W43 - Ferrofluid-linked Acoustic Resonance Enhancement (FLARE) Bidirectional Needle

Introduction: : Microneedles have revolutionized drug delivery by providing minimally invasive and efficacious solutions. The ability to penetrate the skin barrier with minimal discomfort has driven progress in wearable and implantable systems, advancing ongoing therapeutic applications [1]. Many existing designs are limited to superficial treatments or lack the necessary precision for deep tissue targeting, resulting in significant shortcomings in dealing with complex biomedical complications [2].
This work presents a Ferrofluid-Linked Acoustic Resonance Enhancement (FLARE) microneedle (Fig. 1). The needle system induces a thermal gradient via ultrasound between its tip and the base, resulting in fluid flow through the center cylinder. The heating element is a magnetically responsive ferrofluid inside the outer cylinder, which can be strategically positioned via an external magnet. Ferrofluids demonstrate significant thermal and magnetic responsiveness, enabling temperature gradients under ultrasound [3]. Simulated by magnetic actuation and an optimized microneedle design, it allows targeted drug delivery with enhanced stability and precision [4-5]. Henceforth, we will call it Ferrofluid-Linked Acoustic Resonance Enhancement (FLARE) bidirectional needle.

Materials and

Methods: : Fig. 1a presents the working principle of the FLARE bidirectional microneedle system. The system utilizes an external magnetic field generated by a permanent magnet (neodymium) to aggregate magnetic particles in the ferrofluid at the tip of the microneedle. Ultrasound irradiation using a high-intensity focused ultrasound (HIFU) transducer (670 kHz center frequency, Precision Acoustics) agitates the ferrofluid, resulting in localized heating. This induces a temperature gradient between the tip and the base. As the temperature gradient tends to decrease fluid density in the heated area, fluid flows from cold to warmer regions.
To validate the concept, we fabricated the prototype (Fig. 1b) using an SLA 3D printer (Form 3B+, Formlabs). The total length was 30 mm, and the diameter was 5.5 mm (5 mm in diameter for the outer cylinder and 1.5 mm for the inner cylinder). The outer cylinder was filled with a mixture of commercial ferrofluid (Ferrotec EMG 900, USA) and DI water. Fig. 2 shows an experimental setup to measure thermal gradients within the needle while supplying ultrasound from the top. Three K-type thermocouples (AZ Instruments) were inserted inside the outer cylinder of the needle to monitor the temperature gradient.

Results, Conclusions, and Discussions:: The experimental results (Fig. 3) show the temperature gradients of the FLARE needle using standardized tests on three separate samples. A 30 mm microneedle was used, and the ultrasound transducer was aligned with the magnet positions during each reading for 30 min, with an increasing ultrasonic intensity (with Vin of 50, 100, and 150 mV) every 10 min. The conditions tested were: (1) no magnet (NM), (2) magnet at the tip (MT), and (3) magnet at the base (MB).
The results indicated that the temperature gradient (ΔT) at the tip (Fig. 3b) improved by 14.19 °C under the MT as compared to NM. Similarly, the base section (Fig. 3d) showed a gradient of 10.05 °C under the MB. Additionally, the measured temperature gradients (ΔT) are 26.23 °C between the tip and the base with a magnet at the tip, whereas, in the control condition, i.e., the absence of the magnet, it was 11.67 °C. Notably, the temperature remained stable with magnet placement, while significant fluctuations were observed in the non-magnet condition. These findings validate that the strategic alignment of ultrasound and magnets enhances thermal control, achieving consistent temperature gradients for precise drug delivery and biomarker extraction.

Acknowledgements and/or References (Optional):: [1] M. Zheng, et al., Nat Rev Bioeng, 2023.
[2] F. Tehrani et al., Nat Biomed Eng, 2022.
[3] M. Kole and S. Khandekar, J. of Mag Mat, 2021.
[4] S. Kusama et al., Nat Commun, 2021.
[5] J. Chen et al., Front Bioeng Biotechnol, 2022.