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# Self-reporting and self-regulating liquid crystals

## Abstract

Liquid crystals (LCs) are anisotropic fluids that combine the long-range order of crystals with the mobility of liquids1,2. This combination of properties has been widely used to create reconfigurable materials that optically report information about their environment, such as changes in electric fields (smart-phone displays)3, temperature (thermometers)4 or mechanical shear5, and the arrival of chemical and biological stimuli (sensors)6,7. An unmet need exists, however, for responsive materials that not only report their environment but also transform it through self-regulated chemical interactions. Here we show that a range of stimuli can trigger pulsatile (transient) or continuous release of microcargo (aqueous microdroplets or solid microparticles and their chemical contents) that is trapped initially within LCs. The resulting LC materials self-report and self-regulate their chemical response to targeted physical, chemical and biological events in ways that can be preprogrammed through an interplay of elastic, electrical double-layer, buoyant and shear forces in diverse geometries (such as wells, films and emulsion droplets). These LC materials can carry out complex functions that go beyond the capabilities of conventional materials used for controlled microcargo release, such as optically reporting a stimulus (for example, mechanical shear stresses generated by motile bacteria) and then responding in a self-regulated manner via a feedback loop (for example, to release the minimum amount of biocidal agent required to cause bacterial cell death).

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## Acknowledgements

This work was primarily funded by the Army Research Office (through grants W911NF-15-1-0568 and W911NF-17-1-0575), the National Science Foundation (grants CBET- 1263970 and DMR-1435195) and the Wisconsin Materials Research Science and Engineering Center (grant DMR-1720415). We thank R. Trivedi and D. B. Weibel for assistance in the preparation of the bacterial dispersions.

### Reviewer information

Nature thanks T. Ware and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

The experimental strategy was proposed initially by N.L.A. and developed by all authors. Experiments were performed by Y.-K.K. with assistance from X.W., P.M. and E.B. and analysed by Y.-K.K. and N.L.A. Y.-K.K. and N.L.A. wrote the manuscript with input from all co-authors.

Correspondence to Nicholas L. Abbott.

## Ethics declarations

### Competing interests

The University of Wisconsin-Madison has filed a patent application (PCT/US207/037414) on the work described in this manuscript. The inventors listed on the patent application are N.L.A., Y.-K.K., X.W. and E.B.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Thermally triggered ejection of microdroplets.

a, b, Mass of tracer released from LCs (5CB) as a function of time, before (a) and after (b) an N-to-I phase transition (corresponds to Fig. 1). Black and red points indicate the mass released before and after an N–I phase transition, respectively. c–e, Sequential photographs of the release of microdroplets (CSDS = 9 mM and red tracer) from the LC, which is triggered by phase transitions induced by resistive heating. We used Caq = 20 v% and 30 V for heating (to Th = 60 °C, d) and 0 V for cooling (to room temperature, e). Heating of the sample from below was achieved by passing a current through an indium-tin-oxide electrode coated on glass. The motion of the N–I interface was upward for both heating and cooling. f–h, Dependence of the direction of motion of the N–I interface on Th and Tc. f, Optical images showing a propagation of the N–I interface across a mini-well filled with 5CB (containing no microdroplets) in an aqueous bath upon heating from T = 25 °C to Th = 50 °C (N-to-I phase transition). Upon heating the sample from below, the interface moved upwards (towards the interface between the LCs and the overlying aqueous phase) regardless of Th. g, Upward motion of the N–I interface upon cooling from T = 50 °C to Tc = 25 °C (I-to-N phase transition). h, Downward motion of the N–I interface upon cooling from T = 50 °C to Tc = 34 °C (I-to-N phase transition). Scale bars, 5 mm. i, Released mass of tracer as a function of the number of phase transitions with Th = 50 °C and Tc = 34 °C; Caq = 20 v% (CSDS = 9 mM).

### Extended Data Fig. 2 Behaviours of microdroplet clusters during passage of the N–I interface.

a–h, Sequential micrographs showing the behaviours of microdroplet clusters dispersed in an LC (5CB) during passage of an N–I interface upon an N-to-I (heating, a–d) and an I-to-N (cooling, e–h) phase transition. We used Caq = 3 v% (CSDS = 2 mM) and measured vNI = 10 µm s−1 for heating, vNI = 35 µm s−1 for cooling and R* ≈ 10 µm for both cases. Scale bar, 100 µm (see Methods, ‘Additional observations on the transport of microdroplets by propagating N–I interfaces’ for more details). Red and blue arrows indicate the direction of motion of the N–I interface. Solid and dotted circles indicate microdroplets with R > R* (= 10 µm) and R < R*, respectively. White arrows indicate microdroplets that coalesced while being transported by the moving N–I interface. We note that microdroplets with R < R* were left behind the N–I interface in c because they were shed from clusters, as illustrated in i–l. i–l, Illustration of a microdroplet cluster being transported by a moving N–I interface. i, Single microdroplets or microdroplet clusters with R < R* are transported by a moving N–I interface. j, As the moving interface collects more microdroplets, the microdroplet clusters formed at the interface increase in size. k, When the effective radius of a microdroplet cluster exceeds R*, the interface no longer transports the cluster. l, Because some of microdroplets from the cluster are left behind the N–I interface, the cluster becomes smaller than R* and thus is transported again by the interface. m–r, Evidence that microdroplets with R > R* are not transported by an N–I interface moving at high speed (vNI = 100 µm s−1) during N-to-I (m–o) and I-to-N (p–r) phase transitions. Here Caq = 0.5 v% and CSDS = 2 mM. Scale bar, 100 µm. The left (m, p) and right (o, r) columns show optical micrographs before and after passage of the N–I interface, respectively, and the middle column (n, q) shows micrographs taken during passage of the N–I interface. The positions of microdroplets before and after the passage of the N–I interface were unchanged, revealing that R > R* for the rapidly moving interface.

### Extended Data Fig. 3 Transport of microdroplets by an N–I interface propagating across an LC film.

a–j, Sequential micrographs (a–e, top view) and corresponding illustrations (f–j, side view) of microdroplets transported by a moving N–I interface upon heating in an LC film (5CB, 40 µm in thickness). The focal plane is near the interface between the LC and the overlying water (red boxes in f and h). a, f, Microdroplets are dispersed initially in the LC bulk. b, g, When the bottom of the LC film is heated to Th = 50 °C (> TNI), the N-to-I transition first occurs at the LC–glass interface (denoted by the asterisk in b) and the N–I interface propagates upwards, towards the LC–water interface. c, h, Microdroplets that were out of focus (red dashed circles in a and f) move into focus, revealing that the moving interface transported the microdroplets towards the LC–water interface. d, i, As the N–I interface reaches the LC–water interface, the microdroplets disappear, consistent with their fusion with the overlying aqueous phase. e, j, After the phase transition, some microdroplets remain in the LC layer. k–n, Micrographs showing the decrease in the population of microdroplets in the LC at T = 25 °C; before any phase transitions (k) and after 2 (l), 4 (m) and 6 (n) phase transitions. Scale bars, 50 µm. P and A indicate the orientations of the polarizer and analyser, respectively. Th = 50 °C, Tc = 25 °C, Caq = 5 vol% (CSDS = 9 mM) and vNI = 8 µm s−1.

### Extended Data Fig. 4 Calculated net force $${{\boldsymbol{F}}}_{{\bf{c}}}^{{\bf{net}}}{\boldsymbol{(}}{\boldsymbol{z}}{\boldsymbol{)}}$$ acting on a microdroplet and calculated dependence of R* on vNI during an I-to-N phase transition (cooling).

a, $${F}_{{\rm{c}}}^{{\rm{net}}}(z)$$ for a quasi-static microdroplet of R = 1.5 µm in 5CB. The insets show microdroplets at zR (black line; (i)), at –R < z < R (blue line; (ii)) and at z ≤ −R (red line; (iii)). b, Critical radius R* of a microdroplet as a function of vNI upon cooling. See Methods for details.

### Extended Data Fig. 5 Influence of LC phase behaviour and elastic properties on dynamics of release of dispersed microdroplets.

a–h, Sequential photographs showing continuous release (a–d) of dispersed microdroplets (green tracer and CSDS = 2 mM) from a nematic LC (E7) in response to a thermal trigger $${T}_{{\rm{h}}} < {T}_{{\rm{NI}}}^{5{\rm{CB}}}$$ (ρE7 > ρaq), and for pulsatile release of dispersed microdroplets (red tracer and CSDS = 9 mM) from 5CB (ρ5CB < ρaq, $${T}_{{\rm{h}}} > {T}_{{\rm{NI}}}^{5{\rm{CB}}}$$) at 0 min (a, e), 15 min (b, f), 60 min (c, g) and 120 min (d, h) after heating of the baths from below to Th = 59 °C. We used Caq = 30 v% for a–d and Caq = 20 v% for e–h. Scale bar, 5 mm. i, Corresponding time derivative of the released mass m of the tracer (dm/dt; t, time) for pulsatile (red line and circles) and continuous (green line and circles) release. j, Released mass of green tracers from E7 (continuous release) with respect to time at representative temperatures of T = 40 °C (blue triangles), 50 °C (circles) and 59 °C (red triangles). The data are mean values and the error bars are 1 s.d. (n = 5). k, Forces acting on an aqueous microdroplet in E7. l, m, Calculated net force $${F}_{{\rm{b}}}^{{\rm{net}}}$$ acting on a microdroplet in E7 as a function of R at h1 = 0 (l) and as a function of h1 for R = 25 µm (m) at T = 40 °C (blue line), 50 °C (black line) and 59 °C (red line). See Methods for details.

### Extended Data Fig. 6 Isothermal release of microdroplets from an LC by a solute-triggered N-to-I phase transition.

a–d, Sequential photographs of a solute-triggered N-to-I phase transition of 5CB at T = 25 °C $$\left({T}_{{\rm{NI}}}^{5{\rm{CB}}}=35{}^{\circ }{\rm{C}}\right)$$ at 0 h (a), 1 h (b), 2 h (c) and 3 h (d) after the addition of propanol to the overlying water. As the propanol diffused into the 5CB, an N-to-I transition occurred first at the interface between the LC and the aqueous solution and propagated into the LC bulk. e, Illustration of inverted mini-wells filled with 5CB containing microdroplets (red tracer), placed in baths containing pure water (left) and water containing propanol (Cpropanol = 16 v%; right). f–h, Sequential photographs of the mini-wells at 0 min (f), 5 min (g) and 30 min (h) after the mini-wells were submerged into the baths; Caq = 10 v% (CSDS = 9 mM). Although Fb (ρLC < ρaq) promotes the release of tracers, no release of red tracers was observed in the left bath owing to strong elastic sequestration. In the right bath, however, the red tracers were continuously released as the elastic barrier was removed by the solute-induced N-to-I phase transition. Scale bars, 5 mm.

### Extended Data Fig. 7 Influence of the size and clustering of microdroplets on release from the LC.

a–f, Optical micrographs of microdroplets (CSDS = 2 mM and red tracer), with R = 9.5 µm (a–c) and 27 µm (d–f) in an LC (E7) at 25 °C (a, d), 50 °C (b, e) and 59 °C (c, f); ρE7 > ρaq and Caq = 1 v%. Scale bars, 20 µm. The microdroplets were elastically trapped in the nematic LC bulk at 25 °C. As the temperature increased to 50 °C (R > 34 µm for release), the microdroplets moved upwards and into focus, but were not dispensed into the overlying water; the focal plane was near the interface between the LC and the overlying aqueous solution. At 59 °C (R > 23 µm for release), we observed the larger microdroplet (R = 27 µm) to escape into the overlying aqueous phase, whereas the smaller microdroplet (R = 9.5 µm) remained elastically trapped in the nematic bulk. This observation is in good agreement with our theoretical prediction (Extended Data Fig. 5l, m). g–i, Micrographs of the clustering of microdroplets at 0 min (g), 30 min (h) and 180 min (i) after they were dispersed in the LC; Caq = 2 v% (CSDS = 9 mM). Scale bar, 200 µm. j–o, Illustration (j) and sequential micrographs (ko) of thermally triggered, continuous release of microdroplets (CSDS = 9 mM and red tracer) from mini-wells filled with E7 containing microdroplets of different sizes, at 0 min (k), 7 min (l), 10 min (m), 12 min (n) and 15 min (o) after the baths were heated to Th = 59 °C(< TNI); ρE7 > ρaq (Fb > 0) and Caq = 20 v%. Scale bar, 5 mm. The mini-well containing the larger microdroplets (left bath) exhibited a higher release rate due to the facile formation of microdroplet clusters with a radius higher than that for which $${F}_{{\rm{b}}}^{{\rm{net}}} > 0$$, consistent with the theoretical model whose results are shown in Extended Data Fig. 5l, m.

### Extended Data Fig. 8 Role of electrical double-layer interactions in the release of microdroplets from an LC.

a, Calculated repulsive elastic (Fe1, dashed line) and attractive electrical double-layer (Fedl, solid lines) forces acting on a microdroplet (CSDS = 9 mM, R = 1.5 µm) as a function of h1 following the addition of DTAB with CDTAB = 10 mM (red), 5 mM (orange) and 2 mM (black) to the overlying aqueous phase. The inset shows the corresponding net forces $$\left({F}_{{\rm{i}}}^{{\rm{net}}}={F}_{{\rm{e1}}}+{F}_{{\rm{edl}}}\right)$$. b, $${F}_{{\rm{i}}}^{{\rm{net}}}$$ near the interface between the LC and the overlying aqueous phase (h1 = λ), and initial release rate of microdroplets (from 0 to 60 min in Fig. 2g) as a function of CDTAB added into the overlying aqueous phase. c, Illustration of inverted mini-wells in baths with water (pH 7, left bath) and alkaline water (pH 13, right bath) at T = 45 °C (> TNI). d, e, Sequential photographs of mini-wells filled with 5CB containing aqueous microdroplets (red tracer) at 0 min (b) and 60 min (c) after an N-to-I phase transition, with Caq = 10 v% (CSDS = 9 mM) and Th = 50 °C. Scale bar, 5 mm. Because ρLC < ρaq (Fb < 0), tracers are continuously released from an isotropic phase of 5CB (Fe1 = 0) into the pure water (left bath). In the alkaline water (right bath), however, the release is suppressed because of the introduction of repulsive charge interactions between the LC interface in alkaline water (negatively charged) and SDS-containing aqueous microdroplets (negatively charged).

### Extended Data Fig. 9 Triggering of the release of dispersed microdroplets by interfacial charge interactions of biological molecules.

a, Zeta potential ζ at LC–aqueous interface without (white bar) and with lipopolysaccharides (LPS; green bars) from Escherichia coli, or with DTAB (grey bar). The data are mean values and the error bars are 1 s.d. (n > 3). b–e, Micrographs (side view) showing the ejection of microdroplets (CDTAB = 2 mM and red tracer) from LC (5CB) 30 min after addition of CLPS = 0 mg ml−1 (b), 1 mg ml−1 (c), 2 mg ml−1 (d) and 4 mg ml−1 (e) to the overlying aqueous phase. f–i, Sequential micrographs (top view) showing the ejection of microdroplets before (f) and at 0 min (g), 15 min (h) and 30 min (i) after the addition of CLPS = 4 mg ml−1 into the overlying aqueous phase. Scale bars, 200 µm. P and A indicate the orientations of the polarizer and analyser, respectively. In the presence of LPS, microdroplets are ejected continuously from the LC, as evidenced by the release of red tracer (b–e) and the decrease in the population of aqueous microdroplets within the thin LC film (40 µm in thickness; f–i). The release rate is enhanced with an increase in CLPS (b–e), consistent with release controlled by interfacial charge interactions.

### Extended Data Fig. 10 Ejection of microdroplets from the LC by motile bacteria.

a–f, Sequential micrographs showing that microdroplets containing anti-bacterial agent (2 mM DTAB and 1 wt% silver acetate) are not ejected from the LC (5CB) in the absence of bacteria (a–c) and at 0 min (a, d), 15 min (b, e) and 30 min (c, f) after the arrival of weakly motile bacteria (d–f); Caq = 10 v%. g–i, Sequential micrographs showing ejection of microdroplets from the LC at 0 min (g), 30 min (h) and 120 min (i) after the arrival of motile bacteria and subsequent bacterial death and aggregation. Scale bar, 40 µm. g, Motile bacteria generate shear stresses at the LC interfaces, triggering the release of microdroplets containing anti-bacterial agent. h, As the anti-bacterial agents are released, the bacteria become less motile. At 30 min, we observe dead bacteria and cessation of the triggered release due to the decrease in the number of motile bacteria. The amount of silver acetate released is 1–2 µg µl−1. i, Two hours after the arrival of motile bacteria, only dead (non-motile) bacteria are observed.

### Extended Data Fig. 11 LC systems with complex geometries and responsiveness to multiple stimuli.

a–c, Isothermal release of aqueous microdroplets (CSDS = 9 mM and red tracer) from a large LC emulsion droplet (5CB) before (a) and at 50 s (b) and 80 s (c) after the addition of DTAB (10 mM) into surrounding aqueous phase; Caq = 5 v%. Scale bar, 200 µm. The insets in a and b are optical micrographs (crossed polars) of the LC droplet, showing the optical response. d–f, Selective release of two agents, triggered by a combination of chemical and thermal stimuli (see Methods for details). d, In an aqueous SDS bath, the thermal stimulus triggers release of DTAB-doped microdroplets (green tracer) from Well 1, but not of SDS-doped microdroplets (red tracer) from Well 2 (1st to 4th phase transition; left inset in f). e, Addition of DTAB into the bath reverses the charge at the interface between the LCs and the overlying aqueous phase, thus enabling the ejection of microdroplets from Well 2, but not from Well 1 (5th to 8th phase transition; right inset in f). Red and blue points in f correspond to the released mass of tracers after N-to-I and I-to-N phase transitions, respectively. Scale bar, 5 mm.

### Extended Data Fig. 12 Triggered release of water-soluble solid microparticles from an LC film.

a–f, Polarizing (a, d; top view) and fluorescence (b, e; top view) micrographs and schematic illustrations (c, f; side view) of an LC film at 25 °C in the initial N phase (a–c) and after six phase transitions (d–f) with Th = 50 °C and Tc = 25 °C. Scale bar, 200 µm. The LC film (5CB, 40 µm in thickness) contained solid microparticles of FITC-dextran (1–2 wt%). Before any phase transition, a strong fluorescence signal was detected from the FITC-dextran microparticles that were sequestered in the LC film (b), but not from the water bath (g), indicating that the solid microparticles were trapped in the LC. After six N–I phase transitions, however, no fluorescence signal was detected from the LC (e), whereas the water bath showed a strong fluorescence signal (h). The inset in h shows the fluorescence intensity If from the bath as a function of the number of phase transitions.

## Supplementary information

### Video 1: Thermally triggered release of dispersed microdroplets from a LC.

Video showing the pulsatile release of aqueous microdroplets (containing a red tracer) from a LC to an overlying aqueous phase after nematic-to-isotropic (heating from 25 °C to 50 °C, 0 to 7 seconds in the video) and isotropic-to-nematic (cooling from 50 °C to 25 °C, 7 to 14 seconds in the video) phase transitions. This video corresponds to Fig. 1c, d, and f.

### Video 2: Release of microcargo from LC triggered by the touch of a human finger.

Video shows the optical response and the pulsatile release of aqueous microdroplets (containing a red tracer) from a cholesteric LC to a surrounding aqueous phase triggered by the heat of a human finger, which causes a nematic-to-isotropic phase transition. This video corresponds to Fig. 4b-d.

### Video 3: Release of microcargo from LC by motile bacteria that generate interfacial shear stresses and interact with microcargo in a self-regulated manner.

Video shows the release of aqueous microdroplets (containing a red tracer) from a LC triggered by the presence of motile bacteria in an overlying aqueous phase. This video corresponds to Fig. 4g.

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• #### DOI

https://doi.org/10.1038/s41586-018-0098-y

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