Letter pubs.acs.org/macroletters
Electromechanically Responsive Liquid Crystal Elastomer Nanocomposites for Active Cell Culture Aditya Agrawal,† Huiying Chen,‡ Hojin Kim,† Bohan Zhu,† Oluwatomiyin Adetiba,‡ Andrea Miranda,∥ Alin Cristian Chipara,§ Pulickel M. Ajayan,§ Jeffrey G. Jacot,*,‡,⊥,# and Rafael Verduzco*,†,§ Departments of †Chemical and Biomolecular Engineering, ‡Bioengineering, §Materials Science and NanoEngineering, and ∥ Chemistry, Rice University, Houston, Texas 77005, United States ⊥ Congenital Heart Surgery, Texas Children’s Hospital, Houston, Texas 77030, United States S Supporting Information *
ABSTRACT: Liquid crystal elastomers (LCEs) are unique among shape-responsive materials in that they exhibit large and reversible shape changes and can respond to a variety of stimuli. However, only a handful of studies have explored LCEs for biomedical applications. Here, we demonstrate that LCE nanocomposites (LCE-NCs) exhibit a fast and reversible electromechanical response and can be employed as dynamic substrates for cell culture. A two-step method for preparing conductive LCE-NCs is described, which produces materials that exhibit rapid (response times as fast at 0.6 s), large-amplitude (contraction by up to 30%), and fully reversible shape changes (stable to over 5000 cycles) under externally applied voltages (5− 40 V). The electromechanical response of the LCE-NCs is tunable through variation of the electrical potential and LCE-NC composition. We utilize conductive LCE-NCs as responsive substrates to culture neonatal rat ventricular myocytes (NRVM) and find that NRVM remain viable on both stimulated and static LCE-NC substrates. These materials provide a reliable and simple route to materials that exhibit a fast, reversible, and large-amplitude electromechanical response.
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quickly and reversibly and be operated in a cell culture environment. Here, we report biocompatible LCE nanocomposites (LCENCs) capable of cyclic, electromechanical actuation. We show that these materials exhibit rapid response times (as fast as 0.6 s), large-amplitude (contraction by up to 30%), and fully reversible shape changes (stable to over 5000 cycles) under externally applied voltages (5−40 V). We implemented these materials as dynamic substrates and found myocytes remain viable on stimulated LCEs. Our work demonstrates a simple approach to LCEs with a fast electromechanical response and for implementing LCEs in biomedical applications. Carbon black nanoparticles were introduced in the LCE network both during and after synthesis, as shown in Scheme 1. This approach combines prior approaches for the preparation of conductive poly(dimethylsiloxane) (PDMS) elastomers26,27 and conductive LCEs.28 First, carbon black nanoparticles were dispersed throughout the reactive mixture during cross-linking and alignment. Next, additional carbon black nanoparticles were introduced near the sample surface and edges by immersing the elastomer in a chloroform dispersion of carbon black nanoparticles (7 mg/mL). The introduction of carbon black nanoparticles in two separate steps is important for achieving a conductive material
iquid crystal elastomers (LCEs) are unique among shaperesponsive materials in that they exhibit large and reversible shape changes and can respond to a variety of stimuli.1−3 Recent examples have reported complex shapechanges in LCEs in response to a variety of stimuli including light,4−8 heat,9,10 magnetic,11−13 and electric fields.14,15 This enables the further development of LCEs for a variety of applications that require stimuli-responsive shape changes. However, only a handful of studies have explored LCEs for biomedical applications, where LCEs interface with cells or living tissue.10,13,16−18 Such applications require materials that are biocompatible and that can respond to mild stimuli, such as magnetic fields or small changes in temperature. A challenge is that LCEs typically exhibit shape changes over broad temperature ranges or at elevated temperatures, typically 70 °C or higher. Further, shape changes in LCEs are typically slow, occurring over several minutes or hours. This work is motivated by the prospect of developing LCEs as dynamic substrates for culturing myocytes during electrical stimulation. Cardiomyocytes are of interest for regenerative therapies for heart disease,19,20 and a combination of equibiaxial stretching devices21,22 and patterned substrates23−25 have been previously implemented to study cardiomyocyte response to mechanical stresses. These studies have shown that myocytes exhibit morphological changes in response to cyclic stretching with strains of 5−20% and cycling periods of 1−10 s. This presents a challenge in producing LCEs that can respond © XXXX American Chemical Society
Received: July 17, 2016 Accepted: November 21, 2016
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DOI: 10.1021/acsmacrolett.6b00554 ACS Macro Lett. 2016, 5, 1386−1390
Letter
ACS Macro Letters Scheme 1. Synthetic Scheme for the Preparation of LCE Nanocomposites
Table 1. Properties of LCE-NCs with Varying Amounts of Carbon Black Nanoparticles Added bulk
bulk + surface
carbon black contenta (wt %)
resistivity (Ω·m)
strain (%)
0 1 2 4 6 10 15 20
∞ ∞ ∞ ∞ ∞ ∞ 1.1 0.3
35.0 27.7 22.5 17.5 16.3 8.3 5.2 2.2
b
resistivity (Ω·m)
strainb (%)
38.5c 25.1 7.5 4.1 3.5 1.6 0.7 0.2
35.0c 27.7 22.5 17.5 16.3 8.2 5.2 2.2
a
The composition of carbon black in the bulk sample, step 1 of the synthesis procedure. bStrain measured through temperature-dependent dynamic mechanical analysis (DMA) measurements by heating the sample up to 130 °C. cCarbon black was added only through surface infiltration (step 2) in these samples. Figure 2. Photograph of LCE-NC with 2 wt % carbon black on top of conductive carbon rods and a plot of electrical potential (top) and sample strain (bottom). The plots at the bottom show the reversible strain at the beginning and end of the experiment.
nanocomposites by surface infiltration alone, and the resulting materials exhibited rapid loss of conductivity during cycling.15 By introducing carbon black nanoparticles in two steps, LCENCs with both high conductivities and large reversible strains were achieved. As shown in Table 1, for bulk carbon black contents of 2−6 wt %, LCE-NCs with low resistivity (
