Electrospun nanofibers: challenges and opportunities. Saša Baumgartner University of Ljubljana Faculty of Pharmacy Slovenia.

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1 Electrospun nanofibers: challenges and opportunities Saša Baumgartner University of Ljubljana Faculty of Pharmacy Slovenia November, 2014 Outline Nanofibers and their application The electrospinning process Parameters influencing electrospinning Formation of nanofibers depends on polymer solution characteristics example on PVA solutions Investigation of chitosan, PEO and their combinations for successful nanofiber formation Interfacial rheology Mechanical properties of PEO/CS nanofibers The influence of RH on PEO/CS Drug incorporation into nanofibers Conclusion 1

2 Polymer nanofibers Polymer nanofibers are solid fibers with diameter from a few tens up to 1000 nm and theoretically of unlimited length having huge surface per mass and small pore size. Nanofibers express greater flexibility and mechanical strength compared to the same material of other forms and larger dimensions. The applicability of nanofibers is wide due to their unique properties. Polymer nanofibers - applications Cosmetic skin masks Skin cleansing Skin healing Skin therapy Life science applications Drug delivery carrier Haemostatic devices Wound dressing Tissue engineering scaffolds Membranes for skin Tubes for blood vessels 3D scaffolds for bone and cartilage regeneration Military protection clothing Minimal impedance to air Trapping aerosols Anti-bio / -chemical gases Polymer nanofibers Nanosensors Thermal sensor Piezoelectric sensor Biochemical sensor Florescence chemical sensor Filtration media Liquid filtration Gas filtration Molecular filtration Industrial applications (electronic/optical) Micro/nano electronic devices Electrostatic dissipation Electromagnetic interference shielding Photovoltaic devices (nano-solar cell) LCD devices Higher efficiency catalyst carriers Burger C, Hsiao BS, Chu B, Annu. Rev. Mater. Res. 2006, 36, 333 2

3 The process of electrospinning Electrospinning is a widely used, simple, one-step, versatile method for producing nanofibers. The principle of electrospinning: A high voltage is applied to the liquid material (polymer solution or melt). At a certain voltage threshold, the electrostatic repulsion forces prevail over the forces maintaining the spherical shape of the drop, first forming a Taylor cone and then a jet of polymer solution (or melt) is formed. The charged jet accelerates towards the grounded collector, the jet is continuously stretched and whipped, which means its diameter is reduced from several hundred micrometers to as little as tens of nanometers. Simultaneously, the solvent evaporates and dry nanofibers are deposited on the collector A scheme illustrating the basic principle of the electrospinning process Rošic R, et ali. Electrospun hydroxyethyl cellulose nanofibers: the relationship between structure and process. J. Drug Del. Sci. Tech. 2011; 21:

4 Parameters influencing feasibility of production and morphology of the nanofibers produced Electrospinning is the process affected by many different parameters, making this process very challenging. Parameters affecting electrospinning can be divided into three major groups: systemic parameters (solution, melt properties) process parameters ambient parameters. Interplay of numerous known parameters and probably some unknown ones can lead to morphologies, which where not predicted (surprised us ) The effect of a single parameter on the process or morphology can be very different with regard to selected polymer - solvent combination 4

5 Parameters influencing feasibility of production and morphology of the nanofibers produced Pelipenko, PhD Thesis, 2014 Systemic parameters solution parameters Solution parametres depend on solvent and polymer characteristics Solution parametres: polymer concentration polymer molecular weight solution viscosity solution interfacial viscosity solution conductivity solution surface tension 5

6 Systemic parameter: Selection of polymer for nanofiber production by electrospinning In theory, nanofibers can be produced using almost every polymer Characteristics of polymer for biomedical applications: Nontoxic Biocompatible Biodegradable Frequently hydrophilic Polymers used for nanofiber production Chitosan: polycationic polysaccharide, difficult to be electrospun alone due to its ionic character and strong hydrogen bonds between polymer chains. Hyaluronic acid (HA): linear glycosaminoglycan that can be found in the ECM of many soft tissues (skin hydration, induces cell proliferation, migration, and angiogenesis). Nanofibers from HA are difficult to be produced (gel-like structure at very low concentrations). Formulated nanofibers should be stabilized by chemical crosslinking agents prior to application into an aqueous environment. Elastin: a protein of the ECM responsible for tissue elasticity, difficult to be electrospun due to its polyelectrolyte nature. Elastin nanofibers should be stabilized by crosslinking agents (glutaraldehyde) Alginate: polysaccharide, nanofibers from alginate show great potential in tissue engineering. Those nanofibers can be stabilized using only Ca 2+. It cannot be electrospun alone. 6

7 Polymers used for nanofiber production Poly(vinyl alcohol) and poly(ethylene oxide): linear polymers, good spinnability (often added to chitosan, alginate, HA, ). PVA nanofibers can be physically stabilized (with temperature or alcohols). PCL, PLA, and PLGA nanofibers are all stable in water, organic solvents are needed for production. Prolonged release can be acchived. Biodegradible polymers! PVP: easy for electrospinning, PVP nanofibers should be chemically stabilized to prvent degradation in water Polymer properties for electrospun nanofibers Rošic R, PhD Thesis,

8 Systemic parameter: Selection of solvent Selected solvent enables full polymer solubility Solvent should be optimally volatile (needle clogging vs. wet product) Solvent should not chemically interact with the dissolved polymer Solvent should be of optimal viscosity Solvent influence surface tension, conductivity, and dipole moment Water is the most desirable solvent (biocompatible, nontoxic, use is limited to hydrophilic polymers) The most commonly used organic solvents in electrospinning: acetone, dichloromethane, methanol, ethanol, acetic acid, dimethyl formamide, ethyl acetate, trifluoroethanol, tetrahydrofuran, and formic acid. In order to achieve optimal solution properties (viscosity, surface tension, and solvent volatility), a combination of two or more solvents is often used Systemic parameter: Polymer concentration there is no general rule for optimal polymer concentration, because it depends on polymer and solvent characteristics Different explanations, which concentration is the right for nanofiber production Example: polymer solutions can be divided into three regions based on their c/c* ratio (c is the polymer concentration and c* is the polymer concentration at which overlapping occurs between polymer chains): (I) dilute (c/c* = 1), (II) semi-dilute unentangled (1 < c/c* < 2), and (III) semi dilute entangled (c/c* > 2). sufficient cohesion forces between polymer chains leading to uniform nanofibers are achieved when the c/c* ratio is above 6 Some claim, that higher polymer concentration results in more uniform nanofibers, thicker nanofibers 8

9 Increasing PVA concentration smoother and thicker nanofibers SEM images of electrospun nanofibers; A-4% PVA, B-5% PVA, C-6% PVA, D-7% PVA, E-8% PVA, F-9% PVA, G-10% PVA, H-11% PVA, I-12% (w/w) PVA, Systemic parameter: Polymer characteristics Usually, polymers with higher molecular weights are preferable - a sufficient number of intermolecular entanglements. Usually, low-molecular-weight polymers tend to form beads rather than fibers. Linear polymers enable easier electrospinnability. Polyelectrolyte polymers are difficult to be electrospun: For polyelectrolyte polymers intensive swelling is characteristic leading to highly viscous solutions at low concentrations. The polyelectrolyte nature of polymers causes intense repulsive forces during electrospinning, which result in jet instability 9

10 Systemic parameter: Surface tension Surface tension is the contractive tendency of a liquid s surface and is the main force acting against Taylor cone formation and further jet elongation. A decisive link between fiber morphology and the value of surface tension has not been established so far. Generally, at lower values of surface tension fibers without beads are obtained and lower applied voltages can be used. Surface tension can be manipulated by the addition of surface active substances. However, low surface tension cannot solve the problem that occurs due to the molecular weight of a polymer or its concentration being too low. Systemic parameter: Rheological characteristics An increase in solution viscosity results in the formation of thicker fibers with lower bead generation, whereas in a polymer solution with low viscosity there is no continuous fiber formation. Not only viscosity but also the viscoelastic properties of a polymer solution greatly influence jet formation and stability, and therefore nanofiber morphology in general. A balance between the elastic (G ) and plastic (G ) moduli of the polymer solution needs to be achieved. The elasticity should be much lower than plasticity to prevent jet breakup and droplet formation, but still present to enable jet initiation. 10

11 Rheological characterization of polymer solutions (PEO and chitosan) Rošic, R., et al., European Polymer Journal, (8): p Viscosity, storage (G ) and loss (G ) moduli as a function of chitosan:peo solution, cone-plate measuring system; viscosity measured at a shear rate of 2 s -1 ; dynamic moduli measured at a frequency 0.2 s -1. Systemic parameter: Interfacial rheological characteristics Interfacial rheological characteristics correlate well with the product of electrospinning process, because during electrospinning the electrospun jet dramatically thins from around 1 mm to a few nm, causing a more dominant role of surface properties over the bulk characteristics of the polymer solution. Interfacial rheological characteristics combine classical rheological characteristics with surface tension, resulting in a valuable predictive tool for nanofiber formation. Bulk properties are largely determined by polymer concentration and are thus useful for predicting jet and fiber formation, whereas the interfacial properties enable the prediction of jet continuation. 11

12 Interfacial rheological characterization of polymer solutions (PEO and chitosan) Rošic, R., et al., European Polymer Journal, (8): p Why interfacial rheological properties are so important for successful electrospinning? Pelipenko J, et al. Acta Pharmaceutica, 62 (2912)

13 Systemic parameter: Conductivity Polymer solutions with too low or too high conductivity cannot be electrospun In the case of uncharged polymers, the problem of low conductivity can be solved with the addition of salts. Higher conductivities generally result in thinner nanofibers. Higher conductivity enables the use of lower applied voltage. Highly conductive solutions can be very unstable in the applied electric field. Correlation between some systemic parameters and nanofiber morphology Pelipenko J. et al. Acta Pharmaceutica, 62 (2912)

14 Electrospinning process parameters applied high voltage needle tip to collector distance flow rate needle tip design and placement collector composition, geometry, and rotation speed Process parameter: applied voltage Usually used voltage: between 5 and 40 kv Solutions with lower conductivity, higher surface tension, or higher viscosity require higher voltages, and vice versa. The increase in applied voltage results in thinner fibers (increased repulsive electrostatic forces, which lead to more extensive stretching of the electrospun jet) A higher voltage usually results in a higher probability of bead formation (in the nanofiber structure) due to Taylor cone instability. 14

15 Process parameter: distance between tip and collector This process parameter is usually correlated with fiber diameter; however, there is no strict correlation. Minimal distance is required to give the electrospun jet sufficient time to dry before reaching the collector. To short distance leads to nanofiber fusion and polymer film formation. Usually, many different parameters must be optimized to produce nanofibers of suitable characteristics: Increased distance leads to the production of thinner nanofibers; however, it should be accompanied by increased flow rate as well as by increased applied voltage. Process parameter: flow rate The control of flow rate depends mainly on the volatility of the solvent used. Again, many parameters need to be optimized: When highly volatile solvent is used, higher flow rates should be used to prevent clogging. At the same time, higher flow rate must be accompanied by a higher voltage to ensure electrically induced extraction of polymer solution from the needle tip. An increase in the flow rate can lead to formation of thicker nanofibers due to thicker jet formulation; however, higher flow rates generate more beads on nanofibers or deposition of un-dried product on collector 15

16 Process parameter: needle tip design Needle s inner diameter is important (smaller diameter generally results in thinner nanofibers, and vice versa) Increasing the inner diameter usually leads to multiple Taylor cone formation. Using a coaxial needle (two or more concentric needles placed inside each other), core-shell or even multilayered nanofibers can be produced Process parameter: collector geometry 16

17 Ambient parameters influencing electrospinning process Environmental temperature and relative humidity (RH) are the most important T affects the solvent evaporation rate and solution or melt viscosity (opposite effects!!) The higher the T, the faster is solvent evaporation, thicker nanofibers can be formed In contrast, the higher T, the lower the viscosity, thinner nanofibers can be formed The effect of RH on electrospinning depends on the polymer solution composition In the case of hydrophobic polymers - higher RH values lead to the formation of porous nanofibers In the case of aqueous polymer solutions, RH can be used for manipulation of nanofiber diameter and mechanical properties. Electrospinning machine controlling ambient and process parameters 17

18 Solution parameter Effect on nanofiber morphology Concentration concentration leads to in fiber diameter. Viscosity Surface tension viscosity leads to thicker and beadless nanofibers. Too high viscosity causes generation of beads. No clear correlation between parameters. Conductivity Increase in conductivity leads to thinner nanofibers. Molecular weight of Increase in polymer molecular weight leads to formation of a polymer nanofiber with fewer beads. Volatility of solvent volatility requires flow rate and leads to nanofiber with beads. Process parameter Flow rate flow rate results in thinner nanofiber. Too high flow rate causes the generation of beads. Applied voltage Thinner fiber with higher applied voltage. Needle tip to collector Minimum distance required to obtain dry nanofiber. distance Geometry of collector Metal collectors are preferred; random or allingned orientation Ambient parameter Humidity humidity enables flow rate results in nanofiber with beads Temperature A thinner nanofiber is obtained when the temperature is higher. Rošic R, PhD Thesis, 2013 Different morphologies of nanofibers Nanofibers obtained from blended solutions of chitosan/peo mass ratio (A) 10/90, (B) 20/80, (C) 30/70, (D) 40/60, (E) 50/50, (F) 60/40, (G) 70/30, (H) 80/20, (I) 90/10 R. Rošic et al. Electrospun chitosan/peo nanofibers and their relevance in biomedical application, IFMBE Proceedings 2011, 37:

19 Formation of nanofibers depends on polymer solution characteristics Polyvinyl alcohol (PVA) nontoxicity, noncarcinogenicity, biocompatibility, biodegradability, hydrophilicity, appropriate mechanical properties excellent electrospinnability An extensive study of physical characteristics of PVA solutions from 2 to 14 % was carried:. density surface tension conductivity viscosity measurements bulk and interfacial rheology small angle X-ray scattering (SAXS) electrospinning Density and apparent polymer volume v app - parameter indicating a measure for the interaction between segment of polymer and solvent molecules (kg dm -3 ) 1,04 1,03 1,02 1,01 1,00 v app /cm 3 g w PVA concentration % (w/w) 1 d0 d vapp 1 d d0 d 0 water density d - solution density - mass ratio - grams of solute per gram of solvent In the range between ~5 % and ~11 % (w/w) v app is nearly constant, i.e. ~0.76 cm3 g-1 19

20 Surface tension ROŠIC R, PELIPENKO J, KRISTL J, KOCBEK P, BEŠTER-ROGAČ M, BAUMGARTNER S. Physical characteristics of poly (vinyl alcohol) solutions in relation to electrospun nanofiber formation. European Polymer Journal, 2013, vol. 49, iss. 2, str Viscosity,, of PVA solutions as a function of concentration, % (w/w) at 25 C; ( ) capillary viscometer, ( ) rotational viscometer, Insert: same data on linear scale. ROŠIC R, PELIPENKO J, KRISTL J, KOCBEK P, BEŠTER-ROGAČ M, BAUMGARTNER S. Physical characteristics of poly (vinyl alcohol) solutions in relation to electrospun nanofiber formation. European Polymer Journal, 2013, vol. 49, iss. 2, str

21 Rheological characterization of PVA solutions ROŠIC R, PELIPENKO J, KRISTL J, KOCBEK P, BEŠTER-ROGAČ M, BAUMGARTNER S. Physical characteristics of poly (vinyl alcohol) solutions in relation to electrospun nanofiber formation. European Polymer Journal, 2013, vol. 49, iss. 2, str SAXS measurements and radius of gyration SAXS spectra of PVA solutions at 25 C. Insert: the Gunier plot Gyration radii, R g, of PVA solutions as a function of concentration % (w/w). 21

22 SEM images of PVA electrospun nanofibers A-4% PVA, B-5% PVA, C-6% PVA, D-7% PVA, E-8% PVA, F-9% PVA, G-10% PVA, H-11% PVA, I-12%PVA ROŠIC R et al. European Polymer Journal, 2013, vol. 49, iss. 2, str Successful formation of smooth PVA nanofibers is possible only using polymer solutions: in concentration range from 8 to 12 % (w/w), where Newtonian behaviour is expressed the plasticity (G ) of the solutions has to be in predominance of elasticity (G ). an increase in the plasticity of the system indicates the formation of internal structures enabling the jet initiation and elongation fibrous structures are formed only, when properly firm internal structures of polymer are formed in the solution that are still able to orient in the direction of applied electric field and thus enable complete polymer elongation during electrospinning 22

23 Investigation of chitosan and PEO polymers for nanofiber formation ROŠIC R, et al. The role of rheology of polymer solutions in predicting nanofiber formation by electrospinning. European Polymer Journal 2012, vol. 48, iss. 8, str SEM images of nanofibers obtained from blended solutions of chitosan:peo at mass ratio (A) 10:90, (B) 20:80, (C) 30:70, (D) 40:60, (E) 50:50, (F) 60:40, (G) 70:30, (H) 80:20, (I) 90:10 at 25 kv, needle-to-collector distance 17 cm and flow rate for chitosan 1.8 ml/h. Total polymer concentration is 3% (w/w). 23

24 The influence of interfacial parameters on nanofiber morphology ROŠIC R, PELIPENKO J, KOCBEK P, BAUMGARTNER S, BEŠTER-ROGAČ M, KRISTL J. The role of rheology of polymer solutions in predicting nanofiber formation by electrospinning. European Polymer Journal 2012, vol. 48, iss. 8, str Why interface? Pelipenko Jan et al. Acta Pharmaceutica 62 (2012)

25 Correlation between interfacial rheology and nanofiber formation ROŠIC R, PELIPENKO J, KOCBEK P, BAUMGARTNER S, BEŠTER-ROGAČ M, KRISTL J. The role of rheology of polymer solutions in predicting nanofiber formation by electrospinning. European Polymer Journal 2012, vol. 48, iss. 8, str Mechanical properties of PEO/CS nanofibers The objective of this study was to evaluate the morphology, mechanical properties, and thermal and swelling behavior of prepared single electrospun nanofibers with respect to their variations in the composition and size. For the purposes of local and bulk compositional analysis, the following methods were conducted: atomic force microscopy (AFM), differential scanning calorimetry (DSC). 25

26 DSC results of CS: PEO = 10:90 Sample T m ( C) H (J/g) PEO 400K powder PEO 400K nanofibers PEO nanofibers 400K/CS Janković B. International Journal of Pharmaceutics 455 (2013) AFM contact mode images (left) and size distributions (right) of the nanofibers studied: PEO 400K (scan size μm) and PEO 400K/CS (scan size μm). The ranking of the increasing nanofibers height according to the median values: PEO 400K (97.5 nm) < PEO 400K/CS (270 nm) 26

27 PEO 400K PEO 400K/CS The ranking of increasing stiffness of the nanofibers: PEO 400K< PEO 400K/CS. In order to confirm the reliability of the purposed order, a three-point beam bending test on AFM was performed Janković B. International Journal of Pharmaceutics 455 (2013) Young's modulus as a function of nanofiber height Polymeric films Young s modulus Hardness (MPa) (GPa) PEO 400K 0.8 ± ± 6 PEO 400K/CS 1.9 ± ± 20 The mechanical properties of polymeric nanofibers can be tuned by changing the fiber size as well as the composition Janković B. International Journal of Pharmaceutics 455 (2013)

28 The impact of ambient parameters on nanofiber formation Pelipenko J et al. Int J Pharm, 456 (2103) Why decreased RH causes formation of thicker nanofibers? Pelipenko J et al. Int J Pharm, 456 (2103) Correlation between phase shift angle θ ( ), G ( ), G ( ), and concentration of polymer in solution, determined at a shear rate of 100 s 1. The dashed line drawn in each graph indicates the polymer concentration used for electrospinning. 28

29 Incorporation of drugs into PVA nanofibers PVA nanofibers prepared using 10% (w/w) PVA water solution: A) plain nanofibers, B) nanofibers with incorporated 10% (w/w) resveratrol, and C) nanofibers with incorporated 10% (w/w) levofloxacin. Green circles indicate crystals of drug on the nanofiber surface. Electrospun nanofibers from 8% PVA solution containing 10% Nasalicylate Pelipenko PhD, 2014 Incorporation of drugs into nanofibers SEM micrographs of electrospun PCL nanofibers loaded with 10% (A, B) and 15% (C, D) ibuprofen (based on the weight of dry polymer). The nanofibers were visualized at small (A, C) and high (B, D) magnification Potrč T. et al, CESPT abstract book,

30 Conclusion To produce nanofibers of predicted quality, the mechanism of their formation and the influence of process parameters on their formation should be understood Complementary techniques for physical characterization of polymer solutions should be introduced There is a huge potential for drug incorporation into different types of nanofibers different release profiles There is a lot of challenges and opportunities to develop proper nanofibers for biomedical application Thanks to Colleagues from Faculty of Pharmacy Special thanks to Petra Kocbek Romana Rošic Julijana Kristl Jan Pelipenko Marija Bešter Rogač Urša Mikac Biljana Janković PhD students Diploma students 30