Facile Fabrication of Composite Electrospun Nanofibrous Matrices of

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Facile fabrication of composite electrospun nano-fibrous matrices of poly(#-caprolactone)–silica based Pickering emulsion Archana Samanta, Sonam Takkar, Ritu Kulshreshtha, Bhanu Nandan, and Rajiv K. Srivastava Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02119 • Publication Date (Web): 23 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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Facile fabrication of composite electrospun nano-fibrous matrices of poly(εcaprolactone)–silica based Pickering emulsion Archana Samantaa, Sonam Takkarb, Ritu Kulshreshthab, Bhanu Nandana, Rajiv K. Srivastavaa* a

Department of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas, New

Delhi 110016 INDIA b

Department of Biochemical Engineering and Biotechnology, Indian Institute of

Technology Delhi, Hauz Khas, New Delhi 110016 INDIA *Corresponding Author Email: [email protected] Phone: 0091-11-26596680 Fax: 0091-11-26581103

Abstract Functionalized matrices have been sought for their application in sensors, filtration, energy storage, catalysis and tissue engineering. We report formation of an inorganic-organic composite matrix based on poly(ε-caprolactone) (PCL) functionalized with hydrophobically modified silica (m-silica) fabricated with reduced organic solvent usage. The matrix was obtained via electrospinning of a water-in-oil emulsion of PCL that was stabilized by judicious choice of m-silica as a Pickering agent resulting into an emulsifier free matrix. Inclusion of m-silica in PCL matrix resulted in enhancing tensile properties and cell proliferation efficiency. The electrospun composite matrix was free from any emulsifier or template polymer thus any abrupt loss in mechanical properties was prevented when the matrix was subjected to aqueous conditions. The inorganic-organic biodegradable composite matrices thus produced using an emulsifier free emulsion find applications in tissue 1 ACS Paragon Plus Environment

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engineering and may further be evaluated for other areas including selective sorption and separation.

Introduction

Pickering emulsions uses solid particles instead of conventional amphiphilic emulsifiers to kinetically stabilize the oil-water interfaces1. Pickering stabilizers offer better emulsion stability with less quantity of the stabilizers due to their irreversible adsorption at the oilwater interface thus less tendency for leaching out and low toxicity as compared to conventional amphiphilic emulsifiers2. Stabilizers in form of particulates like silica, titania3, clays4–7, starch8–11 and microgels12 have successfully been used as effective Pickering stabilizers. All of these stabilizers needs to fulfil the basic requirement of “wettability by both the phases” of the emulsion that results in their efficient adsorption at the interface and stabilization of the emulsion. Wettability of the Pickering stabilizers at the oil-water interface plays an important role in the stability of emulsion. Particles preferentially wetted by only one phase, either oil or water, remain dispersed in that phase and are unable to adequately deposit at the interface13. In that case it is difficult to obtain a stable emulsion. In general, hydrophilic particles e.g. starch, silica and titania stabilize oil-in-water emulsions and hydrophobic particles e.g. modified clay, rice starch stabilize water-in-oil emulsions14–18. As compared to conventional emulsifiers, Pickering stabilizers are required in very low amount to form a stable emulsion due to their irreversible adsorption at the interface19. Pickering emulsion, being surfactant free, are considered suitable over conventional emulsions and are seemly used in cosmetics, food and pharmaceutical to prevent irritations or allergies caused by most surfactants20. Polymerization of Pickering emulsions has also effectively been used to synthesize functionalized composite matrices and particles with features including drug encapsulation, hydrocolloid food items and others21–23. 2 ACS Paragon Plus Environment

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Porous matrices are desired for their application in tissue engineering, filtration and separation etc. and the technique of electrospinning to produce such matrices has gained immense attention over last decade24,25. Ease of operation and control over fiber fineness makes electrospinning a popular choice to produce micro- and nano-fibrous matrices which are also reported to mimic the extra-cellular matrix environment26,27. Electrospinning is generally carried out using a polymeric solution and use of organic solvents in the process is undesirous not only from environmental point of view but also for the solvent remnants in the matrices thus unsuitability of such matrices for tissue engineering. Emulsion electrospinning has recently been reported to overcome this problem to an extent28. However, matrix contamination with emulsifier(s) and lowering of mechanical properties of electrospun matrices on exposure to an aqueous environment due to leaching or removal of the amphiphilic emulsifier or template polymer are major drawbacks of this process29. Use of Pickering emulsions to prepare fibrous matrices using electrospinning has scantly been researched and the advantage of producing surfactant free, composite matrices using Pickering emulsions has not been exploited thoroughly. The aim of present study was to produce composite electrospun fibrous matrices using a silica stabilized Pickering emulsion of a biodegradable and biocompatible hydrophobic polymer, poly(ε-caprolactone) (PCL). PCL has extensively been used in tissue engineering while silica was also reported to be a biocompatible material, largely used in drug delivery applications30,31. The two materials were thus selected to produce a composite matrix suitable for tissue engineering. Role of silica in emulsion stabilization and formation of uniform fibrous matrices via electrospinning was studied in detail. The surfactant free composite matrices, produced using Pickering emulsions with minimal organic solvent, were characterized for their thermo-mechanical properties and application in the area of tissue engineering.

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Experimental Materials Poly(ε-caprolactone) (PCL) having number average molecular weight (Mn) = 80,000 g/mol (Sigma Aldrich, India) was used for preparation of Pickering emulsions and their electrospinning to obtain fibrous matrices. Hydrophobic fumed silica (m-silica, modified with dimethyldichlorosilane, 3 wt.% as confirmed by thermogravimetric analysis) under the trade name of Aerosil R-792-V with average particle size 23 nm (from TEM) was purchased from Evonik Industries, Singapore and used without any treatment. Deionized (DI) water (Millipore India), toluene, dichloromethane (DCM) and dimethylformide (DMF) (Merck, India) were used as received. L929 mouse fibroblast cells were obtained from National Centre for Cell Science (NCCS), Pune, India and were maintained in DMEM (GIBCO) medium supplemented with 10 % FBS (Fetal Bovine Serum), 100 U/ml penicillin and 100 µg/ml streptomycin. Methods Preparation of Pickering Emulsions In general, a predetermined quantity of m-silica was dispersed in toluene by sonication for 15 minutes. PCL was later dissolved in the silica/toluene dispersion followed by dropwise addition of DI water under a high speed homogenizer (IKA Ultra Turrax T25) operating at 15000 rpm. The emulsion thus formed was stirred for additional two minutes and stored at room temperature for electrospinning and characterization. Amount of m-silica and PCL, and


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volume fraction of oil (m-silica/PCL/toluene) and water was varied at different levels to generate various emulsions as summarized in table 1.

Emulsion Electrospinning For preparation of fibrous matrices, electrospinning of freshly prepared emulsions was conducted on an electrospinning instrument (Physics Equipment Co. India) using a plate collector at 30 ± 3°C and 65 ± 5 % RH. Emulsion was transferred to a 2 ml disposable syringe attached with blunt tip metallic needle and a computer controlled pump was used to flow the emulsion at a constant rate of 0.5 ml/hr. The needle was subjected to a constant high voltage power supply of 25 KV and continuous stream of fibers was collected on a plate collector placed at a distance of 20 cm from needle tip. For electrospinning of neat PCL, a predetermined concentration of PCL was dissolved in DCM/DMF (volume ratio of 70/30) and was electrospun under same conditions (table 1). Characterisation Techniques Optical Microscopy Droplet size analysis of Pickering emulsions were carried out using Leica CH-9435 microscope. A drop of emulsion was spread on glass slide and was observed under the microscope under transmittance mode. An average of 100 readings for every sample was used to report the mean diameter. Scanning Electron Microscopy (SEM) The surface morphology of the fibers in matrices was studied from SEM images acquired using a Zeiss Evo 50 Scanning Electron Microscope. The samples were pre-coated with gold 5 ACS Paragon Plus Environment

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before analysis. For diameter measurement average of 100 readings was taken using Image-J software. Transmission Electron Microscopy (TEM) Fiber morphology was studied using Transmission Electron Microscopy (TEM, FEI, Tecnai G2) at a voltage of 200 KV by directly placing the sample onto copper TEM grid. TEM analysis of silica particles was done by dispersing silica in toluene and drop casting it onto the TEM grid. Tensile Testing The mechanical properties of PCL and silica/PCL electrospun matrices were measured using Instron micro-tensile tester (Model-5848, Singapore). For measurements, the matrices were deposited on a 5 × 5 cm template for 30 minutes. The samples were pre-conditioned at 25±1°C and 65% RH for 24 hours before testing. The samples were axially fixed on a window template with the help of a scotch tape. A 10 N load cell was used at a gauge length of 2.5 cm and cross-head speed of 10 mm/min for testing. An average of 5 measurements was reported. Thermal Gravimetric Analysis (TGA) TGA study was performed on Perkin Elmer 4000 instrument. A known weight of sample was heated from 50°C to 600°C at 10°C/minute under nitrogen environment. Differential Scanning Calorimetry (DSC) DSC of the samples was performed on TA Q2000 instrument. All the samples were first heated at 10°C/min to 100°C and kept under isothermal condition for 5 minutes. The samples were then cooled to -75°C at a cooling rate of 5°C/min and reheated at 10°C/min to 100°C. Melting (Tm) and crystallization (Tc) temperature, as well as change in heats of fusion (∆Hf) 6 ACS Paragon Plus Environment

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and crystallization (∆Hc) were determined from the second heating and cooling thermograms, respectively. Relative crystallinity (wc) of PCL in different samples was calculated according to equation (1) where ∆H0f =136 J/g (heat of fusion of 100% crystalline PCL) was used as reference32,33. ∆ு బ

‫ݓ‬௖ = ೑ ∗ 100% ∆ு

Equation (1)

೑

Cell Proliferation Assay Fibrous matrices were tested for their viability for cell attachment and growth using fibroblast (L929) cells. Cell proliferation was studied using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide) dye assay. The cells (3 × 105 per well in 6 well plate) were seeded over PCL matrices in triplicate and allowed to grow for 4 days. At defined intervals i.e. 0, 2 and 4 days, the medium was supplemented with MTT dye for 2 hours and solubilized using dimethyl sulfoxide. The absorbance was read at 595 nm using iMark microplate reader (Bio-rad) as a measure of cell proliferation. A higher absorbance reflected higher cell growth. L929 cells were grown in the media supplemented with 10% fetal bovine serum, 100 U/ml penicillin and 100 µg/ml streptomycin and incubated at 37°C in 5% CO2 incubator. To study the cell proliferation, cells (1 × 105 cells per well) were seeded over PCL matrices in 6-well plates in duplicates and cell proliferation was scored at different intervals (i.e. 0, 2 and 4 days) using MTT dye, quantified at 595 nm using iMark microplate reader (Bio-Rad).

Results and discussion Stabilization of Emulsions Silica particles were hydrophilic in nature, therefore in order to use them to stabilize a water-in-oil emulsion, hydrophobically modified silica (m-silica, modified using dimethyldichlorosilane, 3 wt.% as confirmed by TGA) was used. Preferential wetting 7 ACS Paragon Plus Environment

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of m-silica by toluene, as compared by water, was also demonstrated (see figure S1 (a), supporting information). A homogeneous dispersion of m-silica was formed in toluene than water. In a 1:1 volume mixture of toluene and water, m-silica particles showed a tendency to accumulate at the toluene-water interface. m-Silica was therefore effectively used to stabilize the water-in-oil emulsion.

A clear visual

evidence of settling of silica particles at water-toluene interface was provided (see figure S1 (a), supporting information). A mixture of water with PCL solution in toluene immediately phase separated in absence of silica (see figure S1 (b), supporting information) – which was an evidence that they did not emulsify. When silica in desired amount was put in PCL solution in toluene a stable emulsion was formed when it was mixed with water. The visual observation along with optical microscopic images presented in figure 1 (and figure S2, supporting information) were therefore clear proofs of emulsion formation. The fact that silica indeed worked as Pickering stabilizer was also proven when a change in droplet diameter was seen with varying silica content. An increased silica resulted in lower droplet size which only occurred when it stabilized the interface (figure 2). For a selected concentration of polymer (i.e. PCL) and volume fraction of waterto-oil, it was desired to use optimal quantity of m-silica for not only producing stable emulsions but also for formation of a uniform and stable jet during electrospinning and ultimately construction of a smooth fibrous matrix. The volume fraction of water-to-oil was fixed at 0.26 : 0.74 and polymer concentration was varied from 10 to 25 wt.% (with respect to toluene) with variation in m-silica content from 1 to 5 wt.% (with respect to toluene) (table 1). The emulsions were considered to be stable if there was no visual phase separation under ambient conditions. At 1 wt.% of m-silica all emulsion phase separated immediately. With 3 wt.% of m-silica, emulsions were found to be stable up to 18 hours and only with use of 5


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wt.% m-silica emulsions stable up to 50 hours were obtained (see figure S2, supporting information). Further increase in m-silica resulted in a too highly viscous oil phase and no emulsion could be produced. Thus for selected PCL concentration and water-to-oil volume ratio, m-silica in 3-5 wt.% was used to prepare fibrous matrices. It was interesting to observe the variation in droplet size with m-silica or PCL content. For a given m-silica amount, increase in PCL concentration resulted in decreasing droplet size. Analogously, for a given PCL amount, increase in m-silica content resulted in decreased droplet size as shown in figure 1, figure 2 and table 1. A higher PCL or m-silica content provided more resistance to droplet coalescence and smaller droplets were therefore stable for longer periods (see scheme S1, supporting information). SEM analysis of cryo-fractured films of emulsions also showed inherent porosity with decreasing pore size as PCL or m-silica content was increased (see figure S3, supporting information). It was also desired to incorporate as much aqueous phase as possible in the emulsions without affecting the stability so that use of organic solvent could be minimized. Maximum amount of aqueous phase which could be incorporated effectively against given silica concentration is shown in figure 3(a) (and the corresponding optical images in figure S4, supporting information). A phase diagram, shown in figure 3(b), reflected a very small window that was available to increase the volume fraction of aqueous phase. Higher aqueous phase at a selected PCL and m-silica content resulted in bigger droplets (table 1). An increase in aqueous phase volume fraction (or decrease in oil phase) resulted in an overall decrease in PCL or m-silica content per unit volume of emulsion and therefore droplets beyond a size limit were not stabilized by the selected level of PCL or m-silica and emulsions started to break. The amount of m-silica particles with average diameter (d) of 23 nm (as confirmed by TEM) for covering the total water-oil interfacial area was assessed from a modified approach to 9 ACS Paragon Plus Environment

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method proposed by Wiley34 and Kim et al

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35

. The number of dispersed aqueous phase

droplets, N, was calculated by N = 6V/πD3 where V represented the total volume of aqueous phase and D was the average diameter of dispersed droplets (as determined from optical microscopy). The total water-oil interfacial area (A) was calculated considering the droplets to be uniform spheres by A = N*4π (D/2)2. The total number of m-silica particles present in the emulsion (n0) was determined by comparing the total volume of n0 particles with volume of one m-silica nanoparticle using following equations. v = 4/3* π (d/2)3 v0 = m0/ρ = n0*v Where v is the volume of single m-silica nanoparticle of diameter (d) = 23 nm, v0 is the total volume of n0 number of m-silica particles of density (ρ = 0.09 g/ cm3) and mass m0 used with respect to the oil phase. The ratio (R*) of surface area of n0 number of m-silica particles per unit area of water-oil interface was calculated by R* = n0d2/ND2 (table 1). A plot of R* with variation in PCL concentration or volume fraction of aqueous phase was made (see figure S5, supporting information). R* was found to be directly depending upon the droplet size which was a function of PCL concentration or volume fraction of water to oil. An increase in wateroil interfacial area due to increased PCL concentration or decreased volume fraction of aqueous phase resulted in lower R* value. Electrospinning of Emulsions A solution of PCL with silica was easily electrospun to form a fiber and it had already been reported in previous studies 36,37. However, no solution electrospinning of PCL in toluene was carried out as the dielectric constant of toluene was too low and therefore it was difficult to spun fibers under electrospinning. Toluene therefore in general was not a preferred solvent for electrospinning. The advantage of using an emulsion was that electrospinning was 10 ACS Paragon Plus Environment

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conducted at a reduced solvent content. In addition, use of silica as a Pickering stabilizer, in an emulsifier free formulation, made the overall approach eco-friendly and viable for incorporation of silica in the polymeric matrix. It was also interesting to see the effect of using a Pickering emulsion on fiber morphology when it was used as precursor for electrospinning (in comparison to solution electrospinning). Scheme 1 showed the role that was played by dispersed water droplets during electrospinning. More than 26% of total volume was occupied by the dispersed aqueous water droplets in the resultant emulsions. When those emulsions were subjected to high electric field in electrospinning unit, a charge got developed on the surface of both the constituent phases (i.e. water and toluene). Since dielectric constant of water was higher than toluene, the charge developed on water was more which pulled it towards the periphery of the emerging jet and ultimately resulted in evaporation of water. The movement of higher charge carrying medium towards outer surface of emerging jet was defined as dielectrophoresis. This was also shown in one of our previous publications in which a high internal phase emulsion (HIPE) was used and fibers of co-continuous morphology originating from dispersed and continuous phase were obtained 29. No major role was played by the water droplets in fiber formation process as there was no polymer present in it and water ultimately evaporated. The stretching of continuous phase (carrying fiber forming polymer i.e. PCL) resulted in formation of fine fibers. A comparison between water droplet size and fiber diameter was presented in figure 4 which showed a general trend of thicker fiber formation with lower droplet size – which primarily was the effect originated from PCL content. Electrospinning of stable emulsions was carried out under optimized conditions to achieve continuous and uniform fiber deposition. Surface morphology of the composites matrices was analysed under SEM (figure 5 and figure S6, supporting information). A 22 gauge needle, having inner diameter of 0.413 mm (413 micro-meter) was used in the electrospinning 11 ACS Paragon Plus Environment

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operation. The minimum diameter of the thread of emulsion ejected out of the syringe was therefore 413 micro-meter, neglecting any die swell behaviour. The final fiber thickness varied from 0.2 to 1.2 micro-meter (table 1) reflecting extensive stretching that was done to the fiber during electrospinning. Thicker fibers were obtained with higher PCL or m-silica content, as expected, due to higher solid content per unit volume of the emulsion (table 1). TEM analysis showed uniform presence of silica across the fiber axis (insert in figure 5). Electrospinning was also carried out for the stable emulsions consisting of maximum volume fraction of aqueous phase with 5 wt.% of m-silica (see figure S7, supporting information). A trend in fiber fineness similar to that observed in previous samples was seen. In addition, formation of long pores and hollow tubular channels along the fiber axis was also observed. Larger water content that resulted in development of polymer rich and poor domains along the stretching jet during electrospinning caused formation of such hollow channels. Though presence of such hollow channels was undesirable as it would have affected the mechanical properties of the matrix, an increased surface area due to hollow channels turned out to be beneficial for cell attachment. Thermal Analysis PCL/m-silica composite matrices were analysed using DSC to check the effect of inclusion of m-silica nanoparticles on crystallization behaviour of PCL. As shown in figure 6, neat PCL electrospun matrix exhibited a crystallization peak (Tc) at 29°C. A melting peak (Tm) at ~ 57°C was observed for every sample (see figure S8, supporting information). Inclusion of 3 wt.% m-silica has commenced an early crystallization of PCL with appearance of Tc at 35°C in the composite fibrous matrices irrespective of PCL concentration and with 5 wt.% msilica the crystallization peak occurred at 39°C. A ~10°C enhancement observed in Tc was


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due to m-silica acting as an external nucleating agent for PCL which facilitated an early crystallization for PCL via heterogeneous nucleation. Enthalpy of crystallization (Hc) was found to increase with increased PCL content in the matrices and the trend matched with their corresponding melting enthalpies (Hf). Tm of all the composite matrices was observed at 57°C, similar to that of neat PCL sample. An important observation was increase in Hf with increased PCL content in the composite matrices resulting in higher crystallinity percentage. Crystallinity of neat PCL fibers was calculated to be ~37% irrespective of initial PCL concentration taken for electrospinning (samples P10, P15, P20 and P25). Crystallinity percentages for the composite fibers with 3 wt.% m-silica were 40, 43 and 45% with PCL content of 15, 20 and 25% respectively (samples P15S3, P20S3 and P25S3). The crystallinity percentages for the composite fibers with 5 wt.% m-silica were 42, 45 and 47% with PCL content of 15, 20 and 25% respectively (samples P15S5, P20S5 and P25S5). Nucleating efficacy of m-silica was marked by higher crystallinity percent with increase in silica concentration for a given value of PCL concentration. Tensile Properties Tensile properties of electrospun samples were majorly affected by the diameter of individual fibers especially in nonaligned fibrous matrices 38. Diameter of fiber increased with increase in total solid content which in turn was affected by PCL and m-silica concentrations. Neat PCL samples showed an increase in diameter and tensile strength with higher polymer concentration. Tensile strength and modulus of PCL/m-silica electrospun composite matrices was found to enhance significantly compared to neat PCL samples with increase in PCL or m-silica content (figure 7). Samples containing 10 wt.% PCL were not tested due to large inhomogeneity in the fiber fineness and presence of agglomerates of m-silica. Samples with constant m-silica but increased PCL concentration showed an increase in diameter as well as tensile strength. The enhancement in mechanical properties by inclusion of m-silica in PCL 13 ACS Paragon Plus Environment

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matrix was attributed to effective load transfer from PCL matrix to m-silica due to strong secondary interactions between the two. Modulus and tensile strength values of 5 wt.% msilica samples were higher than 3 wt.% m-silica samples at constant PCL concentration. This was in correlation with crystallinity percentage values of corresponding samples. Thus msilica particles not only acted as external nucleating agent to enhance crystallization of PCL but also as a good reinforcing agent to provide better strength to the composite matrix. Cell Growth Assessment It has been widely reported that PCL is stable under aqueous environment and hence is being used in tissue engineering applications39–41. Usefulness of silica especially for bone tissue engineering applications has been reported 42–45. We separately captured SEM images of neat PCL and PCL/silica (5 wt.%) samples kept under aqueous simulated body fluid (without cells). PCL and PCL/silica (5 wt.%) samples were found to be stable after a period of 21 days (SEM images, see figure S9). Therefore it was necessary to assess the efficacy of PCL/msilica composite matrices for their application as cell supporting material. Growth of L929 fibroblast cells was carried out on the PCL/m-silica matrices and the results are shown in figure 8 (optical images shown in figure S10, supporting information). An increase in relative cell proliferation was observed on the matrices containing m-silica. Smaller the diameter of fiber, higher is the exposed surface area and hence higher is the cell proliferation efficiency. Thus the proliferation efficacy trends in order of PCL10S5 > PCL15S5 > PCL20S5 > PCL25S5. The fibrous scaffolds thus generated with minimised usage of organic solvents, but with enhanced mechanical and cell proliferation efficiency compared to neat PCL samples can efficiently be used in tissue engineering applications. Conclusions


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PCL/m-silica based composite matrices were efficiently produced via emulsion electrospinning and the efficacy of matrices for sustaining cell proliferation was assessed. The emulsions were made using dimethyldichlorosilane-modified-silica (m-silica) as the Pickering stabilizer in an emulsifier free formulation using as much water as possible to form one of the phases of the emulsion. m-Silica stabilized emulsions were reproducibly electrospun to generate nano-fibrous organic-inorganic composite matrices. Inclusion of msilica resulted in higher PCL crystallinity due to its involvement as external nucleating agents. m-Silica also acted as a reinforcing agent for the composite matrix resulting in enhanced tensile properties of PCL/m-silica composite matrices in comparison to PCL matrix. Growth profile of Fibroblast L929 cells enhanced with inclusion of m-silica in the PCL matrix. The PCL/m-silica based composite fibrous matrices produced via Pickering emulsion electrospinning may further be scaled-up and evaluated for their commercial viability and application in other areas. Acknowledgement Authors gratefully acknowledge the financial support provided by Indian Institute of Technology Delhi and Department of Science and Technology, India (grant no. SERB/F/3988/2014-15) to perform this research.

Supporting Information: Scheme S1 and Figures S1, S2, S3, S4, S5, S6, S7, S8, S9 and S10 are supplied as supporting information.


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Table 1. m-Silica stabilized Pickering emulsions of PCL and their electrospun matrices

Sample ID

PCL a

P10 P15 P20 P25 P10S1 P15S1 P20S1 P25S1 P10S3 P15S3 P20S3 P25S3 P10S5 P15S5 P20S5 P25S5 P10S3M P15S3M P20S3M P25S3M P10S5M P15S5M P20S5M P25S5M

10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25

Silica b

0 0 0 0 1 1 1 1 3 3 3 3 5 5 5 5 3 3 3 3 5 5 5 5

Φo

c

1 1 1 1 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.26 26.5 27.5 29 32.5 28 32 37 41

Emulsion’s stability d Unstable Unstable Unstable Unstable Stable Stable Stable Stable Stable Stable Stable Stable Stable Stable Stable Stable Stable Stable Stable Stable

Droplet Size (D) (µm) e 177±15 140±12 110±25 90±10 120 ± 15 90 ± 12 70 ± 10 60 ± 11 191±17 175±10 120±22 103±22 177±25 120±15 101±12 91±11

N (x 106) f

0.9 1.8 3.7 6.8 2.9 6.8 14.5 23.0 0.7 1.0 3.2 5.7 0.9 3.5 6.7 10.4

no (x 1017)

R* h

0.45 0.45 0.45 0.45 1.36 1.36 1.36 1.36 2.27 2.27 2.27 2.27 1.39 1.44 1.52 1.70 2.44 2.79 3.23 3.58

2565 2029 1594 1304 2899 2174 1691 1449 2768 2536 1739 1493 4275 2899 2440 2198

g

Fiber Diameter (nm) i 520±12 724±15 1011±20 1342±32 178±10 390±12 592±17 829±21 257±12 450±14 665±15 1107±20 310±85 403±32 550±12 812±23 345±17 442±25 588±24 865±37

a

weight percentage of PCL with respect to toluene weight percentage of m-silica with respect to toluene c volume fraction of aqueous phase d emulsion’s stability was checked for no visual phase separation under ambient conditions e calculated using ImageJ analysis on optical microscopic images, average of 100 readings f Number of aqueous phase droplets (N) formed, calculated using N = 6V/πD3, where V is the total volume of aqueous phase and D is the average size of dispersed aqueous phase droplets g Number of m-silica particles (n0), calculated using v0 = m0/ρ = n0*4/3* π (d/2)3, where v0 is the total volume of m-silica particles, m0 = mass of m-silica taken in the formulation with respect to oil phase, ρ is the density of m-silica (0.09 g/cm3) and d is average diameter of msilica (23 nm) h Ratio of surface area of n0 number of m-silica particles per unit water-oil interfacial area, calculated using R* = nd2/ND2. i calculated using ImageJ analysis on SEM images, average of 100 readings b


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H2O

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SiO2

Oil Silica stabilized oil in water emulsion

Resultant PCLsilica composite fiber Coalescence and stretching of droplets Movement of water droplets towards periphery Evaporation of water and toluene

Scheme 1. Fiber formation process from silica stabilized Pickering emulsion


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Figure 1. Optical microscopic images of m-silica stabilized emulsions reflecting the reduction in droplet size with increase in polymer concentration a) P10S3 b) P15S3 c) P20S3 d) P25S3 e) P10S5 f) P15S5 g) P20S5 h) P25S5 25 ACS Paragon Plus Environment

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Figure 2. Effect of silica concentration on dispersed droplet diameter


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Figure 3 (a) Maximum of aqueous phase against specific PCL concentration with variation in m-silica content from 3 to 5 wt.% (b) ternary phase diagram representing stable emulsion regime formed by variation in m-silica or PCL concentration and oil-to-water volume fraction


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Figure 4. Effect of silica and PCL concentration on fiber diameter


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Figure 5. SEM images of electrospun fibers obtained from 5 wt.% m-silica stabilized emulsions. Insets show corresponding TEM images (a) P10S5, (b) P15S5, (c) P20S5 and (d) P25S5 wt.%


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Figure 6. Effect of inclusion of m-silica on crystallization behaviour of PCL matrix


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Figure 7. Mechanical Property analysis of composite electrospun matrices, (a) modulus, (b) tensile strength and (c) diameter of PCL-m-silica composite matrices


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Figure 8. MTT Assay of PCL/m-silica composite fibers against L929 fibroblast cells


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For Table of Contents Use Only


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