Electrical Field Guided Electrospray Deposition for Production of

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Electrical Field Guided Electrospray Deposition for Production of Gradient Particle Patterns Wei-Cheng Yan, Jingwei Xie, and Chi-Hwa Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02914 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Electrical Field Guided Electrospray Deposition for Production of Gradient Particle Patterns Wei-Cheng Yan§,, Jingwei Xie*,‡, and Chi-Hwa Wang*,§ §

Department of Chemical and Biomolecular Engineering, National University of Singapore,

Singapore, 117585 ‡

Department of Surgery-Transplant and Mary & Dick Holland Regenerative Medicine Program,

University of Nebraska Medical Center, Omaha, Nebraska 68198, United States *

To whom correspondence should be addressed. E-mails: (Jingwei Xie) [email protected]

(Chi-Hwa Wang) [email protected]

ABSTRACT: Our previous work demonstrated the uniform particle pattern formation on the substrates using electrical field guided electrospray deposition. In this work, we reported for the first time the fabrication of gradient particle patterns on glass slides using an additional point, line or bar electrode based on our previous electrospray deposition configuration. We also demonstrated that the polydimethylsiloxane (PDMS) coating could result in the formation of uniform particle patterns instead of gradient particle patterns on glass slides using the same experimental setup. Meanwhile, we investigated the effect of experimental configurations on the gradient particle pattern formation by computational simulation. The simulation results are in line with experimental observations. The formation of gradient particle patterns was ascribed to the gradient of electric field and the corresponding focusing effect. Cell patterns can be formed on the particle patterns deposited on PDMS-coated glass slides. The formed particle patterns hold great promise for high-throughput screening of biomaterials-cell interactions and sensing.

KEYWORDS: electrospray deposition, electrical field, gradient particle pattern, cell pattern, cell-material interaction. †

Present address for Wei-Cheng Yan: School of Chemistry and Chemical Engineering,

Jiangsu University, Zhenjiang 212013, China

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1. INTRODUCTION Electrospray, also called electrohydrodynamic atomization, is an enabling technology that can produce particles with various morphologies and sizes, cell-laden microcapsules, functional films, and surface patterns.1-3 Among them, electrospray deposition has been widely examined for production of functional films and surface patterns.1,4-7 Poly(vinylidene fluoride) film fabrication by electrospray deposition was examined based on a droplet evaporation model.8 PLGA films encapsulated with anticancer drug paclitaxel was generated by a combination of electrospray deposition and spin coating for sustained release applications.9 Different electrospray droplet sources including liquid metal ion sources were used for thin film deposition in applications including nuclear instruments, solar cells, fuel cells, lithium batteries, electronic devices, and biotechnology.10 Early studies examined the use of a mask for producing the surface patterns using electrospray deposition. Single- and multi-component microarrays of biologically active substances were prepared on any marginally conductive substrate including membrane, wet glass, and semiconductor using a dielectric mask with an array of holes during electrospray deposition.11-16 In these studies, the electric field is a supraposition of an attracting electric field projecting through the holes and a repelling electric field of the mask charged with deposited particles. At the initial stage of experiment, most of the particles may be deposited on the surface of masks, which led to an inefficient patterning formation. The patterning of nano-sized hydroxyapatite and silicon-doped hydroxyapatite on both metallic (titanium) and non-metallic surfaces (glass) were also demonstrated using a gold mesh mask during electrospray deposition.17,18 Similarly, micropatterns of silica nanoparticles were formed through a stencil mask during the electrospray deposition.15 Based on the similar principle, silver nanoparticles were deposited on the patterned silica nanoparticles, resulting in strong surface-enhanced Raman scattering effects.19 Additional electric field (e.g., applying voltage to the mask or the focusing lens, ions deposition to the mask) was added to the electrospray deposition experimental setup for the precise control of deposition of various materials. A series of electrostatic lenses were used for the fabrication of the thin films of functional organic materials during electrospray deposition using a moving target.20 In separate studies, similar to the electrostatic lens, an ion-induced parallel-focusing approach was developed to pattern silver nanoparticles on both conducting (p2 ACS Paragon Plus Environment

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type silicon) and non-conducting (silica) surfaces through accumulating positive nitrogen ions generated by a corona discharger on the surface of the mask to form “focusing effect”.21 Based on the same setup, the parallel patterning of nanoparticles (Ag, Cu, ZnO, PS) and protein (IgG) with 20-420 nm features on various substrates (glass, PET film) was also fabricated.22,23 Interestingly, extending electrospraying deposition time could result in the generation of 3D assembly of nanoparticles with ordered arrays.24,25 In our previous study, a simpler approach of generate “electrostatic focusing effect” was developed by directly applying a high voltage to the mask.26 We demonstrated the fabrication of biodegradable and biocompatible polymeric particle patterns and subsequent formation of cell microarrays. A mathematical model was also developed to track the trajectories of particle and to better understand electrostatic focusing effect in the process of electrospray deposition on the substrate.27 In this work, we report a novel particle pattern with size gradient fabricated by electrical field guided electrospray deposition with an additional grounded point, line or bar electrode, which were unrealized previously.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly (D, L-lactic-co-glycolic acid) (PLGA) with L:G molar ratio of 50:50 (MW = 90,000–120,000), Poly(ε-caprolactone) (PCL) (MW = 14,000), coumarin 6 for labeling particles, and fluorescein diacetate (FDA) for a labeling living cells were purchased from Sigma Aldrich (St. Louis, MO). Dichloromethane (DCM) was purchased from Tedia Company (Fairfield, OH). Dulbecco's modified Eagle's medium (DMEM), fetal bovine serum (FBS), penicillin and streptomycin were purchased from Invitrogen. Sylgard 184 silicone elastomer kit was bought from Dow Corning (Auburn, MI). 2.2. Gradient Particle Pattern Formation. The experimental setup for the producing gradient particle patterns was shown in Figure 1A. Briefly, A DCM solution containing 5% of PLGA or PCL with and without 0.1% coumarin 6 was used for producing gradient particle patterns on the substrates (1 mm thick glass slides and 0.17 mm thick glass coverslips) by electrospray deposition through a mask (a stainless steel mesh 100 wire per inch and diameter of the wire: 100 microns). In this setup, an additional electrical potential source was applied to the mask mesh. Another electrode (e.g., point electrode, line electrode, or bar electrode) was placed under the substrates, which normally served as a grounding electrode. Stainless steel wires (100 3 ACS Paragon Plus Environment


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microns in diameter), aluminum foil strips (500 microns in width), and tips of a 27 Gauge needle were used as line, bar, and point electrodes in this study. The nozzle (a 27 Gauge needle) was applied a voltage (as compared to the ground) of around 12.0 kV. The distance between the nozzle and the mask was around 10-15 cm. The voltage applied to the mesh was around 3-5 kV. The humidity is around 26-50%. 2.3. Uniform Particle Pattern Formation. The experimental setup and condition are similar to the setup and condition used for producing gradient particle patterns except that the substrates were coated with a podimethylsiloxane (PDMS) film (Figure 1B). The PDMS coating on the glass slides/cover slips was formed by spin coating of mixed two parts of Sylgard 184 silicone elastomer kit at a ratio of 10:1 and curing at 60 oC for 20 min. The coating thickness was controlled by the rotating speed of the spin coater. 2.4. Morphology Characterization. The particle patterns after fabrication were observed by a fluorescence microscope. Fluorescent images were taken using a QICAM Fast Cooled Mono 12-bit camera (Q Imaging, Burnaby, BC, Canada) attached to an Olympus microscope with OCapture 2.90.1 (Olympus, Tokyo, Japan). The morphology of particle patterns was also observed using field emission scanning electron microscopy (FSEM) (200 Nanolab, FEI, Oregon). FSEM required an ion coating with platinum for 40 s in a vacuum at a current intensity of 40 mA after preparing the sample on metallic studs with double-sided conductive tape. The accelerating voltage was 15 kV during scanning. 2.5. Cell Pattern Formation. NIH3T3 cells were cultured in DMEM supplemented with 10% FBS and 1% gentamycin/streptomycin at 37 °C in an atmosphere of 95% air/5% CO2. Cell culture medium was replaced every 2 days. Cells were transferred to patterned glass cover slips at a density of 1 × 105 cells/ml. Cells were incubated for around 3 days, gently rinsed with culture media to remove loosely adherent cells, and imaged under phase-contrast optical microscope. Living cells were stained with FDA following the standard procedures. Subsequently, samples were observed by a fluorescence microscope described above.

3. RESULTS 3.1. Simulation of Electrical Field. In the previous works, we have demonstrated the fabrication of uniform particle patterns by applying voltage on the mask and grounding the collection substrate.26,27 In this work, the electric field distributions for the cases that using 4 ACS Paragon Plus Environment

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additional ground electrode under the collection substrates were simulated and compared with that using grounded collection substrate. The electric field was simulated by solving Poisson’s equation:

∇ 2Φ =

∂ 2Φ ∂ 2Φ ∂ 2Φ + + =0 ∂x 2 ∂y 2 ∂z 2

(1)

E = -∇Ф

(2)

A large-scale computational domain (Figure S1A) was created based on the experimental operating conditions to simulate the electric field distribution first. Subsequently, the simulation of electric field was carried out in a region with dimensions of 10 mm× 10 mm × 10 mm including mask and ground electrode (Figure S1B-C). Figure 2 shows the simulated electric potentials and electric field line distributions with various electrode configurations. As shown in Figure 2A, gradient distribution of electric potential from the mask to the bottom of the grounded substrate can be observed while uniform distributions along the Y- direction were formed when the substrate was grounded. The focusing electric field lines pointing to the substrate exhibited uniform distributions at each mesh hole, suggesting the formation of uniform patterns when particles deposited on the substrate since this is an electric force dominated process. However, a non-uniform distribution of electric potential along the Y-coordinate was formed when using bar (Figure 2B), line (Figure 2C-D) or point (Figure 2E) ground electrode under substrate. Strongly convergent electric field lines can be seen at the region near the grounded electrode while extent of the focusing effect was alleviated with the increase in distance away from the bar electrode (Figure 2B) or line electrode (Figure 2C). Electrosprayed particles mostly deposit along the electric field lines since this is an electric force dominated process;26,31 therefore, gradient particle patterns along the direction away from bar or line electrode may be formed under the same condition. For the case with point ground electrode, the electric field streamlines released from the top surface all point to the grounded electrode (Figure 2E). Similarly, the simulated electric field implies that particle patterns with gradient size along radial direction from the center to the side of substrate are possible (Figure 2E). Generally, the focusing effect is stronger in the vicinity of mask near grounded electrode (bar, line or point electrode), indicating that smaller pattern size would be formed in this region. In addition, the electric field for the case with a coating layer of PDMS on glass slide shown in Figure 1B was simulated. Although the electric potential distribution (Figure 2D) displayed 5 ACS Paragon Plus Environment


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similar trend with that shown in Figure 2C, the focusing effects are much more uniform along Y-direction, indicating that particle patterns with more uniform size may be obtained. 3.2. Fabrication of Particle Patterns. To demonstrate the formation of gradient particle patterns, we performed electrospraying of a DCM solution containing 5% PCL and 0.1% coumarin 6 using experimental setup as shown in Figure 1A. The line electrode, bar electrode, or point electrode placed under the glass slide was grounded. It is observed that the size of particle patterns increased gradually starting from the line electrode to the right side (Figure 3A), confirming the deduction from simulated electric field distribution (Figure 2B).

In

contrast, for bar electrode, particles patterns were uniform on the surface above the bar electrode. Akin to line electrode, further away from bar electrode to the right side or left side, gradient particle patterns were formed (Figure 3B). The particle patterns in size gradient along the radial direction were formed further away from the point electrode (Figure 3C). Similarly, uniform particle patterns were formed on the surface above the point electrode (Figure 3D). To demonstrate the formation of particle patterns in uniform size, we performed electrospraying of a DCM solution containing 5% PCL and 0.1% coumarin 6 using experimental setup as shown in Figure 1B. In this case, the glass slides/cover slips were coated with PDMS. Similar to our previous studies, the pattern size can be readily controlled by adjusting the voltages applied to the mask. It is seen that the pattern size ranged from several microns to one hundred microns when the voltages applied to the mask varied from 5 kV to 1 kV (Figure 4). Interestingly, extending the deposition time could result in different three-dimensional structures. If the deposition time was 5 min, patterns made of particle clusters were formed (Figure 5A,B). By extending the deposition time to 30 min, various three-dimensional structures were formed including rod, needle, or tree like structures due to the electrostatic focusing effect (Figure 5CH). The mechanism for the formation of different structures was not clear yet. 3.3. Simulation of Particle Pattern Formation. In addition to the experimental study, simulation of particle pattern formation was performed under the conditions shown in Figures 1A-1B. Particle patterns formed by using different grounding electrode configurations were also simulated. The particle motion was governed by Newton’s second law with consideration of drag force, electric force, gravity, buoyancy and columbic repulsion (Eq. 3). The simulation was carried out based on 3D Lagrangian model. Air was used as the surrounding fluid. The material properties used in simulation, configuration of 6 ACS Paragon Plus Environment

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the mask, boundary conditions, and operating conditions were summarized in Table 1. The maximum charge on the droplet was estimated by Rayleigh limit of charge.27-29 Droplet size was estimated according to Ganán-Calvo’s work30 and then the magnitude order of particle size was estimated through mass conservation based on the droplet size. Detailed information can be seen in the Supporting Information. ρp (

4π 3 du D +F E +F g +F B +F q ) =F p 3 dt

(3)

Gradient patterns along right side of line electrode were obtained from the simulation (Figure 6A) when glass slide with additional line ground electrode was used. The size of particle patterns near the line electrode was smaller than that far from it which is consistent with the observation from experimental results (Figure 3A). Particles may have high chance to overlap at the limited area and form a 3D structure when the focusing effect is strong. In the region away from the grounding electrode, particles tend to scatter at a larger area due to weaker focusing effect (Figure 2C). However, more uniform particle patterns near the grounding electrode were obtained when using PDMS coated glass slide as substrate (Figure 6B). This simulation results were in line with the finding in experiment (Figure 4), also confirming the deduction according to the electric field line distributions (Figure 2D). The simulated particle patterns on the glass slide using point or plate grounding electrode were shown in Figure 6C-D. The size of the pattern near the center of the substrate is smaller than that far from the point electrode due to the difference in focusing effect. Similarly, instead of overlapping, the particles scatter to larger area forming larger pattern size at the position far from the center.

Gradient particle patterns along radial direction of point electrode were

observed, which is consistent with the results shown in Figure 3C-D. Pretty uniform particle patterns can also be obtained by grounding the bottom plate of collection substrate (Figure 6D). 3.4. Cell Pattern Formation. To demonstrate the cell pattern formation, the glass slides were coated with PDMS with different thicknesses. Owing to the better cell adhesion on the surface of PLGA particles compared with that of PCL particles, we chose PLGA particles for cell pattern formation. For the thick coating (~100 µm), uniform PLGA particle patterns normally were formed on the glass slides/cover slips. For the thin coating (~10 µm), the gradient PLGA particle patterns could be formed on the glass slides/cover slips. Also, the particle disposition efficiency was lower compared to the substrate without PDMS coating. NIH3T3 cells were cultured on the particle patterns formed on the PDMS coated glass slides/cover slips for 3 7 ACS Paragon Plus Environment


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days and stained with FDA. It is observed that cell patterns were nicely formed on the particle patterns (Figure 7). Cell patterns with uniform size were formed on the particle patterns on the glass slides/cover slips with thick PDMS coatings (Figure 7A-D). In addition, the size of particle patterns also determined the size of cell patterns. In contrast, gradient cell patterns were seen on the glass slides/cover slips with thin PDMS coatings (Figure 7E,F). The intrinsic high surface hydrophobicity of PDMS causes very poor cell adhesion. Therefore, cells prefer to attach to the particle patterns and proliferate on the particle patterns.

4. DISCUSSION Electrospray deposition has been used to generate surface patterns with different materials including protein, polymers, inorganic particles and metal particles.1 Only uniform patterns were reported in the literature by electrospray deposition through a mask. This work represents the first study of the fabrication of particle patterns in size gradient using electrospray deposition with additional voltages applied to the mask and grounding line, bar, or point electrode placed under the substrate (glass slides/cover slips). As found in this study, stronger focusing effect would present in the vicinity of ground electrode. Different types of gradient patterns could be formed by controlling the configuration of ground electrode. In addition, the focusing effect would guide the particles to deposit on limited area which may cause the overlap and accumulation of particles in Z-direction and bring it into forming a 3D rod or needle structure. Under a non-uniform distribution of focusing effect, gradient pattern along three dimensions may be also possible. Although we used one condition for electrospraying in this work, the experimental parameters can be varied to tailor the shape, size, and morphology of particles based on our previous studies.32,33 Different structures of particles can be obtained using different nozzles such as co-axial nozzle, side-by-side nozzle etc.1 The particle shape, size and morphology may affect the deposition patterns, which will be investigated in our future studies. Previous studies used contact printing to form patterns with different sizes and examined the effect of pattern size on the cellular response.34-37 Therefore, the particle patterns with gradient size could provide a useful substrate to examine the geometry confinement effect on the cellular behavior/response such as stem cell differentiation. Material arrays/patterns could serve as highthroughput platform for screening purposes.38,39 The particle patterns formed in the current study 8 ACS Paragon Plus Environment

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were made of either PCL or PLGA. To make a high-throughput substrate, different materials could be deposited to one PDMS-coated glass slide/cover slip. Then, cells can be seeded to such a substrate for examining the interaction between cells and deposited materials in terms of adhesion, proliferation, and differentiation. Array or pattern-based sensing using particles has been examined for diagnostics.40 The particle patterns with gradient sizes developed in this study could be used as high-throughput platform to screen assays for diagnostic purpose as well.

5. CONCLUSIONS We have demonstrated a simple and novel approach for fabrication of particle patterns with size gradient using electric field guided electrospray deposition and additional grounded line, bar or point electrode placed under the substrate (e.g., glass slides, cover slips). By designing the configuration of ground electrode, gradient pattern with desired distribution is possible. Simply thick coating of PDMS on the glass slides/cover slips could result in uniform particle patterns. We also demonstrated the cell pattern formation on the particle patterns. These particle patterns could potentially be useful for screening biomaterials based on the cell-material interaction and high-throughput sensing. Further, the formed cell patterns could be used for high throughput of drug screening.

Acknowledgements This project was supported by A*STAR and National University of Singapore under the project/grant numbers APG2013/40A (A*STAR BMRC Strategic Positioning Fund, R279–000487–305) and R261–509-001–646 (NUS FOE 3D Printing Initiatives).

AUTHOR INFORMATION Corresponding Authors *

E-mail address: (Jingwei Xie) [email protected], (Chi-Hwa Wang) [email protected].

Supporting Information Simulation method and mathematical model descriptions; Table representing model governing equations; Figures representing large scale simulation domains, details of small scale simulation

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domain, structure of mask, coating layer and collecting substrate, simulated electric field distributions at large simulation domain and small simulation domain, particle trajectory, and particle deposition patterns on substrate.

Notes The authors declare no competing financial interest.

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(33) Xie, J.; Lim, L. K.; Phua, Y.; Hua, J.; Wang, C. H. Electrohydrodynamic atomization for biodegradable polymeric particle production. J. Colloid Interface Sci. 2006, 302, 103-112. (34) Guilak F.; Cohen D M, Estes B. T.; Gimble J. M.; Liedtke W.; Chen C. S. Control of stem cell fate by physical interactions with the extracellular matrix. Cell Stem Cell 2009, 5, 1726. (35) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Micropatterned surfaces for control of cell shape, position, and function. Biotechnol. Prog. 1998, 14, 356-363. (36) Chen, C. S.; Alonso, J. L.; Ostuni, E.; Whitesides, G. M.; Ingber, D. E. Cell shape provides global control of focal adhesion assembly. Biochem. Biophys. Res. Commun. 2003, 307, 355-361. (37) McBeath, R.; Pirone, D. M.; Nelson, C. M.; Bhadriraju, K.; Chen, C. S. Cell shape, cytoskeletal tension, and RhoA regulate stem cell lineage commitment. Dev. Cell 2004, 6, 483495. (38) Patel, A. K.; Tibbitt, M. W.; Celiz, A. D.; Davies, M. C.; Langer, R.; Denning, D.; Alexander, M. R.; Anderson, D. G. High throughput screening for discovery of materials that control stem cell fate. Curr. Opin. Solid State Mater. Sci. 2016, 20, 202-211. (39) Wang, L. S.; Duncan, B.; Tang, R.; Lee, Y. W.; Creran, B.; Elci, S. G.; Zhu, J.; Tonga, G. Y.; Doble, J.; Fessenden, M. Bayat, M.; Nonnenmann, S.; Vachet, R. W.; Rotello, V. M. Gradient and patterned protein films stabilized via nanoimprint lithography for engineered interactions with cells. ACS Appl. Mater. Interfaces 2017, 9, 42-46. (40) Le, N. D. B.; Yazdani, M.; Rotello, V. M. Array-based sensing using nanoparticles: an alternative approach for cancer diagnostics. Nanomedicine 2014, 9, 1487-1498.



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Figure 1. Schematic illustrating electrical field guided electrospray deposition. (a) Glass slides/cover slips as a substrate for gradient particle pattern formation. (b) PDMS-coated glass slides/cover slips as a substrate for uniform particle pattern formation.


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Figure 2. Electric potential distribution and electric field line. (A) Glass slide as grounded substrate; (B) Glass slide as substrate with bar electrode; (C) Glass slide as substrate with line 15 ACS Paragon Plus Environment


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electrode; (D) PDMS coated glass slide as substrate with line electrode; (E) Glass slide as substrate with point electrode. Simulating conditions: nozzle voltage = 12kV, voltage applied to the mask = 5kV, distance between nozzle and mask = 10 cm. Color contour: Electric potential; Black/blue line: Electric field line.


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Figure 3. Fluorescent images showing particle patterns with size gradient fabricated using experimental setup shown in Figure 1A. PCL particles were labeled with coumarin 6. (A) Line electrode. Only the right side of Figure 1 was shown as the particle patterns are symmetric. (B) Bar electrode. (C, D) Point electrode. The center of (C) was shown in (D). Voltages applied to the mask = 5 kV. Deposition time = 5 min. There is no PDMS coating on the glass slides/cover slips in this case.



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Figure 4. Fluorescent images showing uniform PCL particle patterns deposited on the glass slides fabricated using experimental setup in Figure 1B. Voltages applied to the mask = (A) 1 kV and (B) 5 kV. Deposition time = 5 min. The glass slides/cover slips were coated with a PDMS film with ~100 µm thick.


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Figure 5. SEM image showing uniform PCL particle patterns deposited on the glass slides fabricated using experimental setup in Figure 1B. (A, B): deposition time = 5 min (C-H): deposition time = 30 min. (B, D, F, and H) are the corresponding magnified images of (A, C, E, 19 ACS Paragon Plus Environment


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and G). The voltage applied to the mask was 5 kV. The glass slides/cover slips were coated with a PDMS film with ~100 µm thick.


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Figure 6. Simulated particle patterns. (A) Glass slide as substrate with line electrode. (B) PDMS coated glass slide as substrate with line electrode. Only the right side of Figure 1 was shown as the particle patterns are symmetric. (C) Point ground electrode. (D) Grounded substrate.



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Figure 7. Cell patterns formation on the PLGA particle patterns deposited on PDMS coated glass slides. Living NIH3T3 cells were stained with FDA in green. (A-D) Glass slides with a thick PDMS coating as substrate. (E, F) Glass slides with a thin PDMS coating as substrate.


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Table 1. The parameters used in the simulation Description

Value

Particle number

20000

Vacuum permittivity ( ε 0 )

8.85×10-12 C2.N-1.m-2

Particles radius (Rp)

5.00×10-6 m

Particles density ( ρ p )

1145 kg/m3

Initial particle velocity

0.01 m/s

Fluid density ( ρ f )

1.225 kg/m3

Fluid viscosity ( µ )

1.725×10-5 kg/m.s

Relative permittivity of glass slide

10

Relative permittivity of PDMS coating layer

2.5

Thickness of coating layer

0.1 mm

Thickness of glass slide

1 mm

Hole size of the mask

0.15 mm × 0.15 mm

Nozzle to substrate distance

10-15 cm

Mask voltage

5 kV

Nozzle voltage

12 kV

Ground electrode

0 kV

Particle behavior on collection substrate

freeze

Particle behavior on the boundaries of

Pass through

small scale domain



Table of Content 257x138mm (96 x 96 DPI)

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Electrical Field Guided Electrospray Deposition for Production of

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