Maskless arrayed nano-fibre mats by bipolar pyro-electrospinning

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Applications of Polymer, Composite, and Coating Materials

Maskless arrayed nano-fibre mats by bipolar pyro-electrospinning Romina Rega, Oriella Gennari, Laura Mecozzi, Vito Pagliarulo, Alessia Bramanti, Pietro Ferraro, and Simonetta Grilli ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12513 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Applied Materials & Interfaces

Maskless arrayed nano-fibre mats by bipolar pyro-electrospinning Romina Rega1, Oriella Gennari1, Laura Mecozzi1, Vito Pagliarulo1, Alessia Bramanti1,2, Pietro Ferraro1,* and Simonetta Grilli1 1

National Research Council (CNR), Institute of Applied Sciences & Intelligent Systems (ISASI) ‘E. Caianiello’, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy 2 IRCCS Centro Neurolesi “Bonino-Pulejo”, Contrada Casazza SS113, 98124 Messina, Italy. *Corresponding Author: Pietro Ferraro, National Research Council (CNR), Institute of Applied Sciences & Intelligent Systems (ISASI) ‘E. Caianiello’, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy email address: [email protected]

Keywords: pyro-electrospinning; pyroelectric effect; arrayed mats; PDLLA; polymer fibres.

Abstract The numerous advantages of micro and nanostructures produced by electrospinning (ES) have stimulated enormous interest in this technology with potential application in several fields. However, ES has still some limitations in controlling the geometrical arrangement of the fibre mats so that expensive and time-consuming technologies are usually employed for producing ordered geometries. Here we present a technique that we call ‘bipolar pyro-electrospinning’ (b-PES) for generating ordered-arrays of fibre mats in a direct manner by using the bipolar pyroelectric field produced by a periodically poled lithium niobate crystal (PPLN). The b-PES is free from expensive electrodes, nozzles and masks because it makes use simply of the structured pyroelectric field produced by the PPLN crystal used as collector. The results show clearly the reliability of the technique in producing a wide variety of arrayed fibre mats that could find application in bioengineering or many others field. Preliminary results of live cells patterning under controlled geometrical constraints is also reported and discussed in order to show potential exploitation as scaffold in tissue engineering.

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1. INTRODUCTION In the last decade, the use of electrospinning (ES) for producing micro- and nano-scaled structures has attracted lots of interest for a wide variety of applications including tissue engineering, sensors and energy harvesting.1-4 ES produces continuous polymeric fibers starting from the polymer solution drawn from an appropriate nozzle under the action of an external electric field. The resulting fibers can have diameters ranging from 500 nm down to 50 nm1 and, thanks to the room temperature operation, a wide variety of polymeric materials can be electrospun, including polymer blends, sol–gels, composites and ceramics, with application to various fields2. The significant advantages of these fibers reside in their small diameter, large surface-area-to-volume ratio, high flexibility in surface functionality and superior mechanical properties, that provide numerous opportunities to improve the performance of existing technologies and devices. The polymer solution is drawn electrostatically into a fibre that initially is stable but, due to the millimetric diameter of the nozzle, is relatively thick. Typically the collector is positioned a few centimetres away and, therefore, the fibre thins progressively along the flight path. However, in the same time, the fibre tends to bend randomly and, due to its charged nature, it experiences selfrepulsion in the bending points leading to highly chaotic whippings3. The process leads to the deposition of the so called fibre mats4. These mats are characterized by small pore size, high porosity and high surface area and, therefore, find application in a wide variety of fields such as reinforcing fibres in composite materials5,6 or scaffolds in tissue engineering7, just to cite some. The ability to control truly the deposition of nanofibers by ES extends the range of applications to printed nano-scale electronics, multifunctional materials and conjugation of nanosized particles8– 10

. Other forms of nanofiber patterning would have several important implications in biomedical

fields in particular for tissue engineering11 and regenerative medicine12. In fact, patterned nanofibers can open up new studies into cell directed assembly and growth using nanofibrous tissue scaffolds and also potentially offers an innovative solution to the widely recognized cellular infiltration12. To ACS Paragon Plus Environment

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accommodate diversified application needs, developing facile and cost-effective methods for patterning of electrospun fibers in a defined manner clearly becomes a critical step. However, controlling the alignment of the electrospun fibre or the geometry of the fibre mat is a significant challenge. In fact, the external electric field that governs the fibre spinning induces inevitably a non-negligible charge into the fibre itself, causing its extremely chaotic whipping3,4. Many efforts have showcased the patterning capability of electrospun fibers in the forms of precise positioning13, orientation and stacking,14,15,19 selective deposition,20,21 stamp-based procedures,22 and post-ES microfabrication or treatment.16–18 Several attempts have been made to direct fiber patterning either by controlling the applied electric field or by making use of rotating drums as collector plates26-30 or usage a pre-patterned conductive collector to obtain patterned nanofibrous structures.19,20 Although these methods are practical and produce excellent results, the morphology of the fiber deposition highly depends on expensive and time-consuming fabrication of pre-patterned collectors, masks and molds, furthermore the material selection is limited and do not allow the direct positioning and control of the nanofiber layout pattern. Recently we have demonstrated the ability to control the deposition of single fibers by means of pyro-electrospinning (PES)33 for producing well ordered patterns at microscale, and by µ-pyroelectrospinning (µ-PES)34 for depositing spiraling microfibers. PES is an innovative approach that produces fine polymer fibres by making use of the electric field generated pyroelectrically onto the surface of a pyroelectric crystal (e.g. LiNbO3)35. PES is free from electrodes and high power voltage generators and draws the polymer fibre directly from the free meniscus of a mother drop avoiding micro-machined and clogging-limited nozzles. This peculiar configuration enables the rapid thinning of the fibre and allows us to avoid the typical whipping zone encountered in ES. Here we present a completely new configuration that we call ‘bipolar PES’ (b-PES) able to produce periodic arrays of fibre mats by using a periodically poled lithium niobate (PPLN) slab as driving crystal.36-38 The pyroelectric effect generates an array of surface charges with opposite sign, following the pattern of the reversed domains. This bipolar surface charge makes the electrospun ACS Paragon Plus Environment


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fibre to deposit only into the regions with negative charge, thus obtaining a periodic arrayed mat. We show the results obtained in case of two different polymers, the D,L-poly(lactic acid) (PDLLA), widely used for printing applications,39,40 and the polystyrene (PS). The b-PES extends the ability of PES to spin polymer fibres directly into arrayed mats by a simple and rapid procedure that is free from masks, electro conductive templates and high power voltage sources. The driving PPLN crystal can be re-used indefinitely without detrimental effects and the results show the reliability of the technique in case of different PPLN periods, avoiding repetitive, time consuming and expensive lithographic procedures, or complicated mechanical scanning. We show here the possibility to use these arrays for guiding the live cell adhesion under controlled geometrical constraints, thus opening the route to the development of a tool easily implementable into a biology laboratory for high throughput studies on cell morphology behaviour.

2. EXPERIMENTAL SECTION 2.1 Lithium niobate crystals. The LN crystals were bought from Crystal Technology Inc. in the form of both sides polished 500µm thick c-cut 3-inch wafers and were cut into square samples (2×2) cm2 sized by a standard diamond saw. The PPLN crystals were homemade by standard electric field poling onto photoresist patterned samples36-38 and consisted of a square array of hexagonal reversed domains with different periods: 50 µm; 200 µm; 300 µm; 400 µm. 2.2 Heating system. The heating system consisted of a commercial ITO coated heater purchased by Cell MicroControls (model HI-24p-BT-1xx). 2.3 Polymer solution. We purchased D,L-poly(lactic acid) (PDLLA) pellets (average Mw 10,000, PDI ≤1.1) from Sigma Aldrich (764620) and we dissolved them in N-methyl-2-pyrrolidone (NMP) at a concentration of 80%w/w by stirring at 70°C for 1 hour. Polystyrene (PS) was purchased from Sigma Aldrich in the form of powder and was dissolved at 60% w/w in anisole and stirred at 70 °C for 6 h.


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2.4 Experimental procedure. Figure 1(a) shows the optical microscope image of a typical PPLN crystal with a period of 200 µm, where the hexagonal regions correspond to the ferroelectric domains with reversed polarization. Figure 1(b-d) show the schematic view of the main steps occurring during the b-PES temporal evolution.

Figure 1: Schematic views of the mechanisms occurring during b-PES: (a) Optical microscope image of the PPLN crystal sample after the Electric Field Poling stage (b) First step of the procedure, where the heating plate is off, the crystal polarization is compensated by screening charges and the whole system is electrically at equilibrium; (c) charging step, where the heating plate is switched on, the magnitude of the crystal polarization reduces and the pre-existing screening charges become unbalanced leading to a non-negligible bipolar electric field on the crystal, the polymer gains a positive charge and starts deforming into a cone; (d) spinning step, where the cone leads to the formation of a thin charged fibre that is then attracted by the regions on the crystal with negative polarity (surrounding the hexagons). The inset in (d) shows a large view image of a typical arrayed fiber mat. Note that both the PPLN crystal and the heating plate are shown tilted in the schemes for the sake of clarity.

Figure 1(b) shows the initial state of the process. A drop of PDLLA solution with a volume of 0.5 µL was pipetted onto a base support consisting of a metallic tip fixed firmly onto a silicone substrate as in case of Ref. [34]. As already explained in Ref [34], the tip allows us to expose a thin meniscus to the electric field generated by the pyroelectric effect, thus favouring a rapid shrinking of the Taylor cone apex and, consequently, the formation of a thin fibre. The base drop faces the



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PPLN surface with the hexagonal areas exposing the c- ends of the spontaneous polarization, and viceversa for the areas outside the hexagons. A commercial heating plate (see details above) is positioned on the back face of the PPLN crystal. The PPLN crystal and the heater are depicted slightly tilted for the sake of clarity. We used a standard optical path, equipped with a high-speed CMOS camera, for recording the b-PES temporal evolution, analogously to the setup used in Ref [34]. Initially the heating plate is off and the PPLN crystal is at equilibrium, due to the surface charges that compensate the polarization charges of the crystal. In particular, the PPLN surface facing the base drop has positive charges compensating the negative polarization charge inside the hexagons and viceversa for the regions outside the hexagons. Figure 1(c) shows the charging state induced by the heater switched on at approximately 60 °C. The temperature variation induces the pyroelectric effect, namely the decrease of the net polarization inside the crystal. This produces a transient state in which the surface charges become excessive and no more compensated by the decreased polarization charge. Since the crystal is made of ferroelectric domains with reversed polarization, the regions inside the hexagons exhibit an excess of positive charge and, viceversa, the regions surrounding the hexagons exhibit an excess of negative charge. This effect generates a bipolar electric field pattern facing the PDLLA drop. Figure 1(d) shows the spinning state when the base support approached the PPLN crystal through a conventional vertical translation stage and, at a distance of approximately 80 µm, the intensity of the bipolar electric field pattern was able to charge the surface of the polymer drop.41-43 In particular, since the area surrounding the hexagons (charged negatively) is larger than the total area covered by the hexagons (charged negatively), the surface of the polymer drop is charged positively. The Coulomb repulsion of such positive charges was strong enough to overcome the surface tension of the liquid and to deform it into the so-called Taylor cone from which apex a thin fibre started to be ejected. The positively charged fibre was attracted electrostatically by the negative charge of the regions surrounding the hexagons, thus covering those areas continuously and forming an arrayed mat layer. ACS Paragon Plus Environment

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It is important to note that, in case of stationary base drop, the fibre typically generates an arrayed mat over a relatively large area with a diameter value around 1.5 mm, as shown by the optical microscope image in the inset of Fig.1(d). However, the b-PES can generate arrayed mats even over larger areas simply by translating laterally the base drop during the spinning step. The period of the arrayed mat corresponds to that of the hexagonal domains of the PPLN crystal. Therefore, the b-PES generates arrayed mats with different periods simply by using the driving PPLN crystal with the desired period. 2.5 Cell culture. We used NIH-3T3 mouse embryonic fibroblast cells for demonstrating the possibility of using the arrayed mats for cell patterning applications. The cells cultured details are reported in [34] 2.6 Immunofluorescence. See details in [51].

3. RESULTS AND DISCUSSION We used PPLN crystals with different periods (50 µm; 200 µm; 300 µm; 400 µm) in order to demonstrate the reliability of the b-PES for producing arrayed mats with different periods. The Supplementary Movies 1 and 2 show the typical formation of arrayed mats in case of two different PPLN crystals with periods of 200 µm and 50 µm, respectively. Only a narrow region appears in focus since we tilted the recording camera intentionally in order to visualize as best as possible the surface of the PPLN where the fibre deposited during b-PES. The Supplementary Movies 1,2 show clearly how the fibre covers the regions surrounding the hexagons, thus producing an arrayed mat, which holes correspond exactly to the hexagons of the PPLN crystal. The b-PES produces an arrayed mat, which follows with high fidelity the periodicity and the array distribution of the PPLN crystal driving the process. Figure 2 (a-d) show four significant frames of the process evolution recorded by the Supplementary Movie 1. The thin fiber is slightly out of focus and is indicated by the arrow. The hexagons are indicated by the schematic drawings with the indication of the positive excess charge ACS Paragon Plus Environment


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appearing during heating. The circle indicates the area where the fiber touches the collector surface. Figure 2(d) shows the completion of the hexagon contour writing, marked with the dotted circle. The process is relatively quick and covers a region 1,8 mm2 large in approximately 100 seconds. Figures 2 (e-m) show the optical microscope images of the resulting arrayed mats of PDLLA produced by PPLN crystals with different periods: 50 µm in (e-g); 200 µm in (h-j), 300 µm in (l); 400 µm in (m). The image in Fig.2(i) was recorded at the edge of the patterned area after stopping the process prior to completion of the whole array area.


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Figure 2. (a-d) Camera side view of PDLLA deposition in case of PPNL with period of 200 um; optical microscope images of PDLLA arrayed mats on PPLN with periods of (e-g) 50µm, (h-j) 200µm, (l) 300 µm, (m) 400 µm; (k) optical microscope image of an arrayed mat made of PS. The inset in (m) shows a typical arrayed mat of PDLLA deposited onto a free-standing sheet of PS.

The thickness of the fiber mat increases for larger periods. The magnified views in Fig.2(f,g) show clearly that the fiber surrounds the hexagonal regions (indicated by the red dotted line) few times compared to the case of larger periods (Fig.2(h-m)) where the fiber covers the larger regions ACS Paragon Plus Environment


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between the hexagons, thus producing a thicker arrayed mat (see Fig.2(h-m)). In other words, larger periods produce arrayed mats with voids separated by a thicker fibrous mat compared to the case of shorter periods, thus providing a relatively wide range of applications, depending on the ratio between voids and fibrous areas required. Due to the constant distance between the base drop and the driving crystal, the diameter of the fiber appears relatively constant during the process. The bPES was applied also to a solution of PS (see the Experimental Section for details) and Fig.2(k) shows the corresponding arrayed mat at a period of 200 µm which appears affected by pattern defects consisting of broken fibers deposited on hexagonal regions. We believe that these defects are related to the lower entanglement number exhibited by PS compared to PDLLA at the operation conditions used in this work, in agreement with the studies reported in [44]. All of these images demonstrate the reliability of the technique in case of different periods and different polymer materials. Additional experiments were performed by interposing a solid thin sheet of PS between the PPLN driving crystal and the base drop. The inset in Fig.2(m) shows the corresponding pattern, demonstrating the ability of b-PES to generate such patterns also directly onto flexible free-standing sheets. The ability to modulate the cellular behaviour, such as adhesion and morphology, has attracting lots of interest for cell patterning applications.19,45-49 Here we demonstrate the possibility of using such arrayed mats for controlling the adhesion of live fibroblast cells in vitro (see the Experimental Section for details about the cells) according to the periodic array geometry. A PPLN crystal patterned with an arrayed mat of PDLLA was placed into a Petri dish and incubated with the cells at a density of 1×105 cells/mL for 24 h. The same cell density was seeded onto a conventional glass slide as control. Figure 3(a,c) show the optical microscope images of the live cells adhering onto the control surface and on the arrayed mat of PDLLA after 24 h incubation, under bright field observation. The PDLLA is known to have cytophobic properties50 while LN has been recently demonstrated to be biocompatible and cytophilic.51 As a direct consequence the cells appear well spread without a preferential orientation in the first case, while a significant round-like ACS Paragon Plus Environment

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orientation of the cells is observed in the second case where the cells appear to adhere into the open regions and align along the circular edge of the holes. The morphology of the cells was investigated by immunofluorescence analysis (see the Experimental Section for details). Figure 3(b,d) show the typical fluorescence microscope images of the cells incubated on the control surface and on the arrayed mat, respectively. These images show clearly how the cytoskeleton of the cells (stained red) adhering on the control substrate spread randomly over the surface without a preferential orientation, while the cells incubated on the arrayed mats exhibit stress fibres with a significant round-like orientation along the direction of the hole edge.

Figure 3. (a,c) Optical microscope images of fibroblast cells after 24 h incubation on a glass slide (control) and on an arrayed mat of PDLLA; (b,d) immunofluorescence images of the cells after 48 h incubation on the glass slide (control) and on the arrayed mat of PDLLA.

The b-PES technique would be useful for cell patterning applications, especially when studying the effects on cell physiological behaviour when subjected to a round-like morphology induced by a two-dimensional scaffold. Moreover, the technique is relatively easy to accomplish and costeffective for a non-specialized laboratory and we believe that biologists would use the b-PES as a



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rapid and easy tool for implementing high throughput microarray chips for cell patterning applications regarding the effects of geometrical constraints on cell functionalities.

4. CONCLUSIONS In conclusion, we present here the b-PES as a rapid and easy technique for producing periodic arrays of polymer mats at microscale. The b-PES derives directly from the PES technique developed recently by the same authors with the innovative ability of producing an arrayed mat in a direct manner, avoiding masks and patterned electrodes. The key factor is the use of a PPLN crystal in place of a one-domain LN crystal, as in case of the PES. The PPLN crystal is re-usable indefinitely and produces a bipolar electric field pattern simply under a uniform heating, avoiding expensive, structured and high power electrodes and generators. The arrayed mat reproduces with high fidelity the array distribution of the reversed domains in the PPLN crystal and the reliability of the technique is demonstrated for arrays with periods ranging from 50 µm up to 400 µm, producing arrayed mates with different thickness. The arrayed mats appear clearly useful for cell patterning applications. We believe that b-PES would be implemented easily in a biology laboratory for developing a rapid tool for high throughput studies on cell functionalities under controlled geometrical constraints. Even studies at different cell agglomeration scales would be possible thanks to the ability to vary easily the performance of the mat by changing the PPLN period, with potential applications in the field of bioengineering.

■ ASSOCIATED CONTENT * Supporting Information The Supporting Information is available free of charge on the ACS Publications website •

Supplementary Video 1 in SI Appendix shows the typical formation of arrays of PDLLA fibre mats by bipolar PES in case of PPLN crystals with periods of 200 µm


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•

Supplementary Video 2 in SI Appendix shows the typical formation of arrays of PDLLA fibre mats by bipolar PES in case of PPLN crystals with periods 50 µm

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FINAL TOP VIEW

SIDE VIEW

Heater

PPLN

PDLA polymer solution


+ + +-

Maskless arrayed nano-fibre mats by bipolar pyro-electrospinning

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