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Research Papers

Melt Electrospinning Writing Process Guided by a “Printability Number”

[+] Author and Article Information
Filippos Tourlomousis

Department of Mechanical Engineering,
Highly Filled Materials Institute,
Stevens Institute of Technology,
Hoboken, NJ 07030
e-mail: ftourlom@stevens.edu

Houzhu Ding

Department of Mechanical Engineering,
Stevens Institute of Technology,
Hoboken, NJ 07030
e-mail: ding4@stevens.edu

Dilhan M. Kalyon

Department of Chemical Engineering
and Material Science,
Highly Filled Materials Institute,
Stevens Institute of Technology,
Hoboken, NJ 07030
e-mail: dkalyon@stevens.edu

Robert C. Chang

Department of Mechanical Engineering,
Stevens Institute of Technology,
Hoboken, NJ 07030
e-mail: rchang6@stevens.edu

1Corresponding author.

Manuscript received June 22, 2016; final manuscript received March 6, 2017; published online May 8, 2017. Assoc. Editor: Yong Huang.

J. Manuf. Sci. Eng 139(8), 081004 (May 08, 2017) (15 pages) Paper No: MANU-16-1345; doi: 10.1115/1.4036348 History: Received June 22, 2016; Revised March 06, 2017

The direct electrostatic printing of highly viscous thermoplastic polymers onto movable collectors, a process known as melt electrospinning writing (MEW), has significant potential as an additive biomanufacturing (ABM) technology. MEW has the hitherto unrealized potential of fabricating three-dimensional (3D) porous interconnected fibrous mesh-patterned scaffolds in conjunction with cellular-relevant fiber diameters and interfiber distances without the use of cytotoxic organic solvents. However, this potential cannot be readily fulfilled owing to the large number and complex interplay of the multivariate independent parameters of the melt electrospinning process. To overcome this manufacturing challenge, dimensional analysis is employed to formulate a “Printability Number” (NPR), which correlates with the dimensionless numbers arising from the nondimensionalization of the governing conservation equations of the electrospinning process and the viscoelasticity of the polymer melt. This analysis suggests that the applied voltage potential (Vp), the volumetric flow rate (Q), and the translational stage speed (UT) are the most critical parameters toward efficient printability. Experimental investigations using a poly(ε-caprolactone) (PCL) melt reveal that any perturbations arising from an imbalance between the downstream pulling forces and the upstream resistive forces can be eliminated by systematically tuning Vp and Q for prescribed thermal conditions. This, in concert with appropriate tuning of the translational stage speed, enables steady-state equilibrium conditions to be achieved for the printing of microfibrous woven meshes with precise and reproducible geometries.

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Figures

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Fig. 1

Concept graph plotting the different manufacturing methods employed in the biofabrication field for scaffold-guided tissue engineering applications as a function of their resolution

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Fig. 2

Physics of the melt electrospinning writing process. (a) Digital photographs at different time instances showing the melt electrospinning process performed with a stationary collector and the printing evolution of conical structures under the tip. The collector plate is mounted on an x–y automated stage (translational stage speed—UT = 0 mm/s). (b) (I) Digital photograph showing the jet deposition with the melt electrospinning writing process as the collector plate is moving at the critical stage speed. (II) Digital photograph showing the jet deposition as the collector plate is moving at a slightly higher speed than the critical stage speed, resulting in trailing edge formation due to the viscoelastic nature of the material.

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Fig. 3

Categorization of independent parameters involved in melt electrospinning writing process

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Fig. 4

Schematic illustrating the proposed heating element and the key heat transfer mechanisms in the polymer melt supply and free-flow regime. (a) computer aided design model of the industrial heat gun mounted in the heating tunnel. (b) The hot stream air causes the simultaneous heating of the polymer melt supply regime (syringe barrel and needle tip) and the free-flow regime by forced and free convection, respectively. The surface temperature on the syringe (Ts (SI:  °C)) and the temperature profile (Tt < z < Tc (SI:  °C)) along the spinline coordinate z can be controlled with an industrial heat gun for a prescribed setting of volumetric flow rate and temperature.

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Fig. 5

Melt electrospinning writing system configuration. (a) computer aided design model showing the three distinct process regimes. (b) The polymer supply regime is heated using a heat gun that is calibrated using a thermal FLIR camera. The temperature at the surface of the melt reservoir (Ts) is determined at the position denoted by the crosshatch. (c) A digital photograph showing the thermocouple and the indexed phantom used to measure the temperature profile along the spinline in the free-flow regime. (d) The temperature profile along the free-flow regime is measured for a heat gun setting corresponding to Ts = 77.8 °C.

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Fig. 6

Rheological characterization of PCL at different melting temperatures (Tm: 70–80–90 °C). (a) Elastic (G″) and loss moduli (G′) as a function of angular frequency (ω (SI: rad/s)). Dynamic data and fitting using the Giesekus model. (b) Polymer viscosity as a function of angular frequency (ω (SI: rad/s)) and shear rate (γ˙ (SI: s−1)). Dynamic data, steady torsional data, and fitting using the Giesekus model.

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Fig. 7

Correlation of the Printability Number under a stationary collector with scaled dimensionless groups arising from the governing equations. (a) Normalized Printability Number (NPR,1*  = NPR,1/NPR,1,min versus normalized melting temperature (T* = Tm/Tref), where NPR,min is given for Tref = 70 °C and Q = 50 μL/h. The arrow at T* = 1.10 indicates the melting conditions set in the present study. (b) NPR,1* versus Re number, (c) NPR,1* versus De number, (d) NPR,1* versus Ca number, and (e) NPR,1* versus Ep number. Graphs in (b)–(e) are obtained for Tm = 78 °C and three different voltage potential (Vp) values.

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Fig. 8

Digital photographs showing the temporal evolution of the printing process initiation. (I) Starting point (t = 0 min) is considered the instance at which the polymer melt enters the free-flow regime. The following are initial values of the main process parameters: volumetric flow rate (Q = 50 μL/h), voltage potential (Vp = 12.5 kV), tip to collector distance (d = 20 mm), and experimental temperature at the surface of the melt reservoir (Ts = 78 °C). (II) At t = 12 min, the elongate shape of the jet denotes incremental electrostatic forces along with the gravity forces to overcome the resistive forces (viscous, elastic, and surface tension forces). (III) At t = 15 min, the downstream forces exceed the upstream resistive forces leading to the Taylor cone formation. (IV) Within seconds, a straight cone–jet forms between the needle tip and the collector.

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Fig. 9

Tuning of the Printability Number under a stationary collector toward steady equilibrium conditions in the free-flow regime. (a) The normalized Printability Number (NPR,1*) versus electrostatic force parameter (Ep). (b) Digital photographs corresponding to each Ep setting and associated NPR,1* illustrate the procedural steps followed to obtain a stable melt electrospun jet. This equilibrium printing state is achieved by systematically tuning the process parameters and observing the effect on the jet shape. (I) For the following initial values of the main process parameters: Q = 50 μL/h, Vp = 12.5 kV, d = 20 mm, and excess jetted material disturbs the cone–jet formation. (II) Within minutes, the jet is reformed with elevation of the Taylor cone position. (III) The collector is set closer to the needle tip (d = 15 mm), resulting in an increment of the electrical field strength. The resultant jet shape is altered, yielding stretching of the excess material and Taylor cone formation closer to the needle tip. (IV) The volumetric flow rate is decreased (Q = 25 μl/h), resulting in decreased mass delivery rate at the needle tip and optimized cone–jet formation for the prescribed electrical field strength. However, chaotic fiber jet movement is observed close to the collector plate. (IV) In order to eliminate the instabilities observed close to the collector plate, the applied voltage potential is decreased (Vp = 11.5 kV) to yield stable cone–jet formation.

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Fig. 10

Results of a square-wave experiment showing the effect of translational stage speed (UT (mm/s)) on fiber topography and average fiber diameter (Df (μm)). (a) Bright field microscopy images of fiber topographies printed at various stage speeds (magnification: 20× and scale bar: 50 μm). The first aligned fiber is obtained at a critical stage speed (UCR (mm/s)) equal to 83 mm/s. (b) The average fiber diameter is measured for each stage setting. (Error bars denote the standard deviation of mean Df for five distinct sections along the fiber length.)

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Fig. 11

Results of printing studies. (a) Nonwoven mesh printed with NPR,2*  = 31.9, where UT = 25 mm/s < UCR. (b) Mesh printed with NPR,2*  = 57.63, where UT = 85 mm/s ≥ UCR and nonequilibrium conditions occur in the free-flow regime. (c) Woven mesh with 0–90 deg pore architecture. (d) Woven mesh with 0–45–135–90 deg pore architecture. Both woven meshes are printed at optimum NPR,2*  = 106, where UT = 85 mm/s ≥ UCR and steady-state equilibrium condition is reached in the free-flow regime (magnification: 20×and scale bar: 50 μm).

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Fig. 12

Woven mesh with 0–45–135–90 deg pore architecture. The mesh is printed at optimum NPR,2* = 106, where UT = 51.5 mm/s ≥ UCR and steady-state equilibrium condition is reached in the free-flow regime with Vp = 11 kV and Q = 15 μL/h. White, solid line boxes: magnified area in the middle of the mesh. Dashed boxes: areas with disturbed pore shape from inconsistent fiber deposition owing to residual charge entrapped within the printed fibers (scale bar: 100 μm).

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