Chemically Triggered Coalescence and Reactivity of Droplet Fibers

Article abstract of DOI:10.1021/jacs.1c02576

We describe the role of functional polymer surfactants in the construction and triggered collapse of droplet-based fibers and the use of these macroscopic supracolloidal structures for reagent compartmentalization. Copolymer surfactants containing both zwitterionic and tertiary amine pendent groups were synthesized for stabilization of oil-in-water droplets, in which the self-adherent properties of the selected zwitterions impart interdroplet adherence, while the amine groups provide access to pH-triggered coalescence. Macroscopic fibers, obtained by droplet extrusion, were prepared with reagents embedded in spatially distinct components of the fibers. Upon acidification of the continuous aqueous phase, protonation of the polymer surfactants increases their hydrophilicity and causes rapid fiber disruption and collapse. Cross-linked versions of these supracolloidal fibers were stable upon acidification and appeared to direct interdroplet passage of encapsulants along the fiber length. Overall, these functional, responsive emulsions provide a strategy to impart on-demand chemical reactivity to soft materials structures that benefits from the interfacial chemistry of the system.

Full text of DOI:10.1021/jacs.1c02576

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Chemically Triggered Coalescence and Reactivity of Droplet Fibers
Jing Zhao, Zehao Pan, Deborah Snyder, Howard A. Stone, and Todd Emrick*
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ABSTRACT: We describe the role of functional polymer surfactants in the construction and triggered collapse of droplet-based
fibers and the use of these macroscopic supracolloidal structures for reagent compartmentalization. Copolymer surfactants
containing both zwitterionic and tertiary amine pendent groups were synthesized for stabilization of oil-in-water droplets, in which
the self-adherent properties of the selected zwitterions impart interdroplet adherence, while the amine groups provide access to pH-
triggered coalescence. Macroscopic fibers, obtained by droplet extrusion, were prepared with reagents embedded in spatially distinct
components of the fibers. Upon acidification of the continuous aqueous phase, protonation of the polymer surfactants increases their
hydrophilicity and causes rapid fiber disruption and collapse. Cross-linked versions of these supracolloidal fibers were stable upon
acidification and appeared to direct interdroplet passage of encapsulants along the fiber length. Overall, these functional, responsive
emulsions provide a strategy to impart on-demand chemical reactivity to soft materials structures that benefits from the interfacial
chemistry of the system.
utilized hinge on the presence of hydrophilic groups, in the
form of zwitterions and amines, strung along a hydrophobic
polymer backbone. While zwitterion-stabilized emulsions and
mixed small molecule interfacial assemblies have been
reported,21−23 polymer approaches are advantageously versa-
tile as they offer tunable extents of functionality and molecular
weights that tailor fluid−fluid interactions. In our system,
structural responses to solution stimuli are dictated by the
selected sulfobetaine zwitterions, which are sensitive to ionic
strength, and the tertiary amines, which are pH-responsive.
This zwitterion/amine combination yielded fiber-like supra-
colloidal assemblies of polymer-stabilized droplets that
functioned as multiresponsive macrostructures. Notably, a
pH-triggered self-coalescence, based on altering the interfacial
tension of the polymer, initiated reactions among reagents that
were initially compartmentalized. Additional insight into
interdroplet communication, enabled by the interfacial contact
INTRODUCTION
■
Polymer-stabilized emulsion droplets are long-studied and
have a wide range of ongoing and emerging applications that
span industrial and consumer products, personal care, and
delivery systems.1 As “soft solids”, emulsions create oppor-
tunities for assembly into structures with tunable rheological
properties.2 The interdroplet interactions necessary for
structure formation arising from functional surfactants at the
fluid−fluid interface may involve H-bonding,3−5 electrostatics,6
or other modes of noncovalent connection.7−9 However, many
of these assembly methods are irreversible, or practically so,
such that new discoveries are needed whereby weak, collective
chemical interactions are employed strategically in macro-
structure design. Such reversible assembly represents a balance
between the initial stability required for structure generation
and control over emulsion disruption by employing pH,10−12
temperature,13,14 light,15 or other stimuli. Microfluidic designs
represent one method for controlling droplet contact and
driving coalescence by application of force (mechanical,16,17
electrical,18,19 or chemical20) but are limited by their relatively
sophisticated designs and scale-up challenges.
Received: March 11, 2021
Published: April 1, 2021
In this paper we describe multifunctional polymers at fluid−
fluid interfaces that provide new routes to chemically triggered
droplet coalescence and collapse of soft solids in fluidic
environments. The interfacial properties of the polymers
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of “sticky droplets”, is realized by using these responsive
polymeric interfacial reagents.
amine protonation as the solution was acidified, with an abrupt
change at pH ∼ 9, while the SB-zwitterions were unaffected
across the investigated pH range (a conductivity titration
measurement showed a similarly abrupt transition in this pH
range (Figure S2)). Copolymer solution behavior in salt water
proved more complex, since salt simultaneously impacts
electrostatic screening of both the zwitterions and amines, by
(1) disrupting inter-zwitterion interactions24−27 (to increase
solubility) and (2) screening charge repulsion28 (to decrease
solubility) (Figure 1c). At pH 9.2, where the polymer is
charge-neutral, turbidity measurements suggested “anti-poly-
electrolyte” behavior typical of SB-polymers,24 characterized by
insolubility at very low salt concentration and rapidly
increasing solubility at >50 mM salt. This behavior contrasted
with the more complex behavior of PZA-50 at pH 6.6, when
the amines are protonated, where solubility at low salt
concentration was lost at higher concentrations, a “salting-
out” effect resulting from polyelectrolyte screening and
hydrophobic interactions. However, for the same polymer,
further increasing salt concentration to 300 mM recaptures
clean solution transmittance as the polymer is “salted-in”, since
at this stage the SB-zwitterion component dominates solution
behavior. Such a U-shaped optical transmission curve
qualitatively resembles examples of charge-unbalanced poly-
ampholytes (i.e., with positive and negative charges on separate
monomer units),29 though we are not aware of related findings
for polymer zwitterions. Similar solution studies performed by
using PZA-30 and PZA-67 showed excellent control over the
impact of solution properties on structure by adjusting the SB-
to-amine ratio. In accord with this rationale, polymers with
lower SB percentages required higher salt concentration to
achieve transparent solutions (Figure S3b); this is reflected in
the transmittance data for samples studied under acidic
conditions, in which transitions to optical clarity shifted to
higher salt concentration for polymers with lower SB content
(Figure S3c). Notably, the U-shaped curve describing salt
response depended on salt composition, as presented in Figure
S4 for PZA-50. Within the conventional Hofmeister series,30,31
salts to the right of NaNO3 (e.g., NaBr and NaI) produced U-
shaped transmittance, while those to the left (e.g., NaCl,
sodium acetate, and Na2SO4) solubilized PZA-50 across the
entire concentration range.
RESULTS AND DISCUSSION
■
Polymer Synthesis and Fundamental Solution Prop-
erties. The polymeric zwitterionic/amine (PZA) copolymers
illustrated in Figure 1 integrate both sulfobetaine (SB)
Figure 1. (a) Chemical structure of PZA copolymers containing both
zwitterionic (Z) and amine (A) functionality. (b) Plot of trans-
mittance as a function of pH for PZA-50. (c) Plot of transmittance as
a function of [salt] at pH 6.6 and 9.2. (d) Plot of transmittance as a
function of temperature at pH 6.9 with [salt] = 150 mM (black) and
9.7 (red). [Polymer] = 1 mg/mL in parts b−d.
zwitterions and tertiary amines (quaternary ammoniums)
onto a polystyrene backbone. These copolymers were
synthesized by reversible addition−fragmentation chain-trans-
fer (RAFT) polymerization using 4-cyano-4-[(dodecylsulfanyl-
thiocarbonyl)sulfanyl]pentanoic acid as the chain-transfer
agent and 4,4′-azobis(4-cyanovaleric acid) (ACVA) as the
free radical initiator.24,25 Performing the polymerization in a
water/2,2,2-trifluoroethanol (TFE) mixture at 70 °C con-
tributed monomer and polymer solubility throughout the
course of the polymerization. Continuing the polymerizations
for ∼24 h produced >90% conversion of each monomer, with
the polymer products having substantial molecular weights
(∼40 kDa) and relatively narrow polydispersity index (PDI)
values (∼1.1−1.2), as estimated by gel permeation chromatog-
raphy (GPC). The polymers were isolated as white solids in
gram quantities following dialysis and lyophilization, and the
ratio of monomers integrated into the polymer products was
Combining copolymer protonation levels with temperature
afforded further control over solution transitions, as shown in
Figure 1d. At pH 6.9 and 150 mM NaNO3(aq), room
temperature insolubility quickly yielded to a transparent
solution at >35 °C; in contrast, with neutral amines at pH
9.7, the cloud point was ∼30 °C higher. Irrespective of amine
protonation, salt-induced interruption of inter-zwitterionic
interactions reduced the cloud point well below that of the
SB-based homopolymer (25% transmittance at 90 °C).
Preliminary evidence for additional interesting phase behavior
of these zwitterion/cationic macromolecules is shown in
Figure S5, in which an apparent liquid−liquid phase separation
in water produces micrometer-sized structures that resemble
polymer-based coacervates, structures that typically result from
combining oppositely charged polyelectrolytes at high salt
concentration.32
1
judged by H NMR spectroscopic integration of the benzylic
SB and amine methylene groups at δ 4.5 and 4.2 ppm,
respectively. The incorporated monomer ratios corresponded
closely to the feed ratios employed, affording products with
∼30, 50, and 67 mol % SB groups, denoted PZA-30, PZA-50,
and PZA-67, respectively. Figure S1 and Tables S1 and S2
summarize the characterization of these copolymers.
The fundamental solution properties of PZA polymers were
probed by turbidity measurements as a function of pH, salt
concentration, and temperature, with key characterization data
given in Figures 1b−d for the example of PZA-50. At pH > 9,
when the amines are neutral, the polymer solution appeared
turbid due to insolubility of the SB component in salt-free
water (Figure 1b).24 Transmittance increased rapidly with
PZA-Induced Droplet Adhesion and Triggered Tran-
sitions. The fundamental solution properties described above
were used to underpin our study of macrostructures built from
PZA-stabilized droplets. Figure 2a shows pendant drop
tensiometry measurements of PZA-50 at the oil−water
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in which droplets coalesced at low pH, since the relatively low
zwitterion content could not maintain sufficient surfactant
properties. For PZA-50, the nonadhesive-to-adhesive tran-
sition evident in Figure 2b hinges on the amine protonation
state: a flowing emulsion in a glass vial turned visibly sticky
upon addition of base, with markedly larger interdroplet
contact angles reflecting stickiness (Figure S7b). Rheological
measurements of these emulsions were performed by using a
stress-controlled rheometer equipped with a parallel plate
geometry (plate diameter of 50 mm) and an arithmetic average
roughness (Ra) of the plates of 6−7 μm to minimize wall slip.
The rheology results reflected our visual observations: about 3
times greater elastic modulus (G′) and 4 times greater viscous
modulus (G″) were seen for the sticky emulsions, with a
corresponding increase in crossover shear strain (Figure 2c).
Interestingly, for PZA-30 and PZA-50, acidifying the sticky
emulsions resulted in rapid droplet coalescence rather than
droplet dispersion (Figure 2b and Figure S6c). This is likely
due to the large interfacial tension change upon polymer
protonation (∼15 mN/m for PZA-50) and the associated
changes in pressure (proportional to interfacial tension) in the
Plateau borders of the droplets.33,34 In sharp contrast, for PZA-
67-stabilized droplets, acidifying the adhesive emulsion
resulted in clean droplet dispersion rather than coalescence
(Figure S6c) because of the high zwitterionic content that
dominates fluid−fluid interfacial interactions and leads to a
smaller interfacial tension change (∼6 mN/m).
The U-shaped transmittance vs salt concentration profile
seen for PZA-50 (Figure 1c) translated to the observed droplet
properties. For droplets prepared under acidic conditions, a
distinct transition of nonsticky to sticky to nonsticky (Figures S8a
and S9b) was observed, in agreement with polymer solubility
trends. This finding is particularly interesting, since reversing
the first transition does not require dilution with large volumes
of fluid. Likewise, the thermal solution properties of the
copolymers proved impactful on droplet interfacesat both
high and low pH, heating sticky emulsions above the cloud
point of their constituent polymers caused “melting” of the
emulsion constructs and allowed remolding or reconfiguration
of a given droplet structure (Figures S8c and S9c). These
reconfigurations occurred smoothly, suggesting excellent
thermal and salt stability of PZA-50-stabilized droplets and
the potential for building reconfigurable structures from them.
It is worth noting that these findings bear a conceptual
relationship to recently described systems exhibiting “reentrant
gelation”,35,36 in which stimulus (usually temperature) effects a
gel to sol to gel transition.
Figure 2. (a) PZA-50 (red and blue), SB-substituted polystyrene, and
their measured interfacial tension (IFT) values. (b) Schematic
illustration, optical micrographs, and photographs of emulsions
stabilized by PZA-50 (scale bars = 100 μm). (c) Viscoelasticity of
PZA-50 as a function of the oscillatory shear strain at a fixed
frequency of 10 rad/s. [Polymer] = 10 mg/mL; oil = TCB; oil phase
fraction (φoil) = 0.63 (pH 6.4) and (φoil) = 0.67 (pH 9.6).
interface (oil = trichlorobenzene (TCB)), in which the neutral
polymer produces a much larger reduction in interfacial
tension (σ = 3.6 mN/m) relative to the more hydrophilic
polyelectrolyte (σ = 19.0 mN/m), which is shown in Table S3.
While both the neutral and cationic forms of PZA-50 stabilized
oil-in-water droplets, such modulation of pH-induced inter-
facial tension allowed on-demand transition from self-adhesive
(neutral polymer surfactant) to nonadhesive (cationic
surfactant) droplets (Figure S6 provides data for the range of
polymers studied). Typically, PZA-stabilized droplets were
prepared by vortexing (1200 rpm for ∼1 min) a mixture of oil
(e.g., TCB), water, and polymer (∼10 mg/mL). Optical
microscopy images (Figure 2b) confirmed the formation of
spherical droplets, 50−200 μm in diameter, formed at pH 6,
with stability aided by electrostatic repulsion of the cationic
component. At pH 12, the droplets were smaller (10−100 μm
diameter) and clustered into dense piles, an indication of
apparent interdroplet adhesion that has macroscopic implica-
tions for structure formation and interdroplet communication
(vide infra). Importantly, the surfactant properties of PZA-50
persisted over a wide pH range, in contrast to the narrow pH
window associated with numerous responsive polymer
surfactants.10−12
PZA Droplet MacrostructuresPreparation and
Reagent Compartmentalization. The pH-induced fluidic-
to-adhesive transition of PZA-50-stabilized droplets makes
them uniquely suited for printing soft structures, confining
reagents, and enabling transport along a supracolloidal
pathway. As seen in Figure 3a, attempts to print droplets
from nonsticky (pH 7) conditions failed to produce robust
structures, whereas printing the droplets into a basic solution
produced structures that maintained their shape even upon
agitation of the water. Interestingly, extrusion of droplets in an
already sticky state (i.e., starting from basic conditions) into
aqueous base afforded gels containing discrete, observable
fibers but lacking fiber-to-fiber adhesion and therefore
rupturing into smaller fibers upon agitation. PZA-50 droplets
were also amenable to vertical construction, shown in the
towers in Figure 3b, in which the injected droplets assumed the
Emulsion droplets stabilized by PZA-50 appear nonadhesive
at low pH due to electrostatic repulsion from the cationic
component but adhesive when the amines are neutral, where
hydrophobic and inter-SB interactions dominate droplet
behavior (Figure S6). Notably, the zwitterion-to-amine ratio
determined interfacial behavior, seen for example in PZA-30,
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Figure 3. Photographs of emulsion-based gels assembled from PZA-
50-stabilized droplets. (a) Plastic UMASS mold filled with oil droplets
in water: nonsticky emulsion injected into water (top) and sticky
emulsion injected into aqueous base and then agitated (bottom). (b)
Vertical assembly of emulsion droplet towers and disassembly by
contact with aqueous acid or salt water. [Polymer] = 10 mg/mL; oil =
TCB; oil phase fraction (φoil) of nonsticky emulsion (φoil) = 0.63 (pH
= 6.4); for sticky emulsion, φoil = 0.67 (pH = 9.6).
shape of the plastic container and maintained that shape
following removal of the mold, benefiting from preferable
interdroplet interactions over adhesion to the plastic walls.
Notably, these emulsion gels formed instantly and did not
require additives to maintain their structures.37 The towers
stood in water for 1 day without noticeable change and
remained stable until contacting aqueous acid or salt solutions
(e.g., 500 mM NaNO3) that caused them to “melt” by
coalescence (from acid) or droplet dispersion (from salt
water).
To prepare multisegment droplet fibers, sequential stacking
of adhesive droplets in a capillary tube (i.d. ∼ 1500 μm) was
followed by gentle extrusion into water by applying pressure
through an inserted needle.38 This allowed for compartmen-
talization of encapsulated components (e.g., dyes, reagents,
nanoparticles, etc.) within predefined segments spaced along
the length of the fiber, which was maintained after extrusion, as
shown in Figure 4. The amines of PZAs gave access to pH-
triggered fiber coalescence, whereby addition of aqueous acid
to the yellow end of the fiber in Figure 4a led to its rapid (∼30
s) and complete conversion into one (or a few) large
droplet(s) (see Movie S1 for visualization of triggered
coalescence). The fluorescence of the fluid interface after
coalescence (seen in experiments employing FITC-labeled
PZA-50) suggests that the protonated PZA ultimately
stabilizes the fluid−fluid interface after fiber coalescence
(Figure S10).
Figure 4. (a) Self-coalescence of multisegment supracolloidal fiber.
(b) Fluorescence quenching in a two-segment fiber. (c) Thiol−ene
addition following collapse of a three-segment fiber. Experimental
conditions: in (a−c), [polymer] = 10 mg/mL; in (a), the oil phase =
TCB with 1 mg/mL Dispersed Red 13 or 1 mg/mL Coumarin 153; in
(b), the oil phase is TCB with 0.1 mg/mL Coumarin 153 or 0.5 mg/
mL Au NPs; in (c), the oil phase is α,α,α-trifluorotoluene (1.14 M 1-
pentanethiol; 0.76 M 4-methylstyrene; 0.76 M DMPA).
occurring in the vicinity of the fiber could be utilized to
trigger droplet coalescence and fiber collapse. This was done
by immobilizing glucose oxidase (GOx) to an Au-patterned Si
wafer via a quaternary ammonium-terminated self-assembled
monolayer, as illustrated in Figures S11 and S12. Addition of
D-glucose (20 mg/mL) to the aqueous solution of these
enzyme-containing systems (producing gluconic acid and
hydrogen peroxide) led to slow fiber coalescence over a 24 h
period, while control experiments lacking the immobilized
enzyme caused no change in fiber morphology (Figure S13).
In principle, the triggered self-coalescence that we describe is
useful for designing multisegment microreactors, which we
demonstrate for reagents encapsulated in the droplet
constituents of PZA-50-stabilized fibers. For example, a fiber
in water with distinct segments of dye (Coumarin 153) and Au
NPs, each encapsulated in oil-in-water emulsion droplets (oil =
TCB), collapses after addition of acid (Figure 4b).
Coalescence-induced mixing of these components quenched
We note that a similarly triggered fiber coalescence was
observed when organic acids, such as gluconic or acetic acid,
were added to the aqueous phase or generated in situ.
Therefore, inspired by enzyme micropumps described by Sen
and Balazs,39−41 we tested whether enzymatic reactions
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the dye’s fluorescence, whereas in the dye-only experiment,
fluorescence and efficient dye encapsulation were maintained
following coalescence. In similar fashion, a three-segment
droplet fiber was prepared, in which the segments contained 4-
methylstyrene, 1-pentanethiol, and 2,2-dimethoxy-2-phenyl-
acetophenone segments (Figure 4c). Triggering fiber collapse
caused reagent mixing and UV-induced thiol−ene addition, as
confirmed by 1H NMR spectroscopy of the collected oil phase
(Figure S14). Notably, despite close contact of the droplets
within these fibers, reagent mixing prior to collapse is
negligible over this time frame, as confirmed by a control
experiment in which the three-segment fiber was subjected to
UV irradiation prior to coalescence, resulting in no observable
sulfide product.
Functional versions of PZA-stabilized fibers offer versatility
in their performance, as seen in UV-cross-linked fibers
(enabled by integration of benzophenone methacrylamide
into the PZA polymer), in which chemical cross-linking
prevented droplet coalescence such that the fibers resisted
collapse when in contact with aqueous acid (Figure 5 and
almost no transport was visible at pH 7 and above. Similar
results were seen for two-component fibers, in which the
organic dye was segregated initially to droplets in one part of
the fiber and then spread throughout the fiber over time
(Figure S17). We speculate that protonation of the interfacial
PZA stabilizer under acidic conditions may alter the interfacial
structure and produce channels to allow such interdroplet
transport, though confirming this will require detailed
structural probing of interfacial structure in future work.
CONCLUSION
■
In summary, we have described the preparation of novel,
multiresponsive polymer surfactants by RAFT polymerization
that incorporate both zwitterionic and tertiary amine pendent
groups onto a polystyrene backbone. Emulsion droplets
stabilized by these polymers ranged from self-adhesive to
self-coalescing, depending on the extent of zwitterionic vs
amine functionality, the selected solution conditions and
triggers, and whether cross-linking chemistry was employed. By
tuning these interactions, we shaped emulsions into supra-
colloidal structures by simple extrusion or printing techniques;
multisegment versions of these fibers afforded spatial and
temporal control over reactions, in which reagents remained
separated prior to triggered fiber coalescence. Taken together,
these responsive emulsions represent a new design strategy to
fabricate smart, functional soft materials for any of a variety of
applications involving functional, printable, soft material
encapsulants.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c02576.
■
sı
Experimental section (section S1) with procedures and
spectroscopic characterization (PDF)
Movie S1 (AVI)
Figure 5. pH-induced interdroplet dye transport in cross-linked
supracolloidal fibers at pH 2 (top row) and 12 (bottom row). (a) Z-
slice from fluorescence confocal micrographs (emission 525 nm;
excitation 488 nm). (b) Photographs (left) and fluorescence images
(right) of PZA-stabilized droplet fibers immersed in water with a drop
of dye-containing oil at the end. (c) Schematic illustration of pH-
induced interfacial structural change leading to interdroplet transport.
Conditions: [polymer] = 10 mg/mL; oil = TCB with Nile Red (1
mg/mL for (a) and 0.5 mg/mL for (b)).
AUTHOR INFORMATION
Corresponding Author
■
Todd Emrick − Polymer Science & Engineering Department,
University of Massachusetts Amherst, Amherst, Massachusetts
01003, United States; orcid.org/0000-0003-0460-1797;
Email: tsemrick@mail.pse.umass.edu
Figure S15). The cross-linked fibers proved stable for weeks or
longer under acidic conditions and fluorescence confocal
microscopy images (Figure 5a) produced similar images of
fluorescent droplets along the fiber length at both pH 2 and
pH 12. Photobleaching of the fiber (via confocal microscopy,
Figure S16) confirmed the expected lack of polymer mobility
following cross-linking, in contrast to rapid fluorescence
recovery for individual (un-cross-linked) PZA 50-stabilized
droplets. As shown in Figure 5b, a cross-linked fiber immersed
in water was connected to an oil droplet by injecting the
droplet onto the fiber end with a blunt needle. Interestingly,
cross-linked fibers containing this droplet reservoir showed
evidence of dye transport (Figure 5c) along the length of the
fiber, over the course of days, which we suggest to occur via
direct transport of dye among interconnected droplets, along
the Plateau borders, rather than by micelle-mediated trans-
port.42,43 This dye transport was induced by acidifying the
liquid medium surrounding the fiber, ultimately generating a
fiber with visible dye along the entire length at pH 2, whereas
Authors
Jing Zhao − Polymer Science & Engineering Department,
University of Massachusetts Amherst, Amherst, Massachusetts
01003, United States
Zehao Pan − Department of Mechanical and Aerospace
Engineering, Princeton University, Princeton, New Jersey
08544, United States
Deborah Snyder − Polymer Science & Engineering
Department, University of Massachusetts Amherst, Amherst,
Massachusetts 01003, United States
Howard A. Stone − Department of Mechanical and Aerospace
Engineering, Princeton University, Princeton, New Jersey
08544, United States; orcid.org/0000-0002-9670-0639
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c02576
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors acknowledge funding for this research from the
National Science Foundation Center for Chemomechanical
Assembly (NSF-CCI-1740630), an NSF-supported Phase I
Center for Chemical Innovation (CCI). D.S. acknowledges
support from the National Institutes of Health Biotechnology
Training Program (1 T32 GM135096). We thank Dr. James
Chambers for helpful discussions associated with microscopy
data gathered in the Light Microscopy Facility and Nikon
Center of Excellence at the Institute for Applied Life Sciences
(IALS) at UMass Amherst, with support from the
Massachusetts Life Sciences Center.
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