In materials science and nanotechnology, a nanobursa (also nanobursa mesh) is a class of hierarchically graded, nanofibrous composite material in which sequential layers of porous polymeric nanofibers encapsulate carbon nanotubes (CNTs) that are surface-functionalized with distinct metal nanoparticles in each layer. The name is derived from the Latin bursa ("sac" or "pouch"), reflecting the encapsulating, sac-like relationship of the polymeric shell around the nanotube core. The material was introduced in 2014 by researchers at the Stevens Institute of Technology (Hoboken, New Jersey), with Dilhan M. Kalyon as the corresponding author and principal investigator.
Nanobursa meshes are fabricated by a hybrid process combining twin-screw extrusion with electrospinning, enabling continuous, industrially-scalable production. Demonstrated realizations include graded layers bearing palladium (Pd), cobalt (Co), silver (Ag), and platinum (Pt) nanoparticles. Principal targeted application domains are heterogeneous catalysis (notably hydrocarbon oxidation), filtration, and tissue engineering scaffolds.
The term nanobursa is a portmanteau of the prefix nano- (from Greek nanos, dwarf; denoting the nanometer length-scale of the constituent fibers and tubes) and the Turkish noun bursa (pouch, purse). The anatomical analogy is apt: just as a bursa is a fluid-filled sac that cushions and encloses anatomical structures, each fiber of a nanobursa mesh encloses and protects a CNT core while simultaneously presenting a functionalized interface to the surrounding medium. The qualifier mesh refers to the nonwoven, interlaced architecture of the electrospun fiber mat.
The associated analytical representation used to characterize the rheological behavior of the precursor polymerâÂÂCNT suspension and, by extension, to optimize the electrospinning process, is termed the MooneyâÂÂKalyon plot (by analogy with the MooneyâÂÂRivlin plot of rubber elasticity). In this linearization, a suitably normalized stress measure is plotted against a reciprocal strain variable to extract material constants from slope and intercept, exactly as in the classical MooneyâÂÂRivlin framework applied to elastomers.
Conventional electrospinning operates by applying a high electric field (typically 10âÂÂ30 kV over a 10âÂÂ20 cm gap) across a polymer solution or melt delivered through a capillary needle, drawing a charged jet that attenuates into fibers collected on a grounded substrate. It lacks inherent capacity for solids conveying, compounding, melting of high-viscosity resins, or controlled dispersion of nanoparticulate fillers.
The TSEE process, pioneered at Stevens Institute, integrates a co-rotating twin-screw extruder as the front end of an electrospinning system. The extruder provides:
The multi-nozzle spinneret is necessary to achieve throughputs at industrially relevant rates, since the volumetric flow rate per nozzle in electrospinning is severely limited by hydrodynamic and electrostatic stability constraints (see çElectrospinning jet stability below).
Graded nanobursa meshes are formed by sequentially collecting layers produced from different CNT-suspension feedstocks, each bearing a distinct metallic nanoparticle type, on the same rotating drum collector. This yields a stratified, functionally graded architecture.
Before incorporation into the polymer melt, multi-walled CNTs (MWCNTs) are surface-functionalized. A representative sequence is:
The resulting nanoparticle-on-tube structures are then dispersed in a polymeric carrier (e.g., polycaprolactone, PCL) prior to TSEE processing.
A nanobursa mesh occupies multiple length scales simultaneously:
The porosity of the nonwoven mat is governed by the fiber diameter distribution and mat basis weight, and typically falls in the range 60âÂÂ90% void fraction, providing high specific surface area and low mass-transfer resistance.
The transition from a steady cone-jet (Taylor cone) to an electrically driven bending instability determines the final fiber diameter. The electric field strength E at the capillary tip of radius R charged to potential V relative to a collector at distance d is approximated by
The critical condition for jet ejection (Taylor cone formation) requires that the Maxwell stress exceed the surface tension restoring force:
where is the permittivity of free space, is the polymer solution surface tension, and is the jet radius at the onset of instability. Solving for the jet radius:
The final fiber diameter , accounting for viscoelastic stretching and solvent evaporation, scales as
where is the volumetric flow rate per nozzle, is the apparent viscosity of the spinning solution, and is the applied electric field. This relationship predicts that increasing the applied voltage (raising E) or reducing the flow rate reduces the fiber diameter, consistent with experimental observations on PCLâÂÂCNT nanobursa precursor suspensions.
The precursor suspension of CNTs dispersed in a polymer carrier is a non-Newtonian fluid. For concentrated suspensions exhibiting a yield stress , the HerschelâÂÂBulkley model is appropriate:
where is the flow consistency index, is the flow behavior index ( for shear-thinning behavior typical of CNT suspensions), and is the magnitude of the rate-of-strain tensor .
For the twin-screw extrusion stage, the relevant dimensionless number is the Deborah number
where is the terminal relaxation time of the melt, is the characteristic shear rate imposed by the screw, is a characteristic length of the die/nozzle, and is the mean velocity. Optimal dispersion of CNT agglomerates requires in the kneading zones.
By analogy with the linearization introduced by Melvin Mooney (1940) for rubber elasticity, wherein a normalized stress plotted against (reciprocal stretch) yields the material constants and from intercept and slope respectively, an analogous linearization termed the MooneyâÂÂKalyon plot, is introduced for the rheological characterization of nanobursa precursor suspensions undergoing steady simple shear.
Define the reduced apparent viscosity
and the reciprocal shear rate variable
Then, for a HerschelâÂÂBulkley fluid,
Taking logarithms:
A plot of versus yields a straight line whose slope gives and whose intercept gives . This is the MooneyâÂÂKalyon plot for nanobursa precursor suspensions.
Formally, the analogy with the MooneyâÂÂRivlin framework for hyperelastic solids is structural: just as the MooneyâÂÂRivlin plot
extracts the constants (intercept) and (slope) governing the strain energy density function
the MooneyâÂÂKalyon plot extracts the rheological parameters and governing the viscoplastic constitutive equation of the CNT-laden nanobursa precursor. In both cases the key insight is a linearization that reduces a nonlinear constitutive relationship to a straight-line fit on suitably chosen axes, with slope and intercept directly yielding the material constants.
The primary demonstrated application of the nanobursa mesh in catalysis is the oxidation of hydrocarbons. A generic complete oxidation reaction is
The catalytic rate per unit geometric area of the nanobursa mesh is modeled using a LangmuirâÂÂHinshelwood mechanism. Denoting the hydrocarbon partial pressure as and oxygen partial pressure as , the surface reaction rate (mol mâ»ò sâ»ù) is
where is the surface rate constant following an Arrhenius temperature dependence
with the pre-exponential factor, the activation energy, the universal gas constant, and absolute temperature. and are adsorption equilibrium constants for the hydrocarbon and oxygen, respectively.
The effectiveness factor of the nanobursa mesh accounts for intrafiber mass-transfer limitations. For a flat slab geometry of half-thickness ,
where is the Thiele modulus
Here is the specific surface area of the metallic nanoparticles per unit volume of the fiber (m<sup>2</sup> mâ»ó), and is the effective diffusivity of the reactant within the porous fiber, estimated using
where is the bulk molecular diffusivity and is the Knudsen diffusivity with the mean pore diameter and the molar mass of the diffusing species.
The overall volumetric conversion rate in a differential reactor element of mesh volume is
where is the mesh void fraction and is the skeletal density of the fiber material.
The performance of a multi-layer nanobursa mesh (with layers, each bearing a different metal at concentration ) can be described by a transfer-matrix formalism. Let denote the state vector (reactant concentration, temperature) at the inlet of layer . Then
where is the layer transfer operator encoding the species and energy balances for that layer. The overall performance objective (e.g., total conversion ) is
Optimization of the layer sequenceâÂÂincluding the choice of metal identity, nanoparticle loading, fiber diameter, and void fraction for each layerâÂÂis a combinatorial problem that can be cast as
where is the mass of layer and is the total pressure drop across the mesh, approximated for the fibrous medium by the KozenyâÂÂCarman equation:
where is the fluid dynamic viscosity, is the superficial velocity, and is the total mesh thickness.
The elastic modulus of an electrospun nonwoven mat composed of isotropically distributed fibers of modulus and volume fraction follows from the Cox shear-lag model adapted to fibrous networks:
where is the fiber segment length between junctions, and
with the shear modulus of the surrounding medium (here effectively air, so and the Cox correction reduces to unity for isolated fibers), the mean center-to-center inter-fiber spacing, and the fiber radius.
The addition of MWCNTs (at weight fraction ) to the polymer matrix increases both fiber modulus and ultimate strength. A rule-of-mixtures-based HalpinâÂÂTsai estimate for the fiber modulus gives
where is the CNT volume fraction, is the shape factor proportional to the CNT aspect ratio, and
Experimental uniaxial tensile data for PCL meshes have confirmed an increase in ultimate tensile strength from approximately 0.47 MPa (pure PCL) to 0.79 MPa upon incorporation of inorganic nanoparticles at 35 wt%, consistent with the HalpinâÂÂTsai trend.
For filtration applications (including antiviral or antimicrobial meshes), the single-fiber efficiency of a cylindrical fiber of diameter collecting particles of diameter is given by the sum of independent capture mechanisms:
where:
The overall fractional penetration of the nanobursa mesh of thickness is
and the filtration efficiency is . The presence of metal nanoparticles (notably Ag) on the CNT surface contributes an additional biocidal mechanism that does not appear directly in the mechanical filtration equation above.
The primary application demonstrated in the original nanobursa publication is catalytic hydrocarbon oxidation. The graded architecture allows sequential or cooperative catalytic action: for example, a Pd-rich upstream layer initiates partial oxidation, while a downstream Pt-rich layer drives complete combustion to COâ and HâÂÂO. The mathematical framework for this is the layer transfer operator formalism described above.
By incorporating antiviral nanoparticles (Ag, Au, Pt, Pd) into electrospun PCL nanofibers produced by TSEE, nanobursa type meshes have been proposed as high-performance membrane layers for N95-class respirators. The sub-200-nm fiber diameters accessible via TSEE yield higher single-fiber efficiencies in the most penetrating particle size range (MPPS, approximately 100âÂÂ300 nm) compared with conventional melt-blown polypropylene webs (fiber diameter typically 1âÂÂ10 üm).
The TSEE platform that underlies nanobursa fabrication has been applied to the production of graded tissue engineering scaffolds, including:
In biomedical contexts the encapsulation of CNTs within the polymeric shell of the nanobursa fiber serves the dual purpose of exploiting CNT mechanical reinforcement while limiting direct biological exposure to bare CNT surfaces, which carry cytotoxicity concerns.