An implantable compound-releasing capsule triggered on demand by ultrasound

Abstract

Implantable devices have a large potential to improve human health, but they are often made of biofouling materials that necessitate special coatings, rely on electrical connections for external communication, and require a continuous power source. This paper demonstrates an alternative platform, which we call iTAG (implantable thermally actuated gel), where an implanted capsule can be wirelessly controlled by ultrasound to trigger the release of compounds. We constructed a millimeter-sized capsule containing a co-polymer gel (NiPAAm-co-AAm) that contracts above body temperature (i.e. at 45 °C) to release compounds through an opening. This gel-containing capsule is biocompatible and free of toxic electronic or battery components. An ultrasound hardware, with a focused ultrasound (FUS) transducer and a co-axial A-mode imaging transducer, was used to image the capsule (to monitor in real time its position, temperature, and effectiveness of dose delivery), as well as to trigger a rapid local rise in temperature, contraction of gel, and release of compounds in vitro and in vivo. The combination of this gel-based capsule and compact ultrasound hardware can serve as a platform for triggering local release of compounds, including potentially in deep tissue, to achieve tailored personalized therapy.

Figure 1. Ultrasound-based approach for triggering implanted devices.

(a) Schematic diagram showing the use of focused ultrasound to trigger local temperature change and compound release, using thermally responsive components inside an implantable device. Picture of the FUS transducer used in this study. The man silhouette drawing belongs to SC. (b) Schematic diagram showing the fabrication steps for the implantable microdevice. (c) Picture of a NiPAAm-based gel (80:20 NiPAAm:AAM, 5% MBAAM) fabricated by UV lithography with dimensions on the micron range (scale bar: 500 μm). (d) Cross-section diagram (top) and top-down photograph (bottom) showing ~20% contraction of the gel across 8 °C (scale bar: 1 mm).

Figure 2. Temperature-triggered changes in gel behavior.

(a) Relative decrease of the NiPAAm:AAm gel diameter as a function of different NiPAAm:AAM ratios (left), and as a function of different MBAAM percentages (right). Experiments were performed in water. (b) Relative decrease of the gel diameter (for 85:15 w/v NiPAAm:AAm, 5% MBAAM) in water (○) and in PBS buffer (■) when the temperature is changed from room temperature to 50 °C using a water bath. The diameter of the gel at 24 °C (Do) was 7.3 ± 0.08 mm in water and 7.2 ± 0.11 mm in PBS. All data points correspond to the average of at least 3 different experiments and the error bars are calculated using the standard deviation. (c) Change of the NiPAAm:AAm gel diameter in water when the temperature is cycled between 37 °C and 45 °C, showing that NiPAAm gels can be thermally actuated for several cycles between 37–45 °C. (d) Percentage of TRITC-dextran (20 kDa) released from a device immersed in water when thermally actuated (◻) at 45 °C for 15 minutes once per day for four consecutive days, or not actuated (■). All devices were kept at 37 °C prior to and between actuations. Fluorescence (λ_ex = 540 nm, λ_em = 580 nm) was measured using a plate reader as a function of time. All data points correspond to the average of at least 3 different experiments and the error bars are calculated using the standard deviation. (E) Release of different MW fluorescent dextran from capsule over time.

Figure 3. FUS-triggered local temperature change and compound release of implanted gel.

(a) Schematic diagram of the setup for testing FUS-triggered drug release. (b) Ultrasound image of the device. The image shows a perpendicular cut of the device in the x-z plane. For clarity, a drawn overlay of the device shows its location. (c) Representative trace of a temperature profile inside the NiPAAm-co-AAm gel by immersing the device in a thermal bath, and applying FUS. The FUS was controlled using the temperature reading from a thermocouple inserted in the gel. For all experiments, the starting temperature was 37 °C. (d) Release of TRITC-dextran (20 kDa) as percentage release from the device when actuated (thermally (◻) or by FUS (■)), or no actuation applied (○). FUS-induced heating was able to mimic gel contraction and release produced by submersing the gel in 45 °C water bath. Temperatures of 37 °C (for no actuation) and 45 °C (for thermal bath and FUS) were verified by a temperature probe. All data points correspond to the average of at least 3 different experiments and the error bars represent the standard deviation.

Figure 4. Simulation and characterization of local FUS-induced temperature change.

(a) Simulation of FUS-induced heating in tissue. Region within black dash represents the FUS focal region, and region within white dash represents −6-dB beamwidth of the A-mode transducers. The gel device is shown within green dashes. (b) Experimental validation of ultrasound-based thermometry. Values of sound speed were measured for temperatures from 33 to 50 °C. Consecutive pulse-echo measurements (M-scan) can provide a measure of the apparent strain in the 1D image, which corresponds to thermally induced change in arrival times of echoes from the top and bottom of the PDMS layer. (c) Temperature-time profile of PDMS disks conductively heated by a hot plate, as measured by ultrasound thermometry and a thermocouple. (d) Use of ultrasound thermometry as feedback to guide FUS triggering. A sustained temperature was achieved over time, as measured by a thermocouple (blue).

Figure 5. Compound release in ex vivo chicken and in vivo mouse.

(a) Schematic diagram of the experiment set up used for the FUS-induced ex vivo experiments. During the ex vivo experiments the imaging transducer was used to non-invasively monitor the temperature inside the gel to control the FUS using a feedback loop. (b) Ultrasound image of the device placed between two layers of 10 mm of chicken breast. The image shows a perpendicular cut of the device in the x-z plane. For clarity, a drawn overlay of the device shows its location. (c) Merged bright image and fluorescent image (λex = 670 nm, λem = 702 nm) of a gel capsule sandwiched between a lower layer of chicken tissue and a top layer of mouse skin. The four samples were capsule alone, chicken tissue alone, capsule embedded in tissue at 37 °C on a hot plate for 10 minutes, and FUS-actuated (at set-temperature of 45 °C) for 10 minutes. The ultrasound actuation took place from the top through the mouse skin and the AlexaFluor-dextran release was from the bottom part of the capsule towards the chicken tissue. (d) In vivo results for FUS induced dextran release. Representative bright field and fluorescence images of control (no FUS actuation) and FUS-administered mouse. All mice were sacrificed and NiPAAm gels explanted before imaging. FUS actuation was for all cases 10 minutes at <00 W/cm² (estimated using radiation-force balance as demonstrated in prior studies6768). (e) Photograph of an explanted device after 2 weeks of being implanted in a mouse. (f) TUNEL stained histology samples of the tissue immediately on top of the gel for a mouse treated with FUS and a control one (no FUS).