media (as opposed to the organic solvents required for radio-
labeling with 18F), at pH ꢀ7, and at near-physiological temper-
atures.9
64Cu2+ by 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid
(DOTA) conjugated to the peptide cyclo(Arg-Gly-Asp-DPhe-
Lys)(cyclo(RGDfK)) which is a potential imaging agent in the
detection of cancer via PET.7,12,20–22
Conventional radiolabeling methods for radiometals such as
64Cu2+ typically require the dilution of small quantities of pico-
molar radiometal solution for convenient handling and proper
mixing, resulting in nanomolar concentrations of the radio-
metal.6,10–12 This dilution requires a large excess (ꢀ100-fold
compared to the radiometal concentration) of the potentially
expensive and difficult-to-obtain BFC–BM conjugate to ensure
the desired high percentage of bound radionuclide (>90%) within
a reasonable time (<1 hour). In turn, the use of large excesses of
BFC–BM conjugate necessitates extensive chromatographic
purification to remove unlabeled BFC–BMs and to obtain the
high specific activities that are desirable for application of the
radiopharmaceutical, for example in PET imaging. Chromato-
graphic purification is also potentially required to remove BFC–
BM impurities that may bind more strongly or more quickly to
the radiometal than the desired BFC–BM conjugate. For
instance, if the 100-fold excess of BFC–BM contains 1% impu-
rity, then the molar ratio of impurity to radiometal would be
1 : 1, potentially leading to the synthesis of unwanted radio-
metal–ligand complexes.
2
Design and testing
2.1 Design of the microreactor
We present here a PDMS-based, microfluidic reactor for labeling
BFC–BMs with radiometals (Fig. 1). The microreactor consists
of three key elements: (1) a serpentine microchannel for mixing,
in which we defined staggered herringbone grooves19 using soft
lithography; (2) a series of reservoirs for the incubation of the
radiometal–ligand mixture and (3) a thin-film heater for heating
the mixture.
2.1.1 Serpentine mixing channel. The microreactor incorpo-
rates a passive mixer developed by Stroock et al.19 to minimize
diffusive limitations to the overall rate of the radiolabeling reac-
tion. Generally, in the laminar, low Reynolds number flow that
occurs at the microscale, when two streams of reagents are brought
in contact, a depletion zone forms as a result of the consumption
of the reagents at the interface between the two streams. This
depletion zone grows in the transverse direction as the streams flow
axially along the channel. As a result, the reagents must diffuse
across increasingly longer distances in order for the reaction to
proceed. The small scale of microfluidic systems renders them less
susceptible to the growth of large depletion zones than macro-scale
systems (the distance for diffusion, Dr [m], is ultimately limited to
half the width of a microchannel, w/2 z 100 mm). However, the
time scale for diffusion across the channel in microfluidic systems
sD ¼ Dr2/D, where D [m2 sꢁ1] is the diffusivity of the reagent, can
still be significant (on the order of minutes), particularly for large
molecules that have low diffusivities in water (D < 10ꢁ11 m2 sꢁ1),
such as proteins and antibodies, compared to the time scale of the
In this report, we demonstrate the utility of microfluidics to
overcome some of the drawbacks associated with conventional
radiolabeling methods mentioned above. Previous efforts on
microfluidic approaches for the synthesis of radiopharmaceuti-
cals focused primarily on the synthesis of 18F- and 11C-labeled
agents.13–15 For example, Lee et al.16 designed an intricate pol-
y(dimethyl siloxane) (PDMS)-based microreactor for the multi-
step synthesis of 18F-FDG, a radiotracer commonly used to
detect cancer via PET. An improved version of this microreactor
provided yields of 96% after ꢀ15 minutes of total synthesis time,
compared to 75% yield after ꢀ45 minutes using an automated
apparatus for conventional synthesis.17 However, an off-chip
purification step was still required to give a radiochemical purity
of ꢀ99%, and loss of 18F through reaction with or absorption by
the PDMS was noted as a significant issue. In other work, Lu
et al. investigated the effect of infusion rate, i.e., residence time,
on the yields of several 18F and 11C-labeled carboxylic esters using
a T-junction, flow-through glass microreactor.18 Of relevance to
our work, they observed that incomplete diffusive mixing at
lower residence times adversely affected the yield.
reaction, sR ¼1/kC0 , where k [Mꢁn sꢁ1] is the reaction rate
n
constant, C0 [M] is the initial concentration of the reagent, and n is
the order of the reaction. Thus, depletion zones can impose dif-
fusive mass transport limitations on the rate of a reaction that
occurs in a microchannel and should be avoided, particularly for
high-throughput reactions with fast kinetics.
We introduce passive mixing of reagents, generated by stag-
gered herringbone grooves,19 to further reduce, and potentially
eliminate entirely, the limitation to reaction kinetics caused by
depletion zones. The staggered herringbone grooves in the mix-
ing channel induce chaotic stirring in the cross-section of the flow
that stretches and folds the interface between the co-flowing,
laminar streams. This stretching and folding reduces the max-
imum distance, Dr, that the solutes in the initially separate
streams must diffuse in order to form a homogeneous mixture19
and to react, resulting in smaller depletion zones, and thus a
reduced potential for diffusive mass transport limitations on the
rate of reaction. The maximum diffusive distance for chaotic
stirring can be approximated by, Dr ¼ w/2 exp(ꢁDy/l), where w
[m] is the width of the microchannel, Dy [m] is the distance
traveled along the axis of the microchannel, and l [m] is a char-
acteristic length determined by the geometry of the trajectories of
the chaotic stirring.19 Based on this equation and the estimated
Here we report on the design, fabrication, and validation of a
PDMS/glass-based microreactor for radiolabeling biomolecules
with 64Cu2+ or other radiometals that exploits several key attributes
of microfluidics. (1) The ability to manipulate small volumes of
reagents (from mL to pL), which eliminates the need for the dilu-
tion of radionuclides, potentially obviating the need to use excess
ligand and the associated chromatographic purification steps. (2)
The overall small size of the microreactor and peripherals, which
reduces space requirements and radioactivity shielding costs. (3)
The small volumes of reactants and the small dimensions of the
reactor, which sidestep some of the heat and mass transport
limitations encountered in macro-scale radiolabeling. Here we also
use staggered herringbone grooves19 to passively mix reagents and
thereby further reduce mass transport limitations. We successfully
validate the microreactor-based approach by comparison with
conventional procedures for radiolabeling using the chelation of
3388 | Lab Chip, 2010, 10, 3387–3396
This journal is ª The Royal Society of Chemistry 2010