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Easy Printing of High Viscous Microdots by Spontaneous Breakup of Thin Fibres Laura Mecozzi, Oriella Gennari, Sara Coppola, Federico Olivieri, Romina Rega, Biagio Mandracchia, Veronica Vespini, Alessia Bramanti, Pietro Ferraro, and Simonetta Grilli ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17358 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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ACS Applied Materials & Interfaces

Easy Printing of High Viscous Microdots by Spontaneous Breakup of Thin Fibres L. Mecozzi1, O. Gennari1, S. Coppola1, F. Olivieri1,2, R. Rega1, B. Mandracchia1, V. Vespini1*, A. Bramanti1, P. Ferraro1, and S. Grilli1 1

Institute of Applied Sciences & Intelligent Systems of the National Research Council (CNR-ISASI), Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy 2 Department of Chemical Materials and Production Engineering of the University “Federico II”, P.le Tecchio 80, 80125 Naples, Italy

Abstract Electro-hydrodynamic jetting is emerging as a successful technique for printing inks with resolutions well beyond those offered by conventional inkjet printers. However, the variety of printable inks is still limited to those with relatively low viscosities (typically < 20 mPa s) due to nozzle clogging problems. Here we show the possibility of printing ordered microdots of high viscous inks such as the poly(lactic-co-glycolic acid) (PLGA) by exploiting the spontaneous breakup of a thin fibre generated through nozzle-free pyro-electrospinning (PES). The PLGA fibre is deposited onto a partially wetting surface and the breakup is achieved simply by applying an appropriate thermal stimulation, able to induce polymer melting and hence a mechanism of surface area minimization due to the Plateau-Rayleigh instability. The results show that the technique is a good candidate for extending the printability at microscale to high viscous inks, thus extending their applicability to additional applications, such as cell behaviour under controlled morphological constraints.

Keywords: polymers; micro-printing; thin liquids; pyro-electrospinning; microdots. Corresponding author: *[email protected]

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1. Introduction Techniques for fabricating ordered structures on surfaces at micro and nanoscale are of fundamental importance for many existing and emerging technologies, such as organic semiconductors and flexible electronics1-3 , bio-related and medicinal research ranging from biomolecule sensing and stimulation4-5 to cell and tissue engineering6-7 , generation of masks and templates8 , production of optical components such as microlens arrays.9 The lithographic techniques already available in the semiconductor industry can fabricate features with sizes well below 100 nm but they are not applicable to all of those active materials that are highly sensitive to chemical, mechanical or thermal treatments (i.e. organic semiconductors, biomaterials for sensing). Moreover, the high cost of the lithographic equipment hinders its use in scientific research. Contact approaches such as dip-pen lithography10 and microcontact printing (µCP)11-12 have emerged as valuable alternatives for processing suspensions on the nanoscale. However, they require strictly controlled atmospheres and the stamp deformation in µCP limits dramatically the pattern reproducibility. Recently, non-contact techniques based on inkjet printing have increasingly attracted attention from research and industrial communities due to their relatively low-cost, highefficiency and environmentally friendly features.13-15 Thermal and piezoelectric inkjet printers are the systems most widely available in the market nowadays, but they are impractical for reaching resolutions below 20 µm.16-19 Conversely, the electro-hydrodynamic jet (e-jet) can generate microdots with sizes ranging from microns down to hundreds of nm, by pulling thin electrified jets from the nozzle through external electric fields.20-23

However, when printing high-resolution

patterns, the e-jet still has some limitations. It requires a high-resolution nozzle with inner diameter of a few micrometres, with costly implications in terms of nanotechnology fabrication. The resolution is limited by the translations between the printing nozzle and the target substrate. Neither inkjet nor e-jet has the flexibility to generate various droplets in a continuously tunable manner. Last, but not least, the ink viscosity is limited to values well below 20 mPa s due to nozzle clogging problems.24-25 This means that a range of polymer solutions, with viscosities exceeding this limit, ACS Paragon Plus Environment

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still suffer poor printability. In these cases the ink composition is usually tailored with appropriate solvents in order to reduce the viscosity26-28, but with detrimental effects for the polymer functionalities.29 Therefore, alternative approaches are highly desirable for avoiding such material damages. In fact, some papers have reported that high-resolution printing in the range of a few micrometers or even submicrometers could be realized by commercial inkjet printers, by controlling the interactions between the inks and substrates.30-32 Recently we developed an innovative electro-hydrodynamic jet printing method based on the pyroelectric effect that we call pyro-electro-hydrodynamic jet (p-jet). It allows us to print dots with sizes down to 300 nm

by a revolutionary set-up that is both electrode- and nozzle-free.33

Therefore, the implementation of appropriate electrodes and nozzles is completely avoided and, as a consequence, the clogging problems are easily overcome. Inks with different viscosities have been printed successfully by p-jet, including oils33 , bioinks34 and polymer solutions with various compositions.35-36 Moreover, we developed what we call pyro-electrospinning (PES)37-38 able to print fibres with thickness down to 300 nm using polymer solutions with viscosities up to about 200 mPa s, thanks to the above-mentioned nozzle-free modality. The surface pyroelectric charges were able to draw fine fibres of poly(lactic-co-glycolic acid) (PLGA), while ordered microdots are still relatively complicated to achieve due to the strong contribution of the viscoelastic forces. Here we use the spontaneous breakup of a thin fibre of PLGA printed by PES onto a partially wetting substrate, for printing easily ordered microdots of a high viscous ink. After deposition, the fibre is subjected to an appropriate thermal treatment able to induce phase transition so that, thanks to the reduced friction on the partially wetting substrate, the polymer behaves according to the rules of the Plateau-Rayleigh instability till the breakup into ordered microdots. This approach combines the advantage of PES in printing easily well-ordered microfibers of high viscous polymers, with the phenomena related to surface instability in thin liquids, thus avoiding the typical drawbacks encountered in contact or inkjet approaches. We show here how these PLGA microdots can be adopted for cell patterning applications through a cost-effective procedure easy to accomplish even ACS Paragon Plus Environment

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into a non-specialized biological laboratory and preserving the cytophilic nature of PLGA. We believe that the breakup technique is a good candidate for extending the microdot printability to high viscous polymers, avoiding the manipulation of the ink composition, and thus opening the route to a wider range of applications by preserving the polymer functionalities.

2. Experimental section The experimental procedure. Figure 1(a,b) show the schematic views of the experimental procedure.

Figure 1. Schematic views of the experimental procedure. (a) The PLGA fibre is deposited onto the partially wetting substrate (collector) by PES and, successively, (b) the substrate is positioned onto a conventional hotplate at 80°C inducing progressively PLGA melting and fibre breakup. The schemes are not to scale.

First, a PLGA microscale fibre was deposited onto a partially wetting substrate (labelled ‘collector’ in Fig.1(a)) by PES.37 A sessile drop of PLGA solution (lactide:glycolide (75:25), Mw 40,000:75,000, dissolved in dymethil carbonate (DMC) at a concentration of 25% wt/wt, both from Sigma Aldrich) with a volume around 1 µL was pipetted onto a stable support (e.g. silicone slide fixed by a vacuum stage) and worked as base drop for the PES process. Without going into details, that can be found in reference37, the PES was activated by the thermal stimulation of a lithium niobate crystal (LN, c-cut, both sides polished, 500 µm thick, Crystal Technology Inc.) standing behind the collector (see Fig.1(a)). The thermal source was a conventional resistive heating head.


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The field lines generated by the pyroelectric effect exerted an attractive force on the polymer drop deforming it into the so-called Taylor cone, from which apex a liquid fibre was ejected and drawn by the collector mounted onto a (x,y) motorized translation stage. After rapid solvent evaporation, the deposited fibre was solid and the collector was removed from the PES system and positioned onto a conventional hotplate (RT2 basic, Thermo Scientific) at 80 °C, in order to induce polymer melting (see Fig.1(b)). At this point, thanks to the partial wetting of the substrate that reduced the friction at the polymer/substrate interface, the polymer melt flowed according to the PlateauRayleigh instability. In a few seconds the high viscous PLGA fibre broke up into the aligned microdots shown schematically in Fig.1(b). The partially wetting substrate. The partial wetting of the substrate is a key parameter for the breakup process. Therefore, we measured the contact angle of a PLGA sessile drop (1 µL) onto five microscope slides spin-coated by different well-known hydrophobic solutions, in order to identify the coating that minimized the friction at the PLGA/substrate interface. A magnified side view of the sessile drop was recorded by a standard transverse illumination system with a lamp, a microscope objective and a CCD camera, all perfectly aligned. The open source image-processing program ImageJ, developed at the National Institutes of Health (NIH), was used to evaluate the contact angle in the recorded images. The following 5 solutions were tested for getting partial wetting: 1) PFPE, Fluorolink® S10-PFPE (perfluoropolyether alkoxy silane, Solvay Specialty Polymers); 2) PDMS, poly(dimethylsiloxane) (Sylgard 184 Silicone Elastomer Kit, 10:1 mix ratio, Dow Corning); 3) Sigmacote® (Sigma-Aldrich); 4) PS, polystyrene (average Mw ~192,000, SigmaAldrich) dissolved in anisole at 60% wt/wt (ReagentPlus®, 99%, Sigma-Aldrich); 5) PMMA (average Mw ~996,000, Sigma Aldrich) dissolved in anisole at 30% wt/wt. Five microscope glass slides were spin-coated by each different solution, according to the protocol suggested by the producer. Table 1 shows the list of contact angle values obtained as mean values over three replicates of the experiments.


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Table 1: List of contact angle values of PLGA on five different coatings.

Coating

PLGA contact angle (°C)

PFPE

56

PDMS

52

SIGMACOTE

50

PS

48

PMMA

47

It is worth noting that the contact angles presented in Table 1 refer to the PLGA solution, while the breakup process involves the polymer melt. However, the wetting tendency of the testing substrates can be considered correct since the probe liquid is the same for all the tests. The results show that PFPE coating provides the less wetting conditions for PLGA, and therefore was selected here as ‘gold choice’ for the breakup process. The cell culture. Considering the well-known biocompatibility of PLGA, the patterns of PLGA microdots were used for guiding the adhesion of live fibroblast cells (NIH/3T3) onto ordered islands. The cells were first grown in Petri dishes in Dulbecco’s modified Eagle’s medium (DMEM) containing 4.5 g/L D-glucose and supplemented with 10% FBS (fetal bovine serum), 100 units/mL penicillin, and 100 µg/mL streptomycin. Successively the cells were harvested from the tissue culture flasks by incubation with a 0.05% trypsin–EDTA solution for 5 min. The cells were then centrifuged, re-suspended in a complete medium, and then seeded onto (2×2) cm2 substrates with patterns of PLGA microdots, at a density of 1×105 cells/mL. The cells were then incubated in conventional 50 mm diameter Petri dishes at 37 °C and in humidified 5% CO2 atmosphere. After


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24h incubation and brief washing by sterilized PBS, the slides were observed under an inverted optical microscope (AxioVert, Carl Zeiss, Germany).

3. Results and discussion We printed PLGA fibres with different geometrical characteristics thanks to the versatility of PES37, and we call them ‘starting fibres’. We used microscope glass slides coated by PFPE as collectors in order to have the starting fibre onto a partially wetting substrate (see the Experimental Section). As a consequence, the breakup process was very versatile and capable of producing patterns of PLGA microdots with high regularity, according to different arrangements. The evolution of the breakup process A PLGA fibre, with a multi-shape geometry, was deposited onto the partially wetting substrate in order to follow the temporal evolution of the breakup in case of a complex pattern in which different fibre geometries are present simultaneously. After deposition, the substrate was positioned under an upright optical microscope with a heating plate underneath set at 80°C, in order to record the evolution of the breakup process (see the Supporting Movie S1). Figure 2 shows four sequential frames of the Supporting Movie S1.

Figure 2. Sequential optical microscope images of a multi-shape PLGA fibre evolving from (a) the starting state at room temperature to (b-c) the successive states during heating and to (d) the final pattern at equilibrium after switching off the heater, as recorded in the Supporting Movie S1. The ghost image in (d) is exactly the image in (a) made transparent and superimposed to the clear image in (d), just to show the position of the final microdots respect to that of the initial fibre. The fibre was about 20 µm thick. The whole process, from (a) to (d), lasted a few seconds.


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Figure 2(a) shows the optical microscope image of the starting PLGA fibre at room temperature with the multi-shape geometry including three regions that we call here ‘straight’, ‘loop’ and ‘sinusoidal’. The arrows indicate the ‘nodal points’, corresponding to non-negligible discontinuities in the volume distribution along the fibre. The images in Fig. 2(b-c) correspond to the successive temporal evolution during heating. In particular, Fig. 2(b) shows the fibre while reaching the melt phase, as shown by the slight swelling effect along the fibre boundaries. Thanks to the partial wetting of the substrate, that reduces drastically the friction at the polymer-substrate interface, the liquid-like PLGA starts slipping and flowing. At this stage, the forces related to the PlateauRayleigh instability39-43 regulate the successive flowing behaviour. The large surface tension, resulting from the large specific-surface-area, necks the liquid fibre and breaks it into segments, as shown in Fig. 2(c) and in the Supporting Movie S1. The fragments further shrink from the fusiform shape into approximately circular droplets in order to minimize their surface area. This leads to the final break of the segments into the ordered pattern of high viscous microdots shown in Fig.2(d), where the equilibrium condition is reached, the heater is switched off and the PLGA recovers its solid state at room temperature. The ghost image in Fig.2(d) corresponds to the initial state of the fibre in Fig.2(a) and shows the slipping of the polymer from the starting fibre to the final microdots, due to the fragments shrinking. It is worth noting that the necking effect occurs first in the loop region and, successively, in the sinusoidal and straight regions. This is strictly related to the nature of the nodal points that work as external perturbations to the liquid profile that trigger the necking effect. The nodal points of the loop regions clearly correspond to volume discontinuities higher than in case of those in the sinusoidal region, as indicated by the arrows in Fig.2(a). This means that the nodal points in the loop region favour the necking effect more rapidly respect to those in the sinusoidal one. Lastly, in case of the straight region, the necking effect is governed by microscopic perturbations of the liquid surface, as well investigated by valuable works in this field39-43, thus making it slightly slower compared to the regions with more pronounced discontinuities. ACS Paragon Plus Environment

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The distribution of the microdot diameter depends strictly onto the thickness of the starting fibre, as well shown in Fig.2(d). The intermolecular forces in the nodal points, indicated as example in Fig.2(a), are significantly higher compared to those in the thinner regions of the fibre, thus leading the fragment shrinking effect to the formation of larger microdots. This means that simply by controlling continuously the fibre thickness deposited by PES37 we can produce patterns of PLGA microdots with the desired distribution of both position and diameter. The technique is applicable easily to other polymers (see the Supporting Information). In order to show more clearly the phase variation of the polymer during heating we performed a very simple experiment. We deposited a simple drop of PLGA solution (around 2 µL) on the partially wetting substrate, with the heating plate underneath, and we used a conventional scalpel for observing the phase transition during heating and cooling under the microscope. The Supporting Movie S2 shows the corresponding images. Initially, the heater is off and the scalpel engraves the solid polymer. Successively, the heater is on and the scalpel spreads the liquid polymer that tends to shrink in order to minimize the surface energy. The regions with higher volume simply shrink the boundaries, while the random thin rivulets break into microdots according to the PlateauRayleigh instability that, in fact, occurs in case of thin liquids. The quality of the movie is poor due to file size limitations. It is important to note that, recently, the breakup of thin fibres, through Plateau-Rayleigh instability, has been already used for patterning applications but through a completely different approach, as discussed below. Huang et al.44-45 used electrospinning for depositing fibres of polyethylene oxide (PEO) onto a partially wetting substrate in the form of well-ordered lattices. Furthermore well-defined PEO line and grid patterns can be fabricated via the proposed EHD direct patterning under appropriate conditions, the viscosity varied from ~ 10 mPa s up to ~ 2000 mPa s.46 Since PEO is soluble in water, they developed a water-vapour technique for inducing the solid fibres to swell into liquid lines and therefore for using the subsequent Plateau-Rayleigh instability for producing colloid microarrays. Even though the technique is intriguing and provides a good ACS Paragon Plus Environment

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controllability, it is inadequate when inks different from water-soluble PEO are desired. In fact, their technique makes use of surface instabilities occurring in liquid lines obtained through the water absorption of PEO that makes it to recover the phase of liquid solution. Moreover, the electrospinning is limited dramatically by nozzle clogging when high viscous inks are needed. Conversely, our approach exploits the melting phase of the polymer, thus extending the applicability of the surface instability to a wider range of polymers and especially to those with high viscosity, where nowadays the printability is poor due to nozzle clogging problems. In other words, here we exploit the successful combination of PES, which is able to print high viscous fibres at microscale, with the Plateau-Rayleigh instability occurring in the melting polymer over a partial wetting substrate. The microdot patterns produced by fibre breakup Figure 3 shows the optical microscope images of the typical starting PLGA fibre (a,c,e,g) and the corresponding microdot pattern (b,d,f,h), respectively. The fibres in Figure 3(a, c, e) are straight and with a constant thickness of about 35 µm, 5 µm and 1 µm, respectively. The corresponding patterns in Fig. 3(b, d, f) show clearly the formation of microdots well aligned along the direction of the starting fibre and with diameters and dot-to-dot spacing that appear clearly proportional to the thickness of the starting fibre. In other words, the thicker fibre produces larger and more distant microdots, and viceversa. Satellite microdots are visible in Fig.3(b) in the dot-to-dot interspace. They are due to surface defects of the partial wetting coating that tend to block residual volumes of liquid during the polymer shrinking into the main microdots. The correlation between fibre thickness and microdot diameter is clearly visible also in case of the fibre in Fig. 3(g). This fibre was deposited with a regularly increasing thickness by reducing the base-collector distance in the PES set-up (see details in Ref.37). After breakup, the resulting pattern consists of a sequence of microdots well-aligned along the fibre direction and with diameters increasing with the thickness of the starting fibre (see Fig.3(h)).


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Figure 3: The case of straight fibres. Optical microscope images of the typical (left column) PLGA starting fibre and (right column) the corresponding microdot pattern (b,d,f,h), after breakup on the hotplate at 80°C. These fibres were deposited straight onto PFPE coated glass slides with (a, c, e) constant and (g) increasing thickness. The scale bars are 200 µm long.

Figure 4 shows the results concerning the case of curved starting fibres. Different PLGA fibres were deposited onto the partial wetting substrate along curved regular directions and Fig. 4(a,c) show the optical microscope images of two typical examples.

Figure 4: The case of curved fibres. Optical microscope images of the typical (left column) PLGA starting fibre (a,c) and (right column) the corresponding microdot pattern (b,d), after breakup onto the hotplate at 80°C.


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The fibre in Fig. 4(a) was about 20 µm thick and was deposited along a curved direction with a single main inlet having a relatively long radius of curvature. The fibre in Fig. 4(c) was about 30 µm thick and was deposited along a sinusoidal direction where the inlets have much shorter radius of curvature. The corresponding microdot pattern in Fig. 4(b, d) is obtained by thermal treatment at 80°C and shows the regular distribution of the PLGA microdots along the curved direction of the starting fibre. Also in this case the microdot diameter is related to the fibre thickness and the satellite droplets often appear in between the main microdots due to surface defects. In case of curved fibres, a peculiar phenomeonon occurs. In case of inlets with shorter radius of curvature (see Fig.4(c)), the corresponding microdot, that we call here ‘inlet microdot’, is larger than those developing along the straght direction, namely the ‘straight microdots’ (see Fig.4(d)). This effect is highly repeatable and is related to the volume discontinuity occurring in the inlet, as discussed in the previous section. In order to test the reliability and repeatibility of the process, the fibre breakup was investigated for a series of straight PLGA fibres with thickness values in the approximate range of 3-35 µm. Both the microdot diameter and the dot-to-dot spacing were evaluated for each pattern and Fig.5(a,b) show the corresponding results, averaged over three replicates of the experiments.

Figure 5: Distribution of (a) the microdot diameter and of (b) the dot-to-dot spacing, both as a function of the thickness of the starting fibre. The data fitted well a second degree polynomial regression curve. The values were averaged over three replicates of the experiments.


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The experimental data in Fig. 5(a,b) show clearly the high repeatability of the process for starting fibres up to about 30 µm thick, with both microdot diameter and dot-to-dot spacing that follow a second degree polynomial behaviour, as well shown by the regression curves in both figures. These results demonstrate the ability of the breakup process to produce ordered microdots with controlled diameter and dot-to-dot spacing simply by varying the fibre thickness through the versatile PES process. Moreover, it is noteworthy that PES allows us to vary such thickness in a continuous manner, thus providing microdot arrays with a controlled variation of the diameter and spacing (see for example Fig.3(g,h)). The selective live cell adhesion The microdot patterns were used for guiding the adhesion of live fibroblast cells (NIH/3T3) in vitro. Figure 6 shows the optical microscope images of typical in-vitro cells adhering on microdot patterns belonging to two different ranges of dot-to-dot spacing: around 50 µm in (a,b) and around 100 µm in (c,d).

Figure 6: Optical microscope images of cells adhering on microdot patterns belonging to two typical dot-to-dot spacing values: (a,b) around 50 µm; (c,d) around 100 µm. The images were recorded after 24 h incubation. The dashed circles indicate the boundary of the microdot underneath.


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The cells clearly tended to colonize the PLGA microdots due to its higher cytophilic nature, compared to the surrounding PFPE coating. In fact, thanks to the direct printing of the microdots without any ink manipulation with additives for reducing the viscosity, the cytophilic nature of the PLGA is well preserved. It is worth noting that the partial wetting surface worked simultaneously as a trigger for the breakup process but also for guiding the in vitro live cell adhesion onto the PLGA microdots. Figure 6(a,b) shows clearly that the cells cultured onto the pattern with 50 µm spacing colonized the dots as well as the intermediate space by developing an elongated shape, thus producing a sort of bridging effect, as labelled in the pictures. Conversely, the cells cultured onto the pattern with larger spacing (see Fig.6(c,d)), clearly colonized the microdots and were forced to live onto separated islands with consequent morphology constraints dictated by the PLGA microdots underneath. Moreover, it is worth noting that the satellite microdots, due to their subcellular size, are irrelevant for the cell patterning applications. Valuable works in literature47-48 have demonstrated that cell shape and morphology can govern life and death of live cells. The parallel formation of the microdots, instead of serial, makes the breakup technique very attractive for high throughput studies on cell behaviour under controlled morphology constraints and under cell-to-cell interactions at predetermined ranges of action. In fact, the technique is relatively easy to accomplish into a biological laboratory, thanks to its freedom from expensive lithographic equipment as well as from clogging-limited jet printing.

4. Conclusion We presented here the ability of fibre breakup to produce ordered microdots of high viscous PLGA microdots. The key aspect is the possibility of producing such microdots by an easy to implement technique that makes use of the Plateau-Rayleigh instability occurring onto a partially wetting surface for a melting polymer fibre with micrometric dimension. The nozzle-free character of the PES deposition allows us to produce microscale fibres made of polymers with viscosities well ACS Paragon Plus Environment

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beyond the 20 mPa s limit encountered by conventional inkjet printing technologies due to clogging problems. The results on selective cell adhesion demonstrate clearly a direct exploitation of the technique where the cytophilic nature of PLGA is well preserved by avoiding detrimental manipulation of the ink with additives aimed at reducing the ink viscosity. We believe that, in general, the breakup would open the route to facile patterning of microdots for all of those fields where high viscous polymers are highly desired but poorly printable through the technologies available nowadays in literature as well as in the market.

Supporting Information Additional polymer inks were employed in order to demonstrate the wide applicability of the breakup technique: D,L-poly(lactic acid) (PDLLA, average Mw 10,000, Sigma Aldrich) dissolved in N-Methyl-2-pyrrolydone (NMP, Sigma Aldrich) at a concentration of 80% wt/wt and polystyrene (PS, average Mw 35,000, Sigma Aldrich) dissolved in anisole (Sigma Aldrich) at a concentration of 60% wt/wt. The Supporting Movie S1 shows the typical breakup evolution encountered by a multishape PLGA fibre deposited by PES onto the partially wetting surface, with a thickness around 20 µm. The Supporting Movie S2 shows the behaviour of a simple drop of PLGA (volume around 2 µL) during phase transition.

References 1. Kelley, T. W. ; Baude, P. F. ; Gerlach, C. ; Ender, D. E. ; Muyres, D. ; Haase, M. A. ; Vogel D. E. ; Theiss, S. D. Recent progress in organic electronics: materials, devices, and processes, Chem. Mater. 2004, 16, 4413–4422. 2. Black, C. T. ; Ruiz, R. ; Breyta, G. ; Cheng, J. Y. ; Colburn, M. E. ; Guarini, K. W. ; Kim, H. C.; Zhang, Y.; Polymer self assembly in semiconductor microelectronics, IBM J. Res. Dev. 2007, 55, 605–633.


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