Novel Modeling Approach to Generate a Polymeric Nanofiber Scaffold for Salivary Gland Cells

Abstract

BACKGROUND: Electrospun nanofibers have been utilized in many biomedical applications as biomimetics of extracellular matrix proteins that promote self-organization of cells into 3D tissue constructs. As progress towards an artificial salivary gland tissue construct, we prepared nanofiber scaffolds using PLGA, a biodegradable and biocompatible material. METHOD OF APPROACH: We used electrospinning to prepare nanofiber scaffolds using PLGA with both DMF and HFIP as solvents. Using a design of experiment (DOE) approach, system and process parameters were optimized concurrently and their effects on the diameter of the resulting fibers were computed into a single model. A transfer function was used to reproducibly produce nanofibers of a defined diameter, which was confirmed by SEM. The mouse salivary gland epithelial cell line, SIMS, was seeded on the nanofiber scaffolds, and morphology, cell proliferation, and viability were assayed. RESULTS: Varying two or more parameters simultaneously yielded trends diverging from the linear response predicted by previous studies. Comparison of two solvents revealed that the diameter of PLGA nanofibers generated using HFIP is less sensitive to changes in the system and process parameters than are fibers generated using DMF. Inclusion of NaCl reduced morphological inconsistencies and minimized process variability. The resulting nanofiber scaffolds supported attachment, survival and cell proliferation of a mouse salivary gland epithelial cell line. In comparison with glass and flat PLGA films, the nanofibers promoted self-organization of the salivary gland cells into 3D cell clusters, or aggregates. CONCLUSIONS: These data indicate that nanofiber scaffolds promote salivary gland cell organization, and suggest that a nanofiber scaffold could provide a platform for engineering of an artificial salivary gland tissue construct. This study additionally provides a method for efficient production of nanofiber scaffolds for general application in tissue engineering.

Figure 1

Electrospinning overview. a) Basic apparatus, b) close-up view of polymeric jet formation from a droplet at the end of the needle tip and c) the structure of PLGA, showing lactic acid as the n monomer and glycolic acid as the m monomer of the PLGA polymer.

Figure 2

SEM image of PLGA fibers electrospun in DMF as the potential is decreased from (a) 20 kV to (b) 19 kV. The decrease in the potential drastically changes the morphology and diameter of the fibers, leading to the formation of beads at the lower potential. When the distance between the needle and collector is reduced, (c) a decrease in the potential has almost no effect on the diameter of the fiber, but fewer beads were formed, while (d) an increase in the feeding rate resulted in thicker fibers that were bead-free.

Figure 3

Surface plots of predicted average diameter of PLGA fibers electrospun in DMF solution. Fiber diameter (Z) is displayed as a function of X and Y: a) % PLGA and Potential, b) % PLGA and Feeding Rate, c) % PLGA and Spinneret to Collector distance and d) Feeding Rate and Potential.

Figure 4

Effect of HFIP concentration and salt additives on the diameter and morphology of nanofibers. Increasing the PLGA concentration without any salt additives from a) 5wt% to b) 7wt% then c) 9wt%, increases the diameter slightly, however beads are present at all 3 concentrations. d) Addition of 1% MgSO₄ to an 8.5wt% solution did not significantly decrease beading. e) 1% NaCl in an 8.5wt% solution completely eliminated beading and yielded homogenous thin fibers.

Figure 5

3D Surface plots of predicted average diameter of PLGA fibers electrospun in HFIP solution. Fiber diameter (Z) is displayed as a function of X and Y: a) % PLGA and Potential, b) % PLGA and spinneret-to-collector distance, c) % PLGA and Feeding rate and d) Potential and spinneret-to-collector distance.

Figure 6

Fibers generated from PLGA-HFIP solutions with 1% NaCl using calculated parameter values from the optimized transfer function. a) Target fiber diameter of 250 nm, actual mean 247.2 nm, parameter settings: 8 wt% PLGA, 10 kV potential, 15 cm spinneret-to-collector distance, 5 μl/min feeding rate. b) Target fiber diameter of 1200 nm, actual mean 1276.5 nm, parameter settings: 18 wt% PLGA, 12kV potential, 15 cm spinneret-to-collector distance, 16 μl/min feeding rate.

Figure 7

PLGA nanofibers can support the survival, growth and 3D-like epithelial morphology of salivary gland cells. a) Proliferation assay for SIMS submandibular cells cultured on flat glass, PLGA flat films, or PLGA nanofibers for the indicated time points, detached and counted using a hemocytometer. b) Cell viability on different surfaces was assayed using trypan blue staining and hemocytometer counting at the indicated time points. c) SIMS cells were seeded Seeded on flat glass, PLGA flat films, or PLGA nanofiber for 48 hrs. were stained with calcein to detect live cells (1μM, 488nm excitation) and ethidium homodimer-1 to stain dead cells (2μM, 543 nm excitation). Images were obtained using a Zeiss CellObserver microscope. Scale, 20μm. d) SIMS salivary gland cell morphology on multiple surfaces. SIMS cells were cultured overnight on flat glass, flat PLGA, and PLGA nanofibers (average diameter ~250 nm), respectively, and images of fixed cells were captured by SEM. Scale, 10μm.