Hierarchical Co-Assembly Enhanced Direct Ink Writing

Article abstract of DOI:10.1002/anie.201800593

Integrating intelligent molecular systems into 3D printing materials and transforming their molecular functions to the macroscale with controlled superstructures will unleash great potential for the development of smart materials. Compared to macromolecul

Full text of DOI:10.1002/anie.201800593

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Accepted Article
Title: Hierarchical Co-assembly Enhanced Direct Ink Writing
Authors: Longyu Li, Pengfei Zhang, Zhiyun Zhang, Qianming Lin,
Yuyang Wu, Alexander Cheng, Yunxiao Lin, Christina M.
Thompson, Ronald A Smaldone, and Chenfeng Ke
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800593
Angew. Chem. 10.1002/ange.201800593
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Angewandte Chemie International Edition
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COMMUNICATION
Hierarchical Co-assembly Enhanced Direct Ink Writing
[
a]
[a]
[a]
[a]
[b]
[a]
Longyu Li, Pengfei Zhang, Zhiyun Zhang, Qianming Lin, Yuyang Wu, Alexander Cheng,
[
a]
[c]
[c]
[a]
Yunxiao Lin, Christina M. Thompson, Ronald A. Smaldone, and Chenfeng Ke*
Abstract: Integrating intelligent molecular systems into 3D printing
materials and transforming their molecular functions to the
macroscale with controlled superstructures will unleash the great
potential for the development of smart materials. Compared to those
macromolecular 3D printing materials, self-assembled small-
molecule-based 3D printing materials are very limited due to the
difficulties of facilitating 3D printability as well as preserving their
molecular functions macroscopically. Here we report a general
approach to integrate functional small molecules into direct ink writing
Intelligent small molecular systems, however, often lack the
required features to be integrated into 3D printing materials, which
is a major roadblock that limits the number of smart materials
available for use in 3D printing applications. For example, inks for
direct ink writing[10] (DIW, Figure 1) need to possess shear-
thinning and rapid self-healing properties.[11] Self-assembled
molecular systems are often too weak to be self-supportive due
to the low-degree of non-covalent crosslinking[12] and/or too fast
associtation/dissociation kinetics,[13] or difficult to self-heal rapidly
(within seconds) as a result of high kinetic barrier[14] for the
reformation of the superstructure. In effort to facilitate 3D
printability to small molecules for DIW, Burdick et al. recently
reported[15] a simultaneous photo-crosslinking while DIW method
to print previously non-compatible acrylic monomers. While in situ
photo-crosslinking is desired for rapid prototyping,[ the readily
formed polymer network can limit the control over the assembled
superstructures because (1) the extrusion process will disrupt the
ordered superstructures and those ill-defined structures will be
fixed by rapid photo-crosslinking; and (2) the covalently linked
polymer chains can perturb the molecular moieties from
assembling and dissembling cooperatively, resulting in the loss of
molecular features macroscopically.
3D printing materials through the introduction of a supramolecular
template. A variety of inorganic and organic small molecule-based
inks are 3D printed and their superstructures are refined by post-
printing hierarchical co-assembly to effectively transform their
molecular features to the macroscale. Through the spatial and
temporal control of individual molecular events across nano-to-
macroscale, fine-tuned macroscale features of the monoliths have
been successfully achieved, including hierarchical porous structures
with enhanced printing resolution, dynamic fluorescence color-
change and macroscopic volume expansion/contraction.
16]
Self-assembled molecular systems with stimuli-responsive
behavior[1] and molecular mechanical motions have been
demonstrated with great promise for the advancement of smart
materials. Collectively harnessing their dynamic molecular
[2]
[3]
[4]
features to the macroscale and performing complex tasks,
however, remains a grand challenge,[5] because it requires
hierarchical control of materials’ nanoscale chemical structure,
mesoscale assembly, and macroscale three-dimensional (3D)
architectures. Integrating assembled functional molecules with 3D
printing technology[6] is a promising path to achieve this goal,
since it combines the controlled nanoscale assembly with
designed macroscale 3D geometry.[7] For example, by aligning
cellulose fibers anisotropically at the micrometer scale, Lewis et
al. successfully designed[8] a series of 3D-printed monoliths which
undergo complex shape morphing upon swelling. We recently
reported[ a 3D-printed polyrotaxane monolith with ordered
crystalline domains, which amplifies the nanoscale ring shutting-
to-stationary motion to the macroscale and converts the external
energy input into useful work.
9]
Figure 1. The design principle of hierarchical co-assembly enhanced direct ink
writing. The ink is formed via co-assembly between molecular monomers and
supramolecular templates (step 1). After fabricating into a 3D architecture (step
2), the printed monolith undergoes an evaporation-induced co-assembly (step
) followed by covalent crosslinking (step 4) and templates removal (step 5),
3
producing homogenous functional monoliths with enhanced the printing
resolution and amplified molecular functions at the macroscale.
Dr. L. Li,† Dr. P. Zhang,† Dr. Z. Zhang,† Q. Lin, A. Cheng, Y. Lin,
[
a]
Prof. Dr. C. Ke
Department of Chemistry, Dartmouth College
4
1 College Street, Hanover, NH 03755, USA
To address these challenges, we report herein a general
method (Figure 1) to integrate a variety of assembled inorganic
and organic molecules (molecular monomers) into 3D printing
inks through the introduction of supramolecular templates. The
high-density multivalent supramolecular network formed between
monomers, templates and monomer-templates can rapidly
deform upon extrusion and reform to self-heal,[11] as well as
sufficiently strong to be self-supportive. After 3D printing, these
molecular monomers and templates co-assemble hierarchically to
refine their superstructures across nano-to-macroscale, affording
E-mail: chenfeng.ke@dartmouth.edu
Dr. Y. Wu
IMSERC, Northwestern University
[
b]
c]
2145 Sheridan Road, Evanston, IL 60208, USA
[
Dr. C. M. Thompson, Prof. Dr. R. A. Smaldone
Department of Chemistry and Biochemistry
The University of Texas at Dallas
800 West Campbell Road, Richardson, TX 75080, USA
†
These authors contribute equally to this work
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201800593
COMMUNICATION
homogeneous hard and soft functional monoliths with fine-tuned
features such as high-resolution hierarchical porous structures,
dynamic fluorescence color change, and macroscopic volume
expansion and contraction that are collectively amplified from
their molecular entities.
documented co-assembly behavior,[19] in which the micellar F127
and partially hydrolyzed TEOS monomers co-assemble[20] to form
hydrogels. The polycondensation of TEOS before 3D printing is
undesired because the dynamic post-printing co-assembly
process would be inhibited. A series of TEOS/F127-based
hydrogels were prepared (See the Supporting Information, Table
S1) with various solvent combinations and reaction times. Under
the optimized conditions (Table S1, entries 5-9), a printable
transparent hydrogel G1 with very low degree of oligomerized
TEOS (Figures S1-3), an elastic modulus (Gʹ) of 4000 Pa, and a
yield strain (ε, where Gʹ = G″) of 402 Pa (Figure S6) was obtained.
G1 is formed as a result of the co-assembly between TEOS and
F127 since no hydrogel was obtained in the absence of TEOS
under the same condition (Table S1, entry 13). Although it is
difficult to quantify the binding energy between the monomer and
the template, the measured yield strain of G1 reflects the required
energy to extensively break the non-covalent interactions[
between TEOS and F127 to facilitate the solid-to-liquid transition.
Hydrogel ink G1 was loaded to syringe barriers and printed into
a series of woodpile lattice cubes (Figure 2b). The spacing-to-
diameter ratios between an in-plane center-to-center filament
In this hierarchical co-assembly enhanced direct ink writing
design (Figure 1), molecular monomers and supramolecular
templates co-assemble via multivalent noncovalent interactions to
form a viscoelastic ink (step 1). After 3D printing (step 2), the
molecular monomers and supramolecular templates in the 3D
printed monolith co-assemble in a holistic manner, affording
monoliths with hierarchically ordered superstructures[17] (step 3).
In this process, a localized re-configuration of monomer and
template is critical to maintain the macroscale structure against
gravity. Subsequent chemical crosslinking (step 4) and template
removal (step 5) staple these assembled molecular monomers
together at the macroscale to afford the desired functional
monoliths.
21]
0 0
spacing (Figure 1, L ) and a measured linewidth (d ) are recorded
between L /d = 3 ~ 6. The lattice cubes fabricated using various
0
0
nozzle diameters (25 ‒ 200 μm) exhibit good structure integrity
and accuracy in all x, y and z dimensions (Figures 2 and S9-10).
Since TEOS oligomers can freely translocate in the hydrogel,
evaporating the solvent of the printed monolith initiates the co-
assembly of TEOS and F127 to refine their superstructures
homogeneously. Nearly isotropic shrinkages (Figure 2b-c) with
signficantly reduced linewidth and volume of the lattice cubes
0 0
(d /d = 2.3, V /V = 3.0, L/d from 2.71 to 6.62 in Table S2-3) were
achieved by introducing a few sacrificial layers (Figure S13). The
significant enhancement of printing resolution and L/d is attributed
to a rapid and localized co-assembly of TEOS and F127 during
the evaporation evolution. In contrast, when TEOS have been
polymerized in the hydrogels G124h and G148h before printing, less
size shrinkage as well as macroscale structural defects (Figure
S14) are recorded after evaporation, becasue the dynamics of the
polymer/template co-assembly is not fast enough to compensate
for the local surface tension variation. These results suggest that
preserving the dynamic molecular assembly is critical in forming
ordered structures across the nano-to-macroscale and enhancing
printing resolution macroscopically.
Calcination of these monoliths at different temperatures (Figure
2b) allows for extensive polycondensation of TEOS and the
removal of F127 template, affording silica monolith with good
structural integrity at both macroscopic (Figure 2b), mesoscopic
(
Figure 2c) and nanoscopic scales (Figure 2d) with a measured
Figure 2. (a) Preparation of TEOS/F127 co-assembled supramolecular
hydrogel inks. (b) Post-printing co-assembly of a woodpile lattice cubes (9.0 ×
2
o
Brunauer–Emmett–Teller (BET) surface area of 304 m /g (700 C,
Figure S19). After calcination at 1000 °C, the linewidth (38 μm)
shrunk nearly an order of magnitude compared to the as-printed
linewidth (330 μm) and a 97-99 % volume reduction was achieved
o
9
7
×
.0 × 9.6 mm) composed of G1 at room temperature and calcinated at 200 C,
o
o
00 C and 1000 C, respectively. (c-e) Microscope images of a lattice cube (12
o
12 × 12.2 mm, d
0
o
= 331 μm, L /d
0 0
= 3.6) after calcination at 700 C (c, SEM;
d, TEM) and 1000 C (e, SEM), respectively. (f) “Arc de Triomphe” replicates
printed using polyacrylonitrilebutadienestyrene (ABS), G1, G1 after evaporation
(
Table S2). Our co-assembly method enables high-resolution
o
and calcination (700 C), respectively.
fabrication of functional materials with controlled nanoscopic
structures far beyond printing nozzle physical limit.[
22]
For
example, G1-based ‘Arc de Triomphe’ replicates (Figure 2f)
fabricated by a low-cost customized desktop 3D printer (Figure
S11-12) was transformed into a high-resolution mesoporous silica
monolith.
At the outset, we chose tetraethyl orthosilicate (TEOS) as the
molecular monomer and Pluronic F127 triblock copolymer[18] as
the supramolecular template (Figure 2a) due to their well-
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Angewandte Chemie International Edition
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first 20 min due to the loss of solvent and subsequently quenched
as a result of the aggregation of 2 in G1.[25] Similarly, the emission
of 3@G1 increases in the first 20 min and keep increasing owing
to the aggregation induced emission,[26] suggesting that tracker 3
aggregates in G1 due to its strong inter-molecular hydrogen
bonding interactions. The recorded dynamic process and time-
dependent fluorescence spectra reveal that, (i) the strong
interaction between the supramolecular template and molecular
monomer can effectively localize the dynamic co-assembly
process, which avoids structural failure macroscopically during
the post-printing process; (ii) the stimuli-induced co-assembly
process is largely kinetically controlled and monomer/F127
assemble rapidly at the surface of the hydrogel and progressively
re-organize into the inner sections; (iii) the monomer dynamic self-
assembly feature is well-preserved in a macroscopic monolith
after 3D printing.
The established model system and revealed molecular
understanding of the co-assembly process encouraged us to
spatially and temporally control the monomers in a 3D printed
monolith and alter their macroscopic property, realizing a 4D
printing process.[27] A pair of Förster resonance energy transfer[28]
(
FRET) donor and acceptor monomers (2 and 4) were
synthesized, doped into hydrogel G1 ([TEOS]/[2]/[4]
7900:10:1) and printed into a woodpile lattice cube (Figure 3f).
=
1
Upon solvent evaporation, the fluorescence of lattice cube
changed gradually from sky blue (λex = 365 nm) to greenish blue
and then green color (Figure 3f) along with volume reduction,
suggesting that the FRET donor and acceptor are approaching
each other nanoscopically in the hydrogel, which is also confirmed
by the time-dependent fluorescence experiment (Figure 3g).
The well-controlled molecular co-assembly behavior in G1
suggests the possibility of further expanding the scope of
monomers, given that the selected monomer co-assembles with
the supramolecular template of choice. Indeed, when
organosilicate derivative 3 was introduced as the functional
monomer, an opaque viscoelastic hydrogel G3 (see the
Supporting Information) was obtained with a yield strain (ε = 974
Pa) larger than that of F127 hydrogel (ε = 520 Pa, Figure S30),
indicating that more energy is required to break the
supramolecular co-assembly formed between 3 and F127 than
F127 alone. The functional monolith M3 (Figure 4a) was obtained
after evaporation-induced co-assembly, extensive chemical
cross-linking (Scheme S4) and template removal by EtOH
extraction, with moderately enhanced printing resolution and 85-
Figure 3. (a) Fluorescent trackers 2, 3 and 4 employed in this study. (b and d)
Bright field and fluorescence microscope images of a two-orthogonal-layer
monolith fabricated using 2@G1 (b) and 3@G1 (d). (c and e) Time-dependent
fluorescence spectra of 2@G1 (c) and 3@G1 (e) measured in open air. (f) A
wood pile lattice cube fabricated using 2/4@G1 ([2]/[4] = 10 : 1) underwent
volume reduction and fluorescence color changing upon evaporation. (g) Time-
dependent fluorescence spectra of 2/4@G1 measured in open air.
To provide molecular understanding of this evaporation-
induced hierarchical co-assembly process, fluorescent TEOS
analogs 2 and 3 (Figure 3a) were synthesized as fluorescent
trackers, which were doped into G1. Upon evaporation, these
fluorescent trackers can not only translocate with the TEOS
monomers but also aggregate owing to their hydrophobic
fluorophores and hydrogen bonding motifs (Scheme S3 and
Figure S32). Fluorescent tracker-doped hydrogels[23] 2@G1
92% volume reduction (Table S5). M3 emits green fluorescence
with λmax = 507 nm in the solid-state (Figure S34) as well as
(
[TEOS]/[2] = 1790) and 3@G1 ([TEOS]/[3] = 17900) were printed
possessing nano-sized pores ranging from 5-20 nm (Figure 4a)
with a measured BET surface area of 177 m /g (Figure S35),
2
into two-layer orthogonal lattices (Figure 3b and 3d) and the
evaporation-induced co-assembly of the top layer
[
24]
were
suggesting
a successful transformation of their molecular
monitored in real-time using a fluorescence microscope. In 2@G1
and 3@G1 lattices, after 5 min of solvent evaporation, noticeable
linewidth shrinkages of the top layers were recorded in the bright
field images (Figure 3b and 3d, left), while localized fluorescent
quenching and enhancement (Figure 3b and 3d, right and Movie
S1-2) were observed firstly at the periphery of filament and
progressively expanded to the inner sections, respectively. The
net fluorescent emission changes of the doped hydrogels were
recorded by the time-dependent fluorescence emission spectra.
Upon irradiation, the emission of 2 (Figure 3c) increased in the
properties to the macroscale.
Similarly, monomer 5 and tetra-thiol crosslinker 6 (Figure 4b)
were employed as organic monomers for DIW. 5 possesses a
benzene-1,3,5-tricarboxamide (BTA) hydrogen-bonding donor-
acceptor core, which forms 1D supramolecular polymers.[29] In an
aqueous environment (MeOD:D O = 1:1), monomer 5 co-
2
1
assembles with the backbone of F127 as evident by the H NMR
titration experiment (Figure S38-39). Mixing monomer 5,
crosslinker 6 and F127 in H
2
O/EtOH/THF (1:1:1) mixed solvent
affords a clear solution and subsequently turned into an opaque
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Figure 4. Optical and TEM images of lattice cubes (a) M3 and (b) M5/6. (c) Active insertion of a supramolecular pillar 7 (0.7 M) into monolith M5/6 (7.1 × 7.2 × 5.6
mm) in an aqueous enviornment at 80 oC. The recovered M5/6 was obtained after DMSO extraction followed by EtOH and H
O solvation.
2
viscoelastic hydrogel G5/6 upon solvent evaporation. Hydrogel
G5/6 was fabricated into a lattice cube, which was subsequently
crosslinked through thiol-ene reaction.[30] The F127 template was
removed by EtOH extraction, affording a crosslinked monolith
M5/6 (Figure 4b) with [5]/[6] = 1.08 :1 suggested by elemental
analysis. Different from M3, no significant nanoporous structure
was observed in the TEM analysis of M5/6 (Figure 4b), which may
attribute to the re-organization of the flexible network after the
template removal. Since BTA moieties in M5/6 are connected via
hydrogen bonding network, an active insertion of supramolecular
In this work, we presented a general approach of integrating
functional small molecules into 3D printing materials and
transforming their molecular features to the macroscale through
supramolecular templation, post-printing hierarchical co-
assembly and covalent crosslinking. 3D monoliths made by
inorganic and organic functional molecules are fabricated,
featuring macroscale properties that are introduced by the
correspondent molecular monomers. The co-assembly process
not only enhances the printing resolution up to an order of
magnitude, it also enables the precise spatial control of nanoscale
features such as molecular assembly over a large scale.
[
31]
pillars with BTA core at the nanoscale will cooperatively induce
a macroscale size expansion (Figure 4c). Interestingly, when a
M5/6 was immersed in an aqueous solution[32] of supramolecular
pillar 7 (0.7 M) at room temperature, no noticeable macroscopic
size expansion was observed until the solution temperature was
Fluorescent
tracking
experiments
provide
molecular
understanding of the dynamic co-assembly process at the
macroscale as well as enabling the development of simultaneous
color and shape changing 4D printing in response to the external
o
raised above 80 C. The observation is consistent with previous
stimuli. Furthermore, we showcased
a
benzene-1,3,5-
report,[33] in which an elevated temperature is required to break
the multivalent hydrogen bonding network in M5/6 to allow for a
hetero supramolecular insertion between different BTA-
tricarboxamide-based monolith capable of expanding and
contracting its size by actively inserting and removing the
correspondent supramolecular pillars. We believe this new
approach will initiate the development of small molecule-based
3D printing materials and greatly accelerate the development of
smart materials and devices beyond our current grasp that are
capable of doing complex tasks in response to environmental
stimuli.
o
monomers. After 2 h immersion at 80 C and cooled to room
temperature,[34] an isotropic volume expansion of 156 ± 4 % is
recorded (Figure 4c), and the mass of hydrogel 7@M5/6 is 160 ±
5
% to that of M5. Elemental analysis of the xerogel 7@M5/6
suggests an insertion ratio of 7/5 = 1.4 : 1. The 7-inserted
monolith did not recover to its original size even after washing with
a large excess of water, suggesting that the cooperative
supramolecular interaction between the BTA moieties of M5/6
and 7 is stable against high dilution. Pillar 7 was removed from
M5/6 by DMSO extraction and M5/6 recovers to its macroscopic
Acknowledgements
We acknowledge Dartmouth College for generous start-up funds.
2
size after H O re-solvation (Figure 4c). This process has been
repeated three times and the macroscopic size of M5/6 can be
successfully altered by actively inserting and removing
Keywords: 3D printing • supramolecular chemistry • templated
synthesis • self-assembly • smart material
supramolecular
pillars,
demonstrating
the
successful
amplification of the nanoscale hetero-supramolecular assembly
event to the macroscopic level.
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Angewandte Chemie International Edition
COMMUNICATION
10.1002/anie.201800593
COMMUNICATION
L. Li, P. Zhang, Z. Zhang, Q. Lin, Y. Wu,
A. Cheng, Y. Lin, C. M. Thompson, R. A.
Smaldone, C. Ke*
Page No. – Page No.
Hierarchical Co-assembly Enhanced
Direct Ink Writing
Co-assembled 3D printing inks: Supramolecular template was introduced to
facilitate 3D printability of a variety of functional small molecules. The molecular
functions have been successfully amplified to the macroscale in the fabricated 3D
monoliths, including hierarchical porosity, fluorescent color changing with good
spatial and temporal control, and size-expansion by active supramolecular insertion.
This article is protected by copyright. All rights reserved.