires the use of expensive masks and molds for the fabrication process. These techni‐ ques are effective for the mass fabrication of high resolution microfeatures, but they are limited to 2D geometries [18]. Nanofibers reinforced polymer microstructures can be fabricated by dispersing the photosensitive resin with naomateirals [14]. Figure 2 shows SEM photographs of the photolithographic patterns for photosensitive polyimide with montmor‐

Electrochemical Fabrication (EFAB) was originally invited to addresses the long develop‐ ment time for Optical MEMS, which can go up to few weeks. This method has also been used for the fabrication of nanofibers reinforced polymer microstrucutres. The fabrication of nanofibers reinforced polymer film of polypyrrole (PPY) and TiO2 nanotube (TNT) arrays via electrochemical methods was reported in several articles. A novel dual-layered photore‐

but also the structural accuracy of the final product. [16].

of an SU-8 photoresist [17].

168 Advances in Nanofibers

illonitenanofibers reinforced polymers [19].

EFAB is a solid free-form fabrication technology that creates complex, miniature threedimensional shapes based on 3-D computer aided design CAD data [8]. Inspired by rapid prototyping methods, EFAB can fabricate complex shapes by stacking multiple patterned layers. Unlike rapid prototyping,EFAB is a batch process that is suitable for volume production of fully functional devices in engineering materials, not only models and prototypes [9]. EFAB has some limitations and shortcomings. EFAB is a technique that requires the use of masks to build two-dimensional (2D) planar structures. High aspect ratio microstructures are thus a result of multiple steps of material deposition or removal through the use of several masks, that requires increased fabrication time and cost.

A conducting microelectrode is untilizedin a localized electrochemical deposition (LECD) technique to fabricate high aspect ratio structures. During fabrication, a localized deposition is produced by placing an electrode tip that has micrometre-scale dimensions, near a substrate in an electrolyte and applying an electric potential between them. Confined deposition is produced due to the highly localized electric field in the region between themicroelectrode and the substrate. High aspect ratio microstructures result from the displacement of the end of the electrode along the trajectory of the desired geometry while maintaining continuity with the deposited materials. However, nanocompositemicrostructures fabricated by this method are usually porous and have feature sizes in the tens of micrometers due to the limitation in fabricating and maintaining a sharp conductive probe, and in confining the electric field down to the nanoscale dimensions [10].

Laser sintering is another technique that has been widely used for the microfabrication of nanofibers reinforced polymers. In this method, a high intensity laser is used to ablate the material and form nanoparticles. Chen et al, reported a new technique for conductive nano‐ fibers reinforced polymer microfabrication and they were able to lower the percolation threshold of the nanofibers reinforced polymer [21]. This process takes place inside a liquid polymer resin which is then polymerized using UV light to form nanofibers reinforced polymers layers. The main advantage of this technique is that metals, non-metals, glass, polymers can be utilized to fabricate nanofibers reinforced polymer s with various properties. The main drawback of this technique is its limitation to planar geometries. Stacking of different layers has been tried, but the alignment and handling of the layers contributes to the limitation of higher aspect ratio of the micro device [13].

The nonlinear optical process of multiphoton absorption (MPA) was first predicted in 1931 by the Nobel laureate physicist Marie Goeppert-Mayer in her doctoral dissertation [22]. The technique was not verified experimentally until the advent of laser. An intuitive explanation of MPA is the transition from the ground electronic state to an excited electronic state which is usually achieved by the absorption of a high-energy photon or instead reached by simulta‐ neous absorption of multiple low energy photons. The most common implementation of this method is degenerate two-photon absorption (TPA) where both photons have the same energy. The spatial confinement of MPA excitation is used to induce a chemical reaction at the laser focal point. This polymerization reaction occurs by radical reaction mechanisms that

concentrations can be employed that are about ten times more than would be feasible for single-

**Figure 3.** Center section of a fabricated ORO lens. Nickel matrix with 20 μm wide and 600 μm deep holes. Measured

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Multi Photon Absorption (MPA) is a relatively new and evolving method for the manufac‐ turing of micro/nano structures. Unlike other methods, this technique does not involve any secondary operations or processes for nanofibers reinforced polymer fabrication. Micro/nano structures and devices are fabricated with higher accuracy and complexity using MPA than other prevailing methods of fabrication for various applications. When an ultrashort laser pulse is focused in a photo responsive polymer resin, a solid voxel (volumetric pixel) is generated. The voxel size defines the minimum resolution of the polymer which is converted into solid form. Complex 2D and 3D microstructures can be fabricated by scanning the laser in the photo responsive resin [23]. Research on fabrication of micro/nano structures from different types of polymers using MPA has been conducted in the past decades. In certain applications there is a real need of complex nanofibers reinforced polymer structures fabrica‐ tion [24]. The prevailing methods for nanofibers reinforced polymer manufacturing are good for planar or 2D device fabrication. For maskless manufacturing 2D and 3D nanofibers reinforced polymer devices with accuracy and precision there is a need of identifying or performance research towards the development of new maskless fabrication techniques using

photon excitation without any fear of out-of-plane polymerization.

aspect ratios (sidewall inclination) >400 [15].

MPP due to its advantages over other methods.

**3. Nanofibers reinforced polymers enhanced properties**

using laser ablative synthesis generated nanofibers as reinforcement [25] [26].

The samples that presented in this section are polymer microstructures fabricated with MPA

**Figure 2.** SEM photographs of the photolithographic patterns of (a) pure PSPI, PSPI/MMT nanofibers reinforced poly‐ mers with (b) 2 wt. % and (c) 3 wt. % MMT contents. [19].

depend on the photoinitiator and monomer being used. The photoinitiators used for radical MPA vary from small molecules to large conjugated molecules. A number of groups have reported the successful application of radical MPA using a different kind of resins, homemade and commercial, and different excitation sources. Custom photoinitiators have been designed by several groups and have been shown to be effective both for low threshold powers and for the ability to use less expensive laser systems. While the benefits of custom initiators are clear, their availability is limited. Commercial resins or resins made of commercial components have the advantage of accessibility but suffer from a slightly higher power threshold for fabrication. However, for the entire laser systems used, the threshold for these resins is always well below the available power and therefore their use is completely practical.

MPA can achieve resolution that is considerably better than that predicted by the diffraction limit due to a combination of optical nonlinearity. The probability for MPA is proportional to In,where I is the light intensity and n is the number of absorbed photons. This effectively narrows the point-spread function (PSF) of the beam near the focal point so that it is smaller than the diffraction limit at the excitation wavelength. The real benefit of the optical nonli‐ nearity of MPA lies in the negligible absorption away from the focal point. Photoinitiator

**Figure 3.** Center section of a fabricated ORO lens. Nickel matrix with 20 μm wide and 600 μm deep holes. Measured aspect ratios (sidewall inclination) >400 [15].

concentrations can be employed that are about ten times more than would be feasible for singlephoton excitation without any fear of out-of-plane polymerization.

Multi Photon Absorption (MPA) is a relatively new and evolving method for the manufac‐ turing of micro/nano structures. Unlike other methods, this technique does not involve any secondary operations or processes for nanofibers reinforced polymer fabrication. Micro/nano structures and devices are fabricated with higher accuracy and complexity using MPA than other prevailing methods of fabrication for various applications. When an ultrashort laser pulse is focused in a photo responsive polymer resin, a solid voxel (volumetric pixel) is generated. The voxel size defines the minimum resolution of the polymer which is converted into solid form. Complex 2D and 3D microstructures can be fabricated by scanning the laser in the photo responsive resin [23]. Research on fabrication of micro/nano structures from different types of polymers using MPA has been conducted in the past decades. In certain applications there is a real need of complex nanofibers reinforced polymer structures fabrica‐ tion [24]. The prevailing methods for nanofibers reinforced polymer manufacturing are good for planar or 2D device fabrication. For maskless manufacturing 2D and 3D nanofibers reinforced polymer devices with accuracy and precision there is a need of identifying or performance research towards the development of new maskless fabrication techniques using MPP due to its advantages over other methods.
