Imaging in Stem Cell Transplant and Cell-based Therapy

26

2.4.4 Polymer Nanoparticles

Polymer nanoparticles can be used in stem cell tracking when conjugated with fluo-
rescent organic dyes. The dye can be entrapped in the polymer interior (similarly to
silica nanoparticles), or covalently bound to the polymer chain before the nanopar-
ticle is prepared by dispersion of preformed polymers or polymerization of mono-
mers [ 16 ]. Polymer nanoparticles can be formed as dendrimers, microgels and
modified polysaccharide nanoparticles, however the most common fluorescent
polymer nanoparticle is the polystyrene nanoparticle (Fig. 2.6).
Studies have shown that mesenchymal stem cells uptake polystyrene nanoparti-
cles, however the stability of the dye molecules is sub-optimal with clear evidence
of photo-bleaching, quenching and leaching [ 74 ]—thus polymer nanoparticles still
have some way to go before being applicable to human long-term stem cell track-
ing. They have, however, demonstrated an aptitude for delivering genes and proteins
into stem cells, in acting as carriers for transplanted stem cells, as well as drug
delivery using stem cells as vehicles [ 66 ]. Polymers can provide structural support
for stem cells that are introduced to the body, and control the biomolecules released
for modulation of stem cells in vivo [ 75 , 76 ]
Polymer nanoparticles are most suitable for in vivo imaging with fluorescence
imaging.

a QD@SiO 2

bcd

QD@SiO 2 @lipids

Octadecane

Gd-DTPA-BSA

(paramagnetic lipid)

PEG-DSPE

(PEGylated lipid)

QD@SiO 2 @octadecane

Fig. 2.5 Overview of the intermediate steps involved in the synthesis of lipid-coated, quantum dot
(QD)-containing silica nanoparticles. Silica nanoparticles are first made hydrophobic by cova-
lently attaching octadecane chains to the silica surface. Subsequently, paramagnetic Gd–DTPA–
DSA and poly(ethylene glycol)-containing PEG–DSPE lipids are applied to coat the hydrophobic
QD@SiO2 nanoparticles with a monolayer of lipids. This nanoparticle platform serves as a bio-
applicable, multimodal contrast agent for MRI and fluorescent techniques. Reprinted with permis-
sion from van Schooneveld et al. [ 73 ]

H.A. Jensen et al.