Bioresorbable Microdroplet Lasers As Injectable Systems for Transient Thermal Sensing and Modulation
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Article www.acsnano.org Bioresorbable Microdroplet Lasers as Injectable Systems for Transient Thermal Sensing and Modulation Daniel Franklin, Tyler Ueltschi, Andrea Carlini, Shenglian Yao, Jonathan Reeder, Benjamin Richards, Richard P. Van Duyne, and John A. Rogers* Cite This: ACS Nano 2021, 15, 2327−2339 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: Minimally invasive methods for temperature sensing and thermal modulation in living tissues have extensive applications in biological research and clinical care. As alternatives to bioelectronic devices for this purpose, functional nanomaterials that self-assemble into optically active microstructures offer important features in remote sensing, injectability, and compact size. This paper introduces a transient, or bioresorbable, system based on injectable slurries of well-defined microparticles that serve as photopumped lasers with temperature-sensitive emission wave- lengths (>4−300 nm °C−1). The resulting platforms can act as tissue-embedded thermal sensors and, simultaneously, as distrib- uted vehicles for thermal modulation. Each particle consists of a spherical resonator formed by self-organized cholesteric liquid crystal molecules doped with fluorophores as gain media, encapsulated in thin shells of soft hydrogels that offer adjustable rates of bioresorption through chemical modification. Detailed studies highlight fundamental aspects of these systems including particle sensitivity, lasing threshold, and size. Additional experiments explore functionality as photothermal agents with active temperature feedback (ΔT =1°C) and potential routes in remote evaluation of thermal transport properties. Cytotoxicity evaluations support their biocompatibility, and ex vivo demonstrations in Casper fish illustrate their ability to measure temperature within biological tissues with resolution of 0.01 °C. This collective set of results demonstrates a range of multifunctional capabilities in thermal sensing and modulation. KEYWORDS: temperature, self-organization, biodegradable, sensors, imaging Downloaded via NORTHWESTERN UNIV on February 23, 2021 at 23:17:47 (UTC). he temperature of biological systems and its active diagnostic and/or therapeutic functionality with a limited regulation are essential for life processes in all residence time in the body, engineered to coincide with a See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. advanced organisms. From metabolic and ionic activity transient biological process such as wound healing.11 T fl at the cellular level to uidic perfusion and immunological Recent strategies in remote temperature sensing exploit activity at the macro/organ-level, temperature affects tissue optically active materials, such as molecular probes,12,13 function, and deviations can indicate underlying illness and upconversion nanoparticles,14 or quantum dots15,16 assembled − inform diagnosis. Once detected, active control of temperature into nano/microparticles17 19 for direct injection into bio- can then serve as the basis for therapies to regulate chemical logical tissues. Careful selection of materials and structures imbalances,1 induce regional apoptosis,2,3 or activate/disrupt 4−6 enables excitation at wavelengths of relative tissue trans- neurological pathways. Bioelectronic devices can provide parancy, with consequent radiation of light that can pass back many of these required functions, but they involve invasive through the tissue for remote detection. Fluorescence imaging methods for insertion and require electrical readout with associated interconnected traces, digital electronics, and/or antenna structures that have dimensions in the millimeter to Received: December 7, 2020 centimeter range.7,8 Functional nano/microparticle probes Accepted: January 5, 2021 formed in biocompatible and bioresorbable materials and Published: January 13, 2021 configured for injection, optical imaging, and remote sensing represent promising alternatives.9,10 Additionally, fabrication of such systems from bioresorbable materials can lead to © 2021 American Chemical Society https://dx.doi.org/10.1021/acsnano.0c10234 2327 ACS Nano 2021, 15, 2327−2339 ACS Nano www.acsnano.org Article Figure 1. Bioresorbable microdroplet lasers for biological temperature sensors via complex coacervation. (a) Illustration of a gelatin encapsulated, dye-doped LC microdroplet laser with internal radial helix director profile. (b) Bragg reflectance due to the molecular helix and corresponding band-edge lasing when pumped with a nanosecond laser at 532 nm. Inset micrograph is of a single particle under crossed polarized illumination. Scale bar is 50 μm. (c) Temperature tunable lasing via changes in LC molecular helical pitch. (d) Schematic illustration of the complex coacervation fabrication process utilized to encapsulate LC droplets in shells of gelatin. (e) Micrographs depicting encapsulated particles “en masse” and the large-scale nature of complex coacervation production. Scale bar is 200 μm and inset scale bar is 50 μm. (f) Resulting particle distributions of LC microdroplets measured via laser scattering. of this type allows for measurements of temperature in deep biologically compatible materials that persist within tissues tissue via ratio-metric,20,21 wavelength-shifting,22 and time- and/or are toxic. In addition, such lasers have not been resolved fluorescence23 in which the amplitude, wavelength, or explored for localized thermal modulation. lifetime of fluorescence, respectively, correlate to local This paper introduces a cholesteric liquid crystal (CLC) temperature. However, the wavelength-dependent absorption microdroplet laser composed of and encapsulated in and scattering of biological tissue necessitates excitation power biocompatible materials, including those designed to undergo and depth calibration curves that are impractical for in vivo complete bioresorption over controlled time scales. Here, sensing where the exact optical path is uncertain.24 biologically derived cholesteryl-esters and an organic fluo- In contrast, methods that encode temperature to emission rescent dye form the basis of tunable photopumped lasers via wavelength are undistorted by depth or optical path. Within self-assembly into temperature-sensitive photonic cavities. this category of wavelength-tunable probes, micro/nano- When pumped optically, the particles emit laser light, with particle systems that support lasing provide attractive features wavelengths that depend on the local temperature. Encapsu- such as narrow wavelength operation, high output intensity, lation in gelatin hydrogel shells via complex coacervation and tunability through resonant cavity design.10,17,25,26 These enables the large-scale production of sensors of this type, in properties support enhanced sensitivity and create prospects forms that can be injected with a syringe into tissue locations for functionality in deep tissue locations through engineering of interest. Systematic evaluations of lasing dynamics define the of emission characteristics. Photonic crystal lasers that exploit underlying physics of operation and the functional bounds of dye-doped liquid crystals (LCs) offer the highest temperature this system in terms of key parameters such as sensitivity and sensitivity within this class of optical schemes.27 However, minimum probe size. Quantitative measurements of laser- these systems have largely been demonstrated with non- induced heating lead to three independent modes of operation: 2328 https://dx.doi.org/10.1021/acsnano.0c10234 ACS Nano 2021, 15, 2327−2339 ACS Nano www.acsnano.org Article (1) temperature sensing, where self-heating is minimal, (2) a nonlinear temperature response, with a peak sensitivity in thermal modulation, where temperature sensing accompanies excess of −300 nm °C−1. This large, negative sensitivity laser-induced heating to establish a closed-feedback loop for originates from a phase transition from cholesteric to smectic active control, and (3) rapid and minute modulation of probe A, in which the chiral molecular orientation rapidly “unwinds” temperature, where analysis of decay profiles yields thermal to yield a stratified and layered vertical orientation. The transport properties of the surounding media. Accelerated tunable and exceptionally high sensitivities that are possible in enzymatic degradation studies reveal the chemical mechanisms these systems represent significant advantages over alternative of bioresorption of the hydrogel shell with kinetics that can be materials and modalities for temperature sensing. Supplemen- tuned through shell cross-linking. Cytotoxicity assays using tary Figure S1 also compares the shift in Bragg reflectance mouse L929 cells demonstrate basic biocompatibility of the based on confinement for the CLC compound within a materals and the products of their degradation. Injection and parallel-plate cell and a microsphere, respectively. characterization of lasing within Casper fish models, ex vivo, Encapsulating the CLC microdroplets in shells formed with highlights the functionality of the microdroplets in biological a cross-linked hydrogel (50% porcine skin gelatin, 50% gum tissues. Taken together, the results presented here demonstrate acacia; thicknesses up to 30 μm) via complex coacervation33 multifunctional capabilities in both active and passive thermal stabilizes and protects the cholesteric core from surrounding interfaces to living systems, with possibilities for immediate