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Microstructures, post-TPP processing
CHAPTER 10
Microstructures, post-TPP processing Richard Farrer Chemistry Department, Colorado State University-Pueblo, Pueblo, CO, United States
1 Introduction The previous chapters have shown the unique capabilities associated with two-photon photopolymerization (TPP)—the ability to fabricate complex, free-standing, threedimensional structures with resolution in the tens of nanometers. However, because of the nature of the polymerization process, the physical and chemical properties of the polymeric structures have found fairly limited application. Although some work has been done to expand the nature of the materials employed in the TPP process, the chemical functionality of the products of TPP remains quite limited. Fortunately, a number of techniques that modify the fabricated polymeric structures have been developed. These modifications are applied after a free-standing structure has been produced through the TPP and development process. This chapter will focus on the modifications that have been realized with postpolymerization processing of structures created via TPP. These post-TPP processing steps provide a means to extend the functions and capabilities of the structures fabricated with TPP. If TPP is to become a common technique employed to produce three-dimensional microscopic systems, the ability to extend the function of the polymer structures is necessary, and post-TPP processing provides a seemingly limitless route to this end.
2 Chemical modification of fabricated polymer surfaces Some of the initial work associated with post-TPP processing employed the unreacted functional groups that were present on the surface of the polymer structures after the direct laser writing (DLW) and development steps. In some situations, the reactive species on the surfaces of the polymer structures are unreacted functional groups that were responsible for the polymerization process. In other examples, the functional groups were added to the resins for the purpose of post-TPP processing. In either case, the only functional groups that are accessible for modification are those that exist on the surfaces of the fabricated polymer structures. Additionally, if more than one type of polymer is employed, selective functionalization of one polymer over the other can be accomplished. Three-Dimensional Microfabrication Using Two-Photon Polymerization https://doi.org/10.1016/B978-0-12-817827-0.00010-2
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2.1 Single-polymer functionalization One of the first things to notice when dealing with DLW is that the production of conductive, free-standing, three-dimensional, structures is lacking. While attempts have been made to produce conductive structures through direct laser writing, the fabrication of three-dimensional, free-standing, highly conductive structures from DLW has been elusive [1–4]. Clearly, the ability to produce conductive structures is critical in the development of DLW as a valid process for producing functional 3-D devices. A structure that has been produced by TPP could be coated with metal through one of the traditional methods: sputter coating, vacuum deposition, chemical vapor deposition, etc. However, these methods coat not only the fabricated structure but also the substrate. In most cases, it is desirable to coat only the structure and not the substrate on which the structure was constructed. As a result, a number of groups have developed post-TPP processes that allow for the incorporation of metals onto the surface of the fabricated structures. In the work by Formanek et al. [5], the silver plating occurs via an electroless plating process, which coats only the structure that was produced and not the substrate onto which the structure was fabricated. In order to accomplish this, the group produced a chemically modified polymer resin—a commercially available acrylate monomer with some amount of styrene added. The authors state that the increased aryl functionality from the included styrene improves the metal deposition process. In order to eliminate the deposition of the metal onto the substrates, the glass slides were modified with dimethydichlorosilane prior to the TPP process. The structures produced by TPP are sensitized using a tin(II) chloride solution, and finally treated with Tollen’s reagent to reduce silver onto the sensitized surfaces. A sample of the results is shown in Fig. 10.1. Also notice in Figure 10.1A, the large array of structures that were constructed in parallel with TPP and a microlens array. The substrate remains relatively clear of silver, because the sensitizing agent has little affinity for the modified substrate.
Fig. 10.1 (A) Image of a series of two-turn coils fabricated on the top of a 2-μm cube. (B) A single unit from the series shown in (A)—this structure has not been coated with silver. (C) A single unit shown after electroless deposition was employed to coat the structures with silver [5].
Microstructures, post-TPP processing
The Kuebler group developed a chemical process employed for the post-TPP modification that can be applied to structures fabricated from acrylate-, methacrylate-, and epoxide-based resins [6]. In this work, TPP is employed to fabricate a structure from an acrylate resin on a glass substrate that has been modified with 3-acrylopropyltrimethyoxysilane. The structure is treated with a lithium aminolysis solution that has been prepared from n-butyllithium and 1,3-diaminopropane. This solution will react with remaining acrylate, methacrylate, or epoxide functional groups to leave primary amine groups on the fabricated surfaces. Gold is reduced onto the amines, followed by electroless deposition of silver onto the gold. One important aspect of this process is the silver electroless plating solution is not Tollen’s solution, which is a fairly unstable mixture. The employed solution employed seems to plate at a slower rate than Tollen’s reagent, but it appears much more selective as the substrate shows no signs of metallization, while the structure is significantly reflective as a result of the deposited silver (Fig. 10.2). One last note concerning this modification process—while the article only shows steps to silver plating, the ability to produce primary amine functionality on a fabricated structure opens paths to many chemical processes and, thus, endpoints. While modification of TPP-fabricated surfaces with metal has gained much interest because of the resulting conductive structures, the use of TPP-fabricated structures as scaffold for cellular growth is another area that has received significant interest.
Fig. 10.2 Logstack structure produced by TPP that has been modified and coated with silver. The small image on the bottom-left is an SEM image of the structure, while the larger image is acquired using reflected light. Notice the structure is silver coated, but the substrate remains free of silver [6].
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Work by Klein et al. [7] produced a scaffold onto which cells were deposited. The scaffolds were produced with TPP using Ormocomp as the resist material. After fabrication, the scaffolds were developed using OrmoDev and rinsed with isopropanol and ethanol. To prepare the scaffold for cell adhesion, the structures were incubated in phosphatebuffered saline (PBS) containing poly-L-lysine. The scaffold were rinsed in PBS and treated with a solution containing fibronectin. Cells were introduced to the fibronectin-coated scaffold and incubated for 30 min. Cell force studies were then performed by observing the distortion of the scaffold caused by the cells. The ability to modify the surfaces of the TPP fabricated microstructures opens the door to development of many different uses for the fabricated structures. The chemical modification of the fabricated surfaces appears to be open to all possible chemical functionality, because the chemical nature of the resins used for fabrication allow for simple modification to common functionalities such as amines. These post-TPP processes modify the entire surface of the microstructure, while leaving the substrate on which the structure was fabricated unmodified. While this lack of selectivity is desirable in some cases, the ability to modify only selected portions of the structure is necessary if TPP is to be used to create complex systems.
2.2 Selective functionalization The ability to modify TPP fabricated surfaces in order to incorporate a wider spectrum of functionality is fundamental to expanding the use of TPP as a valid microfabrication technique. However, if one truly wishes to employ TPP to create microscopic devices, more often than not, the device will require more than one type of functionality. Therefore, the work done by the Fourkas group [8] in which structures created through TPP are shown to be selectively functionalized, provides a means by which polymers with orthogonal functionality can be employed to create more than one type of functionality in a single device. In this work [8], TPP is used to create structures from two different, but similar, polymer resins—one an acrylate-based resin and the other a methacrylate-based resin. Methacrylate resins are not often found in TPP processes, because of a lower reactivity rate as compared to acrylate resins; however, this work shows that moderate increases in laser powers can successfully produce methacrylate polymer structures. The substrate employed in this process was a glass coverslip that was modified with (3-methacryloxypropyl) trimethoxysilane—leaving a surface with methacrylate functionality. Serial TPP of the two different resins was performed with DMF and ethanol washes after each fabrication process. After the final wash, the substrate containing both methacrylate and acrylate structures was submerged in an ethanoic solution of ethylenediamine. The ethylenediamine reacts with the remaining acrylate functional groups at a much faster rate than the remaining methacrylate groups. As a result, the acrylic structures
Microstructures, post-TPP processing
Fig. 10.3 The images shows the selective modification of polymer structures produced through TPP methods. The letters “U” and “M” were prepared with acrylate monomers, while the letter “M” was prepared with methacrylate monomers. The resulting polymers were modified and coated with copper. The kinetics associated with the reactions of acrylates and methacrylates differ significantly, and allow the acrylate-based polymer to be modified while the methacrylate-based polymer remains unmodified [9].
were modified to contain amine functionality, while the methacrylic structures remained unmodified (Fig. 10.3). At this point, the amines provide access to a large number of chemical reactions that could end with any number of different chemical functionalities. In this work, the amine groups were used to deposit metals (copper and gold) onto the surfaces of the acrylate structures. In addition to showing the selective modification of the two polymers, the Fourkas group also showed the deposited metal was conductive. However, when the metal was applied to a polymer coil in an attempt to produce a working inductor, it was found that the inductance produced by the coil was significantly less than that predicted of a solid metal coil of similar size. Fig. 10.3 shows the results of this process. The “U” and the “D” were fabricated from an acrylate-based resin, while the “M” was produced from a methacrylate-based resin. A slightly different approach to the selective functionalization of fabricated polymer structures was taken by Takeyasu et al. [10] In this work, silver was deposited onto polymer structures that contained methacrylamide, while polymer structures not containing the amide functionality were left uncoated. A photopolymerizable resin was employed to fabricate the uncoated structures via TPP. The same resin was then impregnated with methacrylamide, and this new mixture was used as the resin for TPP. After development, the structures were treated with a silver nitrate solution. Since silver ions have an affinity for amides, the silver ions preferentially attached to the polymer containing the amide moieties, and thus selective functionalization of the structures was accomplished (Fig. 10.4). Further silver coating was accomplished through electroless plating techniques. The previous examples use metal deposition to prove the ability to selectively modify one polymeric structure over another. As stated previously, these chemical modifications allow for functionality beyond metal deposition and conductivity. For example, Scheiwe et al. [11] produced scaffold onto which chicken embryonic fibroblasts (CEFs) would
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Fig. 10.4 Silver is deposited selectively onto the cubes that contain amide functional groups, while polymeric cubes that do not contain amide functionality are not metal coated [10].
grow. The scaffolds were produced using two separate resins with one of the resins being modifiable to create adhesion sites for the CEFs to attach. The resins employed were Ormocomp and a homemade resin consisting of polyethylene glycol diacrylate, pentaerythritol tetraacrylate, and Irgacure 369 at the photoinitiator. The scaffolds were fabricated onto a glass substrate that had been modified so the fabricated structure would adhere to the surface. The homemade acrylate resin was used to make the vertical posts of the scaffolds, while the Ormocomp was employed to make the cell-attachment pads on each post. After development, the scaffolds were rinsed with phosphate-buffered saline (PBS) and incubated in PBS-containing fibronectin, which preferentially bonded to the Ormocomp polymer pads (Fig. 10.5). Cells were introduced to petri dishes containing the scaffolds, and the systems were incubated for 3 h. Once the cells had attached to the scaffold, a micromanipulator was employed to deform the scaffold, and the cell reaction to the deformation was studied. While the ability to modify the surfaces of TPP fabricated structures opened the door to many new uses of the structures, the selective modification of these structures significantly expands the possible uses of TPP fabrication. Instead of simply employing the structures for a single task, we can begin to imagine the construction of a device that has more than one purpose or function.
3 Double inversion Another area in which TPP fabrication techniques have received a significant amount of attention is in the production of 3-D photonic materials. TPP processes allow for the production of structures that function as photonic bandgaps (PGBs); however, the polymer that is produced by TPP does not provide a range of refractive indices that is sufficient to produce a complete photonic bandgap. At this point, direct writing of materials
Microstructures, post-TPP processing
Fig. 10.5 (A) SEM image of the TPP-fabricated scaffold (the Ormocomp polymer is shown in orange). (B) Reconstruction of confocal image stack of scaffold with the CEF growing in the scaffold. (C) Series of confocal images highlighting different parts of the CEF. The MERGE image is all the confocal images stacks of (C) merged into one image [11].
having a sufficient refractive index has not been realized. A post-TPP processing method known as silicon double inversion developed by Tetreault et al. [12] has been employed to produce silicon structures that provide a refractive index large enough to produce functioning PBGs. In this work [12], a woodpile structure having 24 layers was produced via TPP with SU-8 as the resin. The calculated photon band gap of the SU-8 structure was 8.6%. After fabrication (Fig. 10.6), the unpolymerized resin was removed, and the remaining structure was treated with humidified N2 in a chemical vapor deposition (CVD) chamber. The sample is then treated with N2 that contains SiCl4. An amorphous shell of dense SiO2 is constructed layer by layer during the treatment with SiCl4. This process is continued until the polymeric woodpile structure has been infiltrated by the SiCl4, which reacts with the adsorbed water to produce SiO2. The SiO2 coats the entire sample—the polymeric structure and the substrate—the process is not selective. At this point, the polymeric core is completely coated in SiO2. In order to remove the polymeric core, the SiO2 that is coating the top layer of the woodpile structure (the layer that is farthest from the substrate) is removed through a reactive-ion etching (RIE) process using an SF6 plasma. The RIE treatment exposes a portion of the polymer structure that lies in the core
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Fig. 10.6 Silicon double inversion process. First, the microstructure is fabricated from SU-* via TPP. Then, DVD is used to coat the sample with SiO2. The SiO2 is removed by reactive ion etching to expose the polymer at the top of the microstructure. Complete removal of SU-8 is carried out using oxygen plasma or calcination. Additional CVD of SiO2 can be used at this stage to produce the desired rod diameter. Si infiltration occurs by low-pressure CVD. Finally, an HF-resistant substrate is attached to the top of the microstructure, and the whole assembly is immersed in HF to remove SiO2 [12].
of SiO2 shell. Because the entire polymeric structure is one large network, exposure of a small portion of the polymer provides access to the entire polymeric network. Because of this, the polymer can be removed through one of two methods. First, the polymer can be etched using an O2 plasma, or the polymer can be removed through a calcination process in a furnace at 450°C. The remaining structure is a SiO2 shell of the original polymeric woodpile structure—basically the SiO2 is a template for the original woodpile structure. Since this process is employed to produce PGBs, the diameter of the rods in the woodpile is fundamental in the ability of the final structure to act as a PBG. In order to maximize the PBG, the diameter of the rods is reduced by depositing SiO2 into the woodpile template that has been exposed by the removal of the polymeric woodpile structure. Again, SiCl4 is introduced to the sample; however, this round of SiCl4 deposition is to reduce the diameter of the rods in the final woodpile structure. After the woodpile channels are narrowed, CVD is again employed to fill the template with disiline, Si2H6, at 460°C. This process both infiltrates and coats the SiO2woodpile template. Once the silicon has completely filled the template, the SiO2 is removed through an HF etch. Recall, the TPP structure employs a glass substrate, so the HF etch not only removes the SiO2 inverse woodpile structure, but also the substrate. Therefore, prior to the HF etch, a sapphire substrate is glued top of the structure—the surface that has been previously etched with SF6 plasma. The HF etch produces a silicon structure that is adhered to the sapphire substrate. Ultimately, this process produces a silicon replica of the original SU-8 woodpile that was fabricated with TPP (Fig. 10.7).
Microstructures, post-TPP processing
Fig. 10.7 SEM images of silicon woodpile structure produced through the silicon double inversion process. The images on the right compare the TPP fabricated SU-8 structure at the bottom and the final silicon structure on the top [12].
The resulting silicon:air structure produces a photon band gap of 8.6%. The estimated effective refractive index of the rods in the silicon woodpile is 3.16, which is similar to an 80% silicon and 20% air mixture. The silicon woodpile structure produced suppression at 2.35 μm that was over two orders of magnitude. The double inversion process can be employed to produce photonic crystals from TPP-produced structures. Since TPP can produce any number of different structures, TPP fabrication coupled with the double inversion post-TPP processing can produce photonic crystals with different overall configurations.
4 Atomic layer deposition Recently, works by the groups of Wegener [13], Kraft [14], and Greer [15] have coupled TPP with a process known as atomic layer deposition (ALD). In the work by Kraft and Greer, ALD was employed to produce a thin layer of alumina on the surface of the structure produced by TPP, while Wegener used ALD to introduce both zinc oxide and titania into the microstructures. The final thickness of the ALD deposited material is controlled through the total number of deposition cycles performed. In the work of Kraft [14], the alumina-polymer composite structures were tested for strength and recoverability after compression (Fig. 10.8). The Greer group [15] took the process a step further by removing the polymer core after the deposition of the alumina—leaving free-standing structures constructed of hollow alumina tubes (Fig. 10.9). These final structures have been shown to be lightweight, strong, and ductile, and are able to recover from significant compression events. The final structure produced by Wegener [13] was employed as a photonic bandgap for visible light. The structure produced by the Wegener group [13] was first produced using TPP and a photoresist. After development, ALD was employed to deposit a layer of ZnO onto the structure. The polymer was removed through calcination, and additional ZnO was
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Fig. 10.8 Five different cellular structures produced with TPP. The top row shows computer-aided design models (A through E), while the lower row shows SEM images of the corresponding TPP microstructures (scale bars are 10 μm) [14].
Fig. 10.9 Alumina nanolattices. Image (A) is a computer-aided design model of the octet-truss that was fabricated. (B) shows a single unit of the structure that is colored blue in (A). A diagram of the cross-section of a tube is shown in (C). A SEM image of the actual TPP fabricated structures is shown in (D), while (E) is a closer image of one of the structures with the inset showing the tubular nature of the trusses. A TEM dark-field image of the diffraction grating of the tube wall is shown in (F) [15].
Microstructures, post-TPP processing
deposited though ALD. Titania was then introduced by way of ALD and infiltrated the volume where the photoresist had been prior to calcination. At this point, the structure is sitting on a ZnO layer, so polymer pads are added to the corners of the woodpile to keep it attached to the substrate. The ZnO is etched from the structure, leaving only TiO in a woodpile structure. The work of both the Kraft [14] and Greer [15] groups begins with the fabrication of three-dimensional structures using TPP and a photoresist. The Kraft group produced a series of cellular structures that are shown in Fig. 10.8, while the Greer group produced the structure shown in Fig. 10.9. After removal of the unpolymerized photoresist, the structures were exposed to several atomic layer deposition (ALD) cycles. According to work by Greer, theses cycles consist of four steps: the system is pulsed with water, purged, pulsed with trimethyl aluminum, followed by a final purge. The process is performed at 150°C. The thickness of the alumina that is deposited onto the structures is determined by the number of atomic layer deposition cycles that are performed. The structures produced by the Kraft group are completed at this point. The Greer group uses a focused ion beam to remove a portion of the alumina in order to expose the polymer scaffold. The structure is then exposed to oxygen plasma that removes the polymer scaffold and leaves the three-dimensional alumina structure. The work presented shows that ALD provides a method to produce surfaces and complete structures of silica, alumina, TiO2, and ZnO. Because ALD is completed layer by layer, the thickness of the resulting layers or structures is entirely controlled by the number of deposition cycles. Such processes have opened the door to new photonic materials and structures with increased strength.
5 Electrodeposition in TPP produced template In 2009, the Wegener group produced a gold helix photonic metamaterial using TPP followed by electroplating [16]. The final structure is an array of gold coils that collectively can act as a circular polarizer (Fig. 10.10). The fabrication process involves creation of at template on a conductive surface using TPP. To this end, a positive photoresist was spun onto a glass substrate that has been treated with a thin conductive film of indium-tin oxide (Fig. 10.11). TPP is employed to expose the areas that will contain the coils in the photoresist. The resist is then developed, which leaves a template with air coils that end at the indium-tin oxide (ITO) surface. The structure is placed in a gold electroplating solution and gold is plated into the coils using the ITO as the electrode. This process is very similar to that of electrodeposition of metal into anodized aluminum oxide templates. After the electroplating is completed, the resist material is dissolved leaving an array of gold coils. A similar method was employed by Williams et al. to produce magnetic nanostructures [17]. Again, a positive resist was spun on top of a glass/ITO substrate. The desired geometry of the final structure was produced in the positive resist through TPP, and the
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Fig. 10.10 Array of gold helixes produced through TPP and electroplating. (A) shows a vertical cut of the template that has been filled with gold. (B) and (C) show a close up of the helixes and a top view of the array of helixes [16].
fabricated structure was removed, leaving a void in the positive resist that extend to the conductive ITO substrate. Cobalt was then electrodeposited in the voids, and the resist was removed leaving only the free-standing, cobalt nanostructure. Fig. 10.12 provides SEM images of two different arrays of structures along with higher magnification images of individual structures. The magnetic properties associated with some of the produced nanostructures were evaluated using spin-SEM and magneto-optical Kerr effect (MOKE) magnetometry. Results of the evaluation of the magnetic properties of the structures show that the structures appear to be composed of small crystalline grains within the larger structure.
Fig. 10.11 This image show the process employed to create the array of gold coils. Initially a positive tone photoresist is spun onto a glass surface coated with conductive ITO. TPP is employed to expose coils in the resist, after which the exposed area is removed by a solvent. Electroplating is employed to fill the exposed areas by employing the ITO surface as an electrode. The growth of the gold coils begins at the ITO surface and continues into the coil-shaped voids. After electroplating is completed, the remaining resist material is removed, exposing the array of gold coils [16].
Fig. 10.12 SEM images of magnetic nanostructures. (A) An array of tilted nanowires. (B) and (C) Magnified images of the nanowire array shown in (A). (D) An array of tetrapods. (E) and (F) Magnified images of single tetrapods [17].
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This technique of creating a metal structure through TPP of a positive photoresist and then electrodepositing metal into the created void opens the door to the creation of pure metal structures; however, the process has some limitations. Since the final structures are created by the electroplating of gold into the voids created in the resist and the electroplating must start at the ITO surface, the structures created are fairly limited in scope. As an example, voids that are horizontal to the surface of the substrate would not completely fill during the electroplating process. However, this technique provides a method to produce structures that are composed of pure metal.
6 Pyrolysis Carbon electrodes are employed for use as electrodes in batteries and fuel cells and are often found as supports for electrochemical sensors. Electrodes having complex shapes and high surface areas are highly desirable. Clearly, TPP can produce complex structures; however, direct fabrication of carbon structures is not possible with TPP. Work by the Maruo group [18] has produced a process that allows for the production of complex carbon structures using TPP as the fabrication method to produce the desired architectures. The carbonization of the polymer structures is accomplished through pyrolysis. It was found that commercial photopolymers do not have a high enough carbon content to produce stable three-dimensional structures. Therefore, the Maruo group developed a set of polymer resins based on resorcinol diglycidyl ether that contain higher percentages of carbon. In this research, three new photopolymers having high carbon contents were developed—all three contained resorcinol diglycidyl ether, an oxetane, and a photoinitiator. Additionally, two of the three contained an acrylate resin. Only the two containing acrylates were found to be suitable for TPP processing. These two resins were employed to produce complex, three-dimensional structures through TPP. After fabrication, the structures underwent pyrolysis to convert the polymer structures to carbon (Fig. 10.13). The shrinkage of the structure caused by the pyrolysis process was approximately 20%–30%, depending on the polymer employed. The final carbon structures were small replicas of the structures originally produced with TPP. Interestingly, one of the polymer composites that was produced would not polymerize with TPP; however, use of microtransfer molding techniques allowed for the production of structures using high-carbon content polymers that are unable to be fabricated by way of TPP (Fig. 10.14).
7 Multiphoton-induced spatially resolved functionalization The final post-TPP process to be discussed is multiphoton-induced chemistry on the surface of TPP produced structures. In this modification method, the laser that was
Microstructures, post-TPP processing
Fig. 10.13 SEM images (A) and (C) are polymeric microstructures produced with TPP. Images (B) and (D) are the same structures after carbonization [18].
Fig. 10.14 SEM images (A) and (C) are microstructures produced by microtranser molding. Images (B) and (D) show the same microstructures after carbonization [18]. Note—structures used to create molds were produced via TPP [18].
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employed for the TPP fabrication of the 3-D polymer structure was employed to excite areas on the surface of the structure in order to modify those specific areas [19]. The TPP structures were created using a resin that consisted of o-quinodimethanes and maleimides. The polymerization reaction is a Diels-Alder reaction, and not the typical radical polymerization that is often employed in TPP fabrication. After the main structure had been produced and developed, the structure was submerged in a solution containing maleimides having bromine functionality. The reaction between the dissolved bromomaleimides and the residual, unreacted, o-quinodimethanes on the surfaces of the TPP fabricated structure was photoinduced by two-photon absorption, mediated by the TPP laser. This process provides a method to produce spatially resolved modifications that have high resolution. Additionally, this selective modification does not require two or more types of polymers, since the surface modification is mediated by the DLW laser system.
8 Implosion fabrication (ImpFab) In late 2018, Oran et al. published a TPP process that they deemed ImpFab (Implosion Fabrication) [20]. This technique uses a three-step process to produce 3-D nanomaterials. The first step is TPP to fabricate 3-D structures in solvated polyacrylate/polyacrylamide hydrogels containing fluorescein [21]. After initial fabrication of the 3-D structure, a second step is employed for the modification of the functionality on the hydrogel, which allows for the introduction of a wide variety of desirable components, such as fluorescent dyes, DNA, and other biologically relevant moieties, metal nanoparticles, and quantum dots. As long as the chemistry of modification can be done in an aqueous environment (required to keep the hydrogel system swollen and solvated), it can be used in this process. Following the initial modification of the hydrogel, further amplification can be employed to enhance the modification. For example, the hydrogel structures were modified to introduce amine moieties that have an affinity for gold nanoparticles (AuNPs). In order to increase the metal-to-hydrogel ratio, a silver intensification process was employed, where silver was deposited on the bound AuNPs. The final step of the process involves treatment with acid or divalent cations followed by dehydration, which allows for controlled shrinkage of the fabricated structure to its final dimensions. Ultimately, the overall reduction in the size of the structure is determined by the amount of crosslinker present in the initial hydrogel mixture. Figs. 10.15 and 10.16 provide images during the different steps of the process, and show some of the capabilities of ImpFab. An interesting factor that is shown in image (L) of Fig. 10.15 is that the shrinkage in the lateral dimensions is different than that in the axial dimension. While this may be of some concern, the shrink factors are reproducible, so that the difference in shrinkage associated with each dimension can be corrected during the fabrication process. Fig. 10.16 shows structures that have been modified with
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Fig. 10.15 (A), (C), (E), and (G) The steps in the ImpFab process, along with SEM images associated with each step, (B), (D), (F), and (H), respectively. Note that the shrink step reduces the scale of the structure by a factor of 10 in both the in-plane dimensions. (I) shows fluorescent patterns that were created by different laser powers. (J) Two different patterns that have been modified with different components. The yellow pad contains metal nanoparticles and the blue pad contains CdTe quantum dots. (K) Large image is a fluorescent pattern prior to shrinking and dehydration, and the two images to the right are after shrinking and dehydration. The top image had a shrink factor of 10 , and the bottom image had a shrink factor of 20. (L) The shrink factors associated with the lateral and axial dimension of a 10 gel. (M) The lateral shrink factor associated with a 20 gel [20].
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Fig. 10.16 Structures in which the hydrogen has been modified with gold nanoparticles and intensification was carried out with silver. (A) and (B) are SEM images that show the metal structure prior to and after sintering. (C) Shows an array of structures after shrinking and dehydration, but before sintering. (D) shows a fluorescent image of a complex 3-D structure, (E) is the same structure after deposition of silver, and (F) shows the structure after dehydration [20].
gold nanoparticles and intensified with silver. Other structures produced following this method are highly conductive after the shrinking, dehydration, and sintering processes are complete. One important note is that the initial hydrogel scaffold is not removed; therefore, the final structures shown in Fig. 10.16 contain the hydrogel so that the 3-D structure is retained.
9 Conclusions The use of post-TPP processing techniques has gained significant interest in the past decade. The ability to extend the functionality and viability of three-dimensional microstructures produced through TPP provides for a much broader use of the produced
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structures. As can be seen, the processing of the fabricated structures can be completed in many different ways: chemical modification of the surface of the structure, atomic layer deposition, pyrolysis, double inversion, etc. Additionally, the structure can be employed as a template for the production of pure metal structures. This post-TPP processing produces structures with new abilities: conductivity, higher refractive index, favorable cellular interactions, higher strength, etc. In addition to the modifications presented, the capability of selective modification of one area of a structure over another is also a significant step for TPP. Recent work involving TPP of hydrogels followed by modification and shrinking may provide a method to produce complex 3-D nanomaterials with a wide variety of functionalities. Although post-TPP processing appears to be nearly limitless in its ability to provide any functionality necessary to a system and the work presented in the chapter provides a significant improvement in the functions of structures produced through TPP, much more work needs to be done if TPP is to realize its potential. If TPP is going to move forward, processes must be developed that allow for the production of a system and not simply one or two structures with orthogonal functionalities. The production of a system of structures having different functionalities (conductivity, magnetism, varying optical properties, fluid flow, etc.) that work together in a concerted manner must to be shown to be possible.
References [1] F. Stellacci, C.A. Bauer, T. Meyer-Friedrichsen, W. Wenseleers, V. Alain, S.M. Kuebler, S.J. K. Pond, Y. Zhang, S.R. Marder, J.W. Perry, Adv. Mat. 14 (2002) 194. [2] C.N. LaFratta, D. Lim, K. O’Malley, T. Baldacchini, J.T. Fourkas, Chem. Mater. 18 (8) (2006) 2038–2042. [3] T. Tanaka, A. Ishikawa, S. Kawata, Appl. Phys. Lett. 88 (8) (2006). [4] K. Kurselis, R. Kiyan, V.N. Bagratashvili, V.K. Popov, B.N. Chichkov, Opt. Exp. 21 (25) (2013) 31029–31035. [5] F. Formanek, N. Takeyasu, T. Tanaka, K. Chiyoda, A. Ishikawa, S. Kawata, Opt. Exp. 14 (2) (2006) 800. [6] Y.-S. Chen, A. T, D.B. Torrance, S.M. Kuebler, Adv. Funct. Mat. 16 (2006) 1739–1744. [7] F. Klein, T. Striebel, J. Fischer, Z. Jiang, C.M. Franz, G. von Freymann, M. Wegener, M. Bastmeyer, Adv. Mat. 22 (2010) 868–871. [8] R.A. Farrer, C.N. LaFratta, L. Li, J. Praino, M.J. Naughton, B.E.A. Saleh, M.C. Teich, J.T. Fourkas, J. Am. Chem. Soc. 128 (2006) 1796–1797. [9] L. Li, J.T. Fourkas, Mat. Today 10 (6) (2007) 30–37. [10] N. Takeyasu, T. Tanaka, S. Kawata, Appl. Phys. A Mater. Sci. Process. 90 (2008) 205–209. [11] A.C. Scheiwe, S.C. Frank, T.J. Autenrieth, M. Bastmeyer, M. Wegener, Biomaterials 44 (2015) 186–194. [12] N. Tetreault, G. von Freymann, M. Deubel, M. Hermatschweiler, F. Perez-Willard, S. John, M. Wegener, G.A. Ozin, Adv. Mat. 18 (2006) 457–460. [13] A. Fr€ olich, J. Fischer, T. Zebrowski, K. Busch, M. Wegener, Adv. Mat. 25 (2013) 3588–3592. [14] J. Bauer, S. Hengsbach, I. Tesari, R. Schwaige, O. Kraft, Proc. Nat. Acad. Sci. U S A 111 (7) (2014) 2453–2458. [15] L.R. Meza, S. Das, J.R. Greer, Science 345 (6202) (2014) 1322–1326.
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[16] J.K. Gansel, M. Thiel, M.S. Rill, M. DeckeR, K. Bade, V. Saile, G. von Freymann, S. Linden, M. Wegener, Science 325 (2009) 1513–1515. [17] G. Williams, M. Hunt, B. Benedikt, A. May, M. Taverne, D. Ho, S. Giblin, D. Read, J. Rarity, R. Allenspach, S. Ladak, Nano Res. 11 (2) (2018) 845–854. [18] Y. Daicho, T. Murakami, T. Hagiwara, S. Maruo, Opt. Mat. Exp. 3 (6) (2013) 875–883. [19] A.S. Quick, H. Rothfuss, A. Welle, B. Richter, J. Fischer, M. Wegener, C. Barner-Kowollik, Adv. Funct. Mater. 24 (2014) 3571–3580. [20] D. Oran, S.G. Rodriques, R. Gao, S. Asano, M.A. Skylar-Scott, F. Chen, P.W. Tillberg, Science 362 (2018) 1281–1285. [21] M.A. Scott, Z.D. Wissner-Gross, M.F. Yanik, Lab Chip 12 (2012) 2265–2276.
Microstructures, post-TPP processing Richard Farrer Chemistry Department, Colorado State University-Pueblo, Pueblo, CO, United States
1 Introduction The previous chapters have shown the unique capabilities associated with two-photon photopolymerization (TPP)—the ability to fabricate complex, free-standing, threedimensional structures with resolution in the tens of nanometers. However, because of the nature of the polymerization process, the physical and chemical properties of the polymeric structures have found fairly limited application. Although some work has been done to expand the nature of the materials employed in the TPP process, the chemical functionality of the products of TPP remains quite limited. Fortunately, a number of techniques that modify the fabricated polymeric structures have been developed. These modifications are applied after a free-standing structure has been produced through the TPP and development process. This chapter will focus on the modifications that have been realized with postpolymerization processing of structures created via TPP. These post-TPP processing steps provide a means to extend the functions and capabilities of the structures fabricated with TPP. If TPP is to become a common technique employed to produce three-dimensional microscopic systems, the ability to extend the function of the polymer structures is necessary, and post-TPP processing provides a seemingly limitless route to this end.
2 Chemical modification of fabricated polymer surfaces Some of the initial work associated with post-TPP processing employed the unreacted functional groups that were present on the surface of the polymer structures after the direct laser writing (DLW) and development steps. In some situations, the reactive species on the surfaces of the polymer structures are unreacted functional groups that were responsible for the polymerization process. In other examples, the functional groups were added to the resins for the purpose of post-TPP processing. In either case, the only functional groups that are accessible for modification are those that exist on the surfaces of the fabricated polymer structures. Additionally, if more than one type of polymer is employed, selective functionalization of one polymer over the other can be accomplished. Three-Dimensional Microfabrication Using Two-Photon Polymerization https://doi.org/10.1016/B978-0-12-817827-0.00010-2
© 2020 Elsevier Inc. All rights reserved.
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2.1 Single-polymer functionalization One of the first things to notice when dealing with DLW is that the production of conductive, free-standing, three-dimensional, structures is lacking. While attempts have been made to produce conductive structures through direct laser writing, the fabrication of three-dimensional, free-standing, highly conductive structures from DLW has been elusive [1–4]. Clearly, the ability to produce conductive structures is critical in the development of DLW as a valid process for producing functional 3-D devices. A structure that has been produced by TPP could be coated with metal through one of the traditional methods: sputter coating, vacuum deposition, chemical vapor deposition, etc. However, these methods coat not only the fabricated structure but also the substrate. In most cases, it is desirable to coat only the structure and not the substrate on which the structure was constructed. As a result, a number of groups have developed post-TPP processes that allow for the incorporation of metals onto the surface of the fabricated structures. In the work by Formanek et al. [5], the silver plating occurs via an electroless plating process, which coats only the structure that was produced and not the substrate onto which the structure was fabricated. In order to accomplish this, the group produced a chemically modified polymer resin—a commercially available acrylate monomer with some amount of styrene added. The authors state that the increased aryl functionality from the included styrene improves the metal deposition process. In order to eliminate the deposition of the metal onto the substrates, the glass slides were modified with dimethydichlorosilane prior to the TPP process. The structures produced by TPP are sensitized using a tin(II) chloride solution, and finally treated with Tollen’s reagent to reduce silver onto the sensitized surfaces. A sample of the results is shown in Fig. 10.1. Also notice in Figure 10.1A, the large array of structures that were constructed in parallel with TPP and a microlens array. The substrate remains relatively clear of silver, because the sensitizing agent has little affinity for the modified substrate.
Fig. 10.1 (A) Image of a series of two-turn coils fabricated on the top of a 2-μm cube. (B) A single unit from the series shown in (A)—this structure has not been coated with silver. (C) A single unit shown after electroless deposition was employed to coat the structures with silver [5].
Microstructures, post-TPP processing
The Kuebler group developed a chemical process employed for the post-TPP modification that can be applied to structures fabricated from acrylate-, methacrylate-, and epoxide-based resins [6]. In this work, TPP is employed to fabricate a structure from an acrylate resin on a glass substrate that has been modified with 3-acrylopropyltrimethyoxysilane. The structure is treated with a lithium aminolysis solution that has been prepared from n-butyllithium and 1,3-diaminopropane. This solution will react with remaining acrylate, methacrylate, or epoxide functional groups to leave primary amine groups on the fabricated surfaces. Gold is reduced onto the amines, followed by electroless deposition of silver onto the gold. One important aspect of this process is the silver electroless plating solution is not Tollen’s solution, which is a fairly unstable mixture. The employed solution employed seems to plate at a slower rate than Tollen’s reagent, but it appears much more selective as the substrate shows no signs of metallization, while the structure is significantly reflective as a result of the deposited silver (Fig. 10.2). One last note concerning this modification process—while the article only shows steps to silver plating, the ability to produce primary amine functionality on a fabricated structure opens paths to many chemical processes and, thus, endpoints. While modification of TPP-fabricated surfaces with metal has gained much interest because of the resulting conductive structures, the use of TPP-fabricated structures as scaffold for cellular growth is another area that has received significant interest.
Fig. 10.2 Logstack structure produced by TPP that has been modified and coated with silver. The small image on the bottom-left is an SEM image of the structure, while the larger image is acquired using reflected light. Notice the structure is silver coated, but the substrate remains free of silver [6].
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Work by Klein et al. [7] produced a scaffold onto which cells were deposited. The scaffolds were produced with TPP using Ormocomp as the resist material. After fabrication, the scaffolds were developed using OrmoDev and rinsed with isopropanol and ethanol. To prepare the scaffold for cell adhesion, the structures were incubated in phosphatebuffered saline (PBS) containing poly-L-lysine. The scaffold were rinsed in PBS and treated with a solution containing fibronectin. Cells were introduced to the fibronectin-coated scaffold and incubated for 30 min. Cell force studies were then performed by observing the distortion of the scaffold caused by the cells. The ability to modify the surfaces of the TPP fabricated microstructures opens the door to development of many different uses for the fabricated structures. The chemical modification of the fabricated surfaces appears to be open to all possible chemical functionality, because the chemical nature of the resins used for fabrication allow for simple modification to common functionalities such as amines. These post-TPP processes modify the entire surface of the microstructure, while leaving the substrate on which the structure was fabricated unmodified. While this lack of selectivity is desirable in some cases, the ability to modify only selected portions of the structure is necessary if TPP is to be used to create complex systems.
2.2 Selective functionalization The ability to modify TPP fabricated surfaces in order to incorporate a wider spectrum of functionality is fundamental to expanding the use of TPP as a valid microfabrication technique. However, if one truly wishes to employ TPP to create microscopic devices, more often than not, the device will require more than one type of functionality. Therefore, the work done by the Fourkas group [8] in which structures created through TPP are shown to be selectively functionalized, provides a means by which polymers with orthogonal functionality can be employed to create more than one type of functionality in a single device. In this work [8], TPP is used to create structures from two different, but similar, polymer resins—one an acrylate-based resin and the other a methacrylate-based resin. Methacrylate resins are not often found in TPP processes, because of a lower reactivity rate as compared to acrylate resins; however, this work shows that moderate increases in laser powers can successfully produce methacrylate polymer structures. The substrate employed in this process was a glass coverslip that was modified with (3-methacryloxypropyl) trimethoxysilane—leaving a surface with methacrylate functionality. Serial TPP of the two different resins was performed with DMF and ethanol washes after each fabrication process. After the final wash, the substrate containing both methacrylate and acrylate structures was submerged in an ethanoic solution of ethylenediamine. The ethylenediamine reacts with the remaining acrylate functional groups at a much faster rate than the remaining methacrylate groups. As a result, the acrylic structures
Microstructures, post-TPP processing
Fig. 10.3 The images shows the selective modification of polymer structures produced through TPP methods. The letters “U” and “M” were prepared with acrylate monomers, while the letter “M” was prepared with methacrylate monomers. The resulting polymers were modified and coated with copper. The kinetics associated with the reactions of acrylates and methacrylates differ significantly, and allow the acrylate-based polymer to be modified while the methacrylate-based polymer remains unmodified [9].
were modified to contain amine functionality, while the methacrylic structures remained unmodified (Fig. 10.3). At this point, the amines provide access to a large number of chemical reactions that could end with any number of different chemical functionalities. In this work, the amine groups were used to deposit metals (copper and gold) onto the surfaces of the acrylate structures. In addition to showing the selective modification of the two polymers, the Fourkas group also showed the deposited metal was conductive. However, when the metal was applied to a polymer coil in an attempt to produce a working inductor, it was found that the inductance produced by the coil was significantly less than that predicted of a solid metal coil of similar size. Fig. 10.3 shows the results of this process. The “U” and the “D” were fabricated from an acrylate-based resin, while the “M” was produced from a methacrylate-based resin. A slightly different approach to the selective functionalization of fabricated polymer structures was taken by Takeyasu et al. [10] In this work, silver was deposited onto polymer structures that contained methacrylamide, while polymer structures not containing the amide functionality were left uncoated. A photopolymerizable resin was employed to fabricate the uncoated structures via TPP. The same resin was then impregnated with methacrylamide, and this new mixture was used as the resin for TPP. After development, the structures were treated with a silver nitrate solution. Since silver ions have an affinity for amides, the silver ions preferentially attached to the polymer containing the amide moieties, and thus selective functionalization of the structures was accomplished (Fig. 10.4). Further silver coating was accomplished through electroless plating techniques. The previous examples use metal deposition to prove the ability to selectively modify one polymeric structure over another. As stated previously, these chemical modifications allow for functionality beyond metal deposition and conductivity. For example, Scheiwe et al. [11] produced scaffold onto which chicken embryonic fibroblasts (CEFs) would
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Fig. 10.4 Silver is deposited selectively onto the cubes that contain amide functional groups, while polymeric cubes that do not contain amide functionality are not metal coated [10].
grow. The scaffolds were produced using two separate resins with one of the resins being modifiable to create adhesion sites for the CEFs to attach. The resins employed were Ormocomp and a homemade resin consisting of polyethylene glycol diacrylate, pentaerythritol tetraacrylate, and Irgacure 369 at the photoinitiator. The scaffolds were fabricated onto a glass substrate that had been modified so the fabricated structure would adhere to the surface. The homemade acrylate resin was used to make the vertical posts of the scaffolds, while the Ormocomp was employed to make the cell-attachment pads on each post. After development, the scaffolds were rinsed with phosphate-buffered saline (PBS) and incubated in PBS-containing fibronectin, which preferentially bonded to the Ormocomp polymer pads (Fig. 10.5). Cells were introduced to petri dishes containing the scaffolds, and the systems were incubated for 3 h. Once the cells had attached to the scaffold, a micromanipulator was employed to deform the scaffold, and the cell reaction to the deformation was studied. While the ability to modify the surfaces of TPP fabricated structures opened the door to many new uses of the structures, the selective modification of these structures significantly expands the possible uses of TPP fabrication. Instead of simply employing the structures for a single task, we can begin to imagine the construction of a device that has more than one purpose or function.
3 Double inversion Another area in which TPP fabrication techniques have received a significant amount of attention is in the production of 3-D photonic materials. TPP processes allow for the production of structures that function as photonic bandgaps (PGBs); however, the polymer that is produced by TPP does not provide a range of refractive indices that is sufficient to produce a complete photonic bandgap. At this point, direct writing of materials
Microstructures, post-TPP processing
Fig. 10.5 (A) SEM image of the TPP-fabricated scaffold (the Ormocomp polymer is shown in orange). (B) Reconstruction of confocal image stack of scaffold with the CEF growing in the scaffold. (C) Series of confocal images highlighting different parts of the CEF. The MERGE image is all the confocal images stacks of (C) merged into one image [11].
having a sufficient refractive index has not been realized. A post-TPP processing method known as silicon double inversion developed by Tetreault et al. [12] has been employed to produce silicon structures that provide a refractive index large enough to produce functioning PBGs. In this work [12], a woodpile structure having 24 layers was produced via TPP with SU-8 as the resin. The calculated photon band gap of the SU-8 structure was 8.6%. After fabrication (Fig. 10.6), the unpolymerized resin was removed, and the remaining structure was treated with humidified N2 in a chemical vapor deposition (CVD) chamber. The sample is then treated with N2 that contains SiCl4. An amorphous shell of dense SiO2 is constructed layer by layer during the treatment with SiCl4. This process is continued until the polymeric woodpile structure has been infiltrated by the SiCl4, which reacts with the adsorbed water to produce SiO2. The SiO2 coats the entire sample—the polymeric structure and the substrate—the process is not selective. At this point, the polymeric core is completely coated in SiO2. In order to remove the polymeric core, the SiO2 that is coating the top layer of the woodpile structure (the layer that is farthest from the substrate) is removed through a reactive-ion etching (RIE) process using an SF6 plasma. The RIE treatment exposes a portion of the polymer structure that lies in the core
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Fig. 10.6 Silicon double inversion process. First, the microstructure is fabricated from SU-* via TPP. Then, DVD is used to coat the sample with SiO2. The SiO2 is removed by reactive ion etching to expose the polymer at the top of the microstructure. Complete removal of SU-8 is carried out using oxygen plasma or calcination. Additional CVD of SiO2 can be used at this stage to produce the desired rod diameter. Si infiltration occurs by low-pressure CVD. Finally, an HF-resistant substrate is attached to the top of the microstructure, and the whole assembly is immersed in HF to remove SiO2 [12].
of SiO2 shell. Because the entire polymeric structure is one large network, exposure of a small portion of the polymer provides access to the entire polymeric network. Because of this, the polymer can be removed through one of two methods. First, the polymer can be etched using an O2 plasma, or the polymer can be removed through a calcination process in a furnace at 450°C. The remaining structure is a SiO2 shell of the original polymeric woodpile structure—basically the SiO2 is a template for the original woodpile structure. Since this process is employed to produce PGBs, the diameter of the rods in the woodpile is fundamental in the ability of the final structure to act as a PBG. In order to maximize the PBG, the diameter of the rods is reduced by depositing SiO2 into the woodpile template that has been exposed by the removal of the polymeric woodpile structure. Again, SiCl4 is introduced to the sample; however, this round of SiCl4 deposition is to reduce the diameter of the rods in the final woodpile structure. After the woodpile channels are narrowed, CVD is again employed to fill the template with disiline, Si2H6, at 460°C. This process both infiltrates and coats the SiO2woodpile template. Once the silicon has completely filled the template, the SiO2 is removed through an HF etch. Recall, the TPP structure employs a glass substrate, so the HF etch not only removes the SiO2 inverse woodpile structure, but also the substrate. Therefore, prior to the HF etch, a sapphire substrate is glued top of the structure—the surface that has been previously etched with SF6 plasma. The HF etch produces a silicon structure that is adhered to the sapphire substrate. Ultimately, this process produces a silicon replica of the original SU-8 woodpile that was fabricated with TPP (Fig. 10.7).
Microstructures, post-TPP processing
Fig. 10.7 SEM images of silicon woodpile structure produced through the silicon double inversion process. The images on the right compare the TPP fabricated SU-8 structure at the bottom and the final silicon structure on the top [12].
The resulting silicon:air structure produces a photon band gap of 8.6%. The estimated effective refractive index of the rods in the silicon woodpile is 3.16, which is similar to an 80% silicon and 20% air mixture. The silicon woodpile structure produced suppression at 2.35 μm that was over two orders of magnitude. The double inversion process can be employed to produce photonic crystals from TPP-produced structures. Since TPP can produce any number of different structures, TPP fabrication coupled with the double inversion post-TPP processing can produce photonic crystals with different overall configurations.
4 Atomic layer deposition Recently, works by the groups of Wegener [13], Kraft [14], and Greer [15] have coupled TPP with a process known as atomic layer deposition (ALD). In the work by Kraft and Greer, ALD was employed to produce a thin layer of alumina on the surface of the structure produced by TPP, while Wegener used ALD to introduce both zinc oxide and titania into the microstructures. The final thickness of the ALD deposited material is controlled through the total number of deposition cycles performed. In the work of Kraft [14], the alumina-polymer composite structures were tested for strength and recoverability after compression (Fig. 10.8). The Greer group [15] took the process a step further by removing the polymer core after the deposition of the alumina—leaving free-standing structures constructed of hollow alumina tubes (Fig. 10.9). These final structures have been shown to be lightweight, strong, and ductile, and are able to recover from significant compression events. The final structure produced by Wegener [13] was employed as a photonic bandgap for visible light. The structure produced by the Wegener group [13] was first produced using TPP and a photoresist. After development, ALD was employed to deposit a layer of ZnO onto the structure. The polymer was removed through calcination, and additional ZnO was
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Fig. 10.8 Five different cellular structures produced with TPP. The top row shows computer-aided design models (A through E), while the lower row shows SEM images of the corresponding TPP microstructures (scale bars are 10 μm) [14].
Fig. 10.9 Alumina nanolattices. Image (A) is a computer-aided design model of the octet-truss that was fabricated. (B) shows a single unit of the structure that is colored blue in (A). A diagram of the cross-section of a tube is shown in (C). A SEM image of the actual TPP fabricated structures is shown in (D), while (E) is a closer image of one of the structures with the inset showing the tubular nature of the trusses. A TEM dark-field image of the diffraction grating of the tube wall is shown in (F) [15].
Microstructures, post-TPP processing
deposited though ALD. Titania was then introduced by way of ALD and infiltrated the volume where the photoresist had been prior to calcination. At this point, the structure is sitting on a ZnO layer, so polymer pads are added to the corners of the woodpile to keep it attached to the substrate. The ZnO is etched from the structure, leaving only TiO in a woodpile structure. The work of both the Kraft [14] and Greer [15] groups begins with the fabrication of three-dimensional structures using TPP and a photoresist. The Kraft group produced a series of cellular structures that are shown in Fig. 10.8, while the Greer group produced the structure shown in Fig. 10.9. After removal of the unpolymerized photoresist, the structures were exposed to several atomic layer deposition (ALD) cycles. According to work by Greer, theses cycles consist of four steps: the system is pulsed with water, purged, pulsed with trimethyl aluminum, followed by a final purge. The process is performed at 150°C. The thickness of the alumina that is deposited onto the structures is determined by the number of atomic layer deposition cycles that are performed. The structures produced by the Kraft group are completed at this point. The Greer group uses a focused ion beam to remove a portion of the alumina in order to expose the polymer scaffold. The structure is then exposed to oxygen plasma that removes the polymer scaffold and leaves the three-dimensional alumina structure. The work presented shows that ALD provides a method to produce surfaces and complete structures of silica, alumina, TiO2, and ZnO. Because ALD is completed layer by layer, the thickness of the resulting layers or structures is entirely controlled by the number of deposition cycles. Such processes have opened the door to new photonic materials and structures with increased strength.
5 Electrodeposition in TPP produced template In 2009, the Wegener group produced a gold helix photonic metamaterial using TPP followed by electroplating [16]. The final structure is an array of gold coils that collectively can act as a circular polarizer (Fig. 10.10). The fabrication process involves creation of at template on a conductive surface using TPP. To this end, a positive photoresist was spun onto a glass substrate that has been treated with a thin conductive film of indium-tin oxide (Fig. 10.11). TPP is employed to expose the areas that will contain the coils in the photoresist. The resist is then developed, which leaves a template with air coils that end at the indium-tin oxide (ITO) surface. The structure is placed in a gold electroplating solution and gold is plated into the coils using the ITO as the electrode. This process is very similar to that of electrodeposition of metal into anodized aluminum oxide templates. After the electroplating is completed, the resist material is dissolved leaving an array of gold coils. A similar method was employed by Williams et al. to produce magnetic nanostructures [17]. Again, a positive resist was spun on top of a glass/ITO substrate. The desired geometry of the final structure was produced in the positive resist through TPP, and the
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Fig. 10.10 Array of gold helixes produced through TPP and electroplating. (A) shows a vertical cut of the template that has been filled with gold. (B) and (C) show a close up of the helixes and a top view of the array of helixes [16].
fabricated structure was removed, leaving a void in the positive resist that extend to the conductive ITO substrate. Cobalt was then electrodeposited in the voids, and the resist was removed leaving only the free-standing, cobalt nanostructure. Fig. 10.12 provides SEM images of two different arrays of structures along with higher magnification images of individual structures. The magnetic properties associated with some of the produced nanostructures were evaluated using spin-SEM and magneto-optical Kerr effect (MOKE) magnetometry. Results of the evaluation of the magnetic properties of the structures show that the structures appear to be composed of small crystalline grains within the larger structure.
Fig. 10.11 This image show the process employed to create the array of gold coils. Initially a positive tone photoresist is spun onto a glass surface coated with conductive ITO. TPP is employed to expose coils in the resist, after which the exposed area is removed by a solvent. Electroplating is employed to fill the exposed areas by employing the ITO surface as an electrode. The growth of the gold coils begins at the ITO surface and continues into the coil-shaped voids. After electroplating is completed, the remaining resist material is removed, exposing the array of gold coils [16].
Fig. 10.12 SEM images of magnetic nanostructures. (A) An array of tilted nanowires. (B) and (C) Magnified images of the nanowire array shown in (A). (D) An array of tetrapods. (E) and (F) Magnified images of single tetrapods [17].
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This technique of creating a metal structure through TPP of a positive photoresist and then electrodepositing metal into the created void opens the door to the creation of pure metal structures; however, the process has some limitations. Since the final structures are created by the electroplating of gold into the voids created in the resist and the electroplating must start at the ITO surface, the structures created are fairly limited in scope. As an example, voids that are horizontal to the surface of the substrate would not completely fill during the electroplating process. However, this technique provides a method to produce structures that are composed of pure metal.
6 Pyrolysis Carbon electrodes are employed for use as electrodes in batteries and fuel cells and are often found as supports for electrochemical sensors. Electrodes having complex shapes and high surface areas are highly desirable. Clearly, TPP can produce complex structures; however, direct fabrication of carbon structures is not possible with TPP. Work by the Maruo group [18] has produced a process that allows for the production of complex carbon structures using TPP as the fabrication method to produce the desired architectures. The carbonization of the polymer structures is accomplished through pyrolysis. It was found that commercial photopolymers do not have a high enough carbon content to produce stable three-dimensional structures. Therefore, the Maruo group developed a set of polymer resins based on resorcinol diglycidyl ether that contain higher percentages of carbon. In this research, three new photopolymers having high carbon contents were developed—all three contained resorcinol diglycidyl ether, an oxetane, and a photoinitiator. Additionally, two of the three contained an acrylate resin. Only the two containing acrylates were found to be suitable for TPP processing. These two resins were employed to produce complex, three-dimensional structures through TPP. After fabrication, the structures underwent pyrolysis to convert the polymer structures to carbon (Fig. 10.13). The shrinkage of the structure caused by the pyrolysis process was approximately 20%–30%, depending on the polymer employed. The final carbon structures were small replicas of the structures originally produced with TPP. Interestingly, one of the polymer composites that was produced would not polymerize with TPP; however, use of microtransfer molding techniques allowed for the production of structures using high-carbon content polymers that are unable to be fabricated by way of TPP (Fig. 10.14).
7 Multiphoton-induced spatially resolved functionalization The final post-TPP process to be discussed is multiphoton-induced chemistry on the surface of TPP produced structures. In this modification method, the laser that was
Microstructures, post-TPP processing
Fig. 10.13 SEM images (A) and (C) are polymeric microstructures produced with TPP. Images (B) and (D) are the same structures after carbonization [18].
Fig. 10.14 SEM images (A) and (C) are microstructures produced by microtranser molding. Images (B) and (D) show the same microstructures after carbonization [18]. Note—structures used to create molds were produced via TPP [18].
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employed for the TPP fabrication of the 3-D polymer structure was employed to excite areas on the surface of the structure in order to modify those specific areas [19]. The TPP structures were created using a resin that consisted of o-quinodimethanes and maleimides. The polymerization reaction is a Diels-Alder reaction, and not the typical radical polymerization that is often employed in TPP fabrication. After the main structure had been produced and developed, the structure was submerged in a solution containing maleimides having bromine functionality. The reaction between the dissolved bromomaleimides and the residual, unreacted, o-quinodimethanes on the surfaces of the TPP fabricated structure was photoinduced by two-photon absorption, mediated by the TPP laser. This process provides a method to produce spatially resolved modifications that have high resolution. Additionally, this selective modification does not require two or more types of polymers, since the surface modification is mediated by the DLW laser system.
8 Implosion fabrication (ImpFab) In late 2018, Oran et al. published a TPP process that they deemed ImpFab (Implosion Fabrication) [20]. This technique uses a three-step process to produce 3-D nanomaterials. The first step is TPP to fabricate 3-D structures in solvated polyacrylate/polyacrylamide hydrogels containing fluorescein [21]. After initial fabrication of the 3-D structure, a second step is employed for the modification of the functionality on the hydrogel, which allows for the introduction of a wide variety of desirable components, such as fluorescent dyes, DNA, and other biologically relevant moieties, metal nanoparticles, and quantum dots. As long as the chemistry of modification can be done in an aqueous environment (required to keep the hydrogel system swollen and solvated), it can be used in this process. Following the initial modification of the hydrogel, further amplification can be employed to enhance the modification. For example, the hydrogel structures were modified to introduce amine moieties that have an affinity for gold nanoparticles (AuNPs). In order to increase the metal-to-hydrogel ratio, a silver intensification process was employed, where silver was deposited on the bound AuNPs. The final step of the process involves treatment with acid or divalent cations followed by dehydration, which allows for controlled shrinkage of the fabricated structure to its final dimensions. Ultimately, the overall reduction in the size of the structure is determined by the amount of crosslinker present in the initial hydrogel mixture. Figs. 10.15 and 10.16 provide images during the different steps of the process, and show some of the capabilities of ImpFab. An interesting factor that is shown in image (L) of Fig. 10.15 is that the shrinkage in the lateral dimensions is different than that in the axial dimension. While this may be of some concern, the shrink factors are reproducible, so that the difference in shrinkage associated with each dimension can be corrected during the fabrication process. Fig. 10.16 shows structures that have been modified with
Microstructures, post-TPP processing
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Fig. 10.15 (A), (C), (E), and (G) The steps in the ImpFab process, along with SEM images associated with each step, (B), (D), (F), and (H), respectively. Note that the shrink step reduces the scale of the structure by a factor of 10 in both the in-plane dimensions. (I) shows fluorescent patterns that were created by different laser powers. (J) Two different patterns that have been modified with different components. The yellow pad contains metal nanoparticles and the blue pad contains CdTe quantum dots. (K) Large image is a fluorescent pattern prior to shrinking and dehydration, and the two images to the right are after shrinking and dehydration. The top image had a shrink factor of 10 , and the bottom image had a shrink factor of 20. (L) The shrink factors associated with the lateral and axial dimension of a 10 gel. (M) The lateral shrink factor associated with a 20 gel [20].
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Fig. 10.16 Structures in which the hydrogen has been modified with gold nanoparticles and intensification was carried out with silver. (A) and (B) are SEM images that show the metal structure prior to and after sintering. (C) Shows an array of structures after shrinking and dehydration, but before sintering. (D) shows a fluorescent image of a complex 3-D structure, (E) is the same structure after deposition of silver, and (F) shows the structure after dehydration [20].
gold nanoparticles and intensified with silver. Other structures produced following this method are highly conductive after the shrinking, dehydration, and sintering processes are complete. One important note is that the initial hydrogel scaffold is not removed; therefore, the final structures shown in Fig. 10.16 contain the hydrogel so that the 3-D structure is retained.
9 Conclusions The use of post-TPP processing techniques has gained significant interest in the past decade. The ability to extend the functionality and viability of three-dimensional microstructures produced through TPP provides for a much broader use of the produced
Microstructures, post-TPP processing
structures. As can be seen, the processing of the fabricated structures can be completed in many different ways: chemical modification of the surface of the structure, atomic layer deposition, pyrolysis, double inversion, etc. Additionally, the structure can be employed as a template for the production of pure metal structures. This post-TPP processing produces structures with new abilities: conductivity, higher refractive index, favorable cellular interactions, higher strength, etc. In addition to the modifications presented, the capability of selective modification of one area of a structure over another is also a significant step for TPP. Recent work involving TPP of hydrogels followed by modification and shrinking may provide a method to produce complex 3-D nanomaterials with a wide variety of functionalities. Although post-TPP processing appears to be nearly limitless in its ability to provide any functionality necessary to a system and the work presented in the chapter provides a significant improvement in the functions of structures produced through TPP, much more work needs to be done if TPP is to realize its potential. If TPP is going to move forward, processes must be developed that allow for the production of a system and not simply one or two structures with orthogonal functionalities. The production of a system of structures having different functionalities (conductivity, magnetism, varying optical properties, fluid flow, etc.) that work together in a concerted manner must to be shown to be possible.
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