טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBoas Mor
SubjectStimuli Responsive Polyelectrolyte Complex Electrospun
Fibers
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Professor Eyal Zussman


Abstract

Polyelectrolyte complexes (PECs) are the association formed between oppositely charged polyelectrolytes. Electrostatic interactions between the polyelectrolytes and increased entropy resulting from counterion release, are the main driving forces for such complexation. Increasing the order and packaging of PECs may influence the microstructure and properties of the resultant complex.

The main goal of this work is to enhance PEC packaging and organization by applying an electrical field and extensive elongational flow. Amorphous Poly (acrylic acid) (PAA) and Poly (allylamine hydrochloride) (PAH) were selected as a model system. A mixed solution of these oppositely charged polyelectrolytes was electrospun to form nanofibers. 

Differential Scanning Calorimetry (DSC) analysis of electrospun fibers demonstrated no indication of glass transition. Tg. Infrared spectroscopy (FTIR) analysis of the fibers as a function of temperature, demonstrated an amidation process at lower temperature compared to amidation in cast film. Polarized FTIR indicated a preference of functional groups perpendicular to the fiber axis. X-ray microstructure analysis demonstrated an increase in macromolecules order as implied by narrower major peak, than in cast film. Solid State Nuclear Magnetic Resonance (NMR) revealed that carboxylic acid groups in the as-spun fibers, are free to interact with ammonium functional groups, implying spatial accessibility for intermolecular interactions. These results imply mixed phase fibers with enhanced conditions for intermolecular interactions, due to the highly confined assembly of the macromolecules.

 The fibers underwent reversible, pH-dependent chemo-mechano-morphological deformation. The degree of ionization of PAA at pH 5.5 and pH 1.8 varied from 85% to 18%. This lead to changes of ionic interactions into hydrogen bonding between the functional groups. These chemical changes led to massive water diffusion of 500% by weight, and a marked 400% increase in fiber diameter at a rate of 0.50 μm/s. Under neutral pH, the fibers became stiff and brittle, with elevated elastic modulus and low mechanical deformation, whereas, under low pH, a gel-like assembly formed, featuring a four-fold decrease in Young's modulus and increased elongation of several hundred percent.

Exposing a circumferentially clamped fiber mat to modulated ion flux, generated by applying direct current on an exchange membrane located parallel to the mat, resulted in deformation changes over time at a variable rate. Fiber mat deformation was found to be affected by the combined effects of electro-osmotic flow and variable fibers diameter and stiffness. 

The PAA/PAH fibers mat were also examined for loading capacity and in-vitro drug release behavior of curcumin, a nutrient with low water solubility.  The fibers demonstrated a high efficiency for loading and uniform drug distribution throughout the fiber structure. Under neutral conditions (e.g., human saliva), 30% curcumin release was observed, whereas under acidic conditions (e.g., in the stomach), 60% release was achieved. Fibers were examined in vivo study. Fibers mat was implanted into the abdominal muscle of mice aiming to enhance the mechanical properties. Blood vessel penetration through the mat was not detected, and inflammation was not observed within two weeks. The hybrid structure, muscle with attached fibers mat, presented slightly superior mechanical performance over a stomach muscle at low strain.