Microporous scaffolds assembled from microgel building blocks for hMSC delivery and bone tissue engineering
Shangjing Xin1, Omar M. Wyman1, Daniel L. Alge1, 2.
1
Department of Biomedical Engineering, Texas A&M University.
2
Department of Materials Science and Engineering, Texas A&M University.
Statement of Purpose: There is a clear clinical need for
increased as microgels assembled together. These results
the development of biomaterials to deliver therapeutic
verified that sequential thiol-ene click chemistry was
cells such as human mesenchymal stem cells (hMSCs) for
successful to assemble microgel building blocks into
bone repair. Hydrogels have been widely studied for this
microporous scaffolds. The cytocompatibility of
purpose because of their ability to encapsulate cells and
microporous scaffolds was also examined by live/dead
guide their fate. Conventional hydrogels are designed
assay (Fig. 2b-d). hMSCs exhibited a high cell viability in
with degradable crosslinkers to enable cell spreading,
microporous scaffolds from each size microgels, which
migration, proliferation, and tissue deposition. However,
indicated these scaffolds were promising to be developed
decoupling these cellular processes from degradation is a
as cell-instructive materials for further applications.
promising strategy for accelerating healing, because the
microenvironmental cues from hydrogels can be more
precisely controlled. Toward this goal, we introduce here
a microporous hydrogel platform prepared by
photopolymerizing electrosprayed poly(ethylene glycol)
(PEG) microgels into hMSC-laden 3D structures. By
tuning the sizes and stiffness of the microparticles, we
aim to develop cell-instructive scaffolds that enhance cell
spreading, migration, and osteogenesis.
Figure 1 – Size distribution of microparticles prepared using different
molecular weight PEG-Nb and electrospraying parameters and confocal
Methods: Multifunctional PEG-norbornene (PEG-Nb, 4images of 10kDa PEG-Nb microparticles with each size. The scale bar is
arm) and PEG-dithiol (PEG-DT, 3,400 Da) were used to
100 µm.
fabricate microgels via thiol-ene photopolymerization and
submerged electrospraying. Cell adhesive peptide,
CGRGDS, was also incorporated in the microparticles
utilizing the thiol groups of cysteine. The ratio of thiol to
norbornene groups was controlled to 0.75:1 so that there
would be free norbornene groups available for subsequent
photopolymerization of the microgels into 3D scaffolds.
Different molecular weights of PEG-NB (5, 10 and 20
kDa) were used to tune the stiffness of microgels. The
electrospraying voltage (V) was adjusted according to the
viscosity of gel solutions and to fit in right
electrospraying mode. Flow rate (Q) and the tip-tocollector distance (d) were adjusted for size control. Asprepared microgels were added into a mold with PEG-DT
and photoinitiator and assembled into microporous
scaffolds via photopolymerization. hMSCs were
incorporated during the assembly process and cultured for
up to 7 days. The cytocompatibility of microporous
Figure 2 - a) Plot of storage modulus evolution during
scaffolds were investigated by live/dead staining.
photopolymerization for microparticles prepared from 5kDa, 10kDa and
Results: Fig.1 shows the microgel sizes and morphology
20kDa PEG-Nb; b-d) Live/dead assay of hMSCs encapsulated in the
under different electrospraying parameters. Microparticles
scaffold pores after 1-day culture. Green represents live cells and red
represents dead cells. The scale bar is 100 µm.
with a size range of 100- 500 µm were obtained through
electrospraying. The spindle mode of electrospraying was
Conclusions: PEG electrosprayed microgels with tunable
used to fabricate microgels by adjusting the voltage. The
size were prepared via thiol-ene photopolymerization and
sizes of microgels were successfully tuned by varying
submerged electrospraying. These microgels were
flow rates. Since the microgel sizes were important in
successfully assembled into microporous scaffolds using
determining pore sizes and surface-to-volume ratios of
sequential thiol-ene click photopolymerization. hMSCs
scaffolds after assembly, these results indicated that we
preferred to grow and spread within stiffer scaffolds.
were able to tune the inner pore structure of microporous
Future studies will investigate the use of these scaffolds
scaffolds.
as cell-instructive materials for hMSC delivery and bone
During the microgel assembly, the kinetics of
tissue engineering.
photopolymerization was monitored by measuring the
modulus change during shear testing on a rheometer (Fig.
2a). After applying UV light, photopolymerization
occurred within 10 seconds and the mechanical properties