Enhancement of Transdermal Absorption of drugs via Phonophoresis and TENS: An in vitro Study

Background: Neuromusculoskeletal pain is a complex condition often requiring multifaceted approaches for effective management. Gabapentin is frequently prescribed for such conditions due to its ability to modulate nerve transmission. However, systemic administration may lead to significant side effects. Enhancing transdermal drug delivery via non-invasive modalities presents a novel strategy for local pain control with minimized systemic risks. Objective: The aim of this study was to compare the effectiveness of Phonophoresis (ultrasound-assisted) and Transcutaneous Electrical Nerve Stimulation (TENS)-assisted techniques in enhancing transdermal drug absorption using an in vitro model, to support their application in pain management. Methods: A 5% gel formulation of gabapentin was prepared using Carbopol 934, ethanol, and standard gelling agents. Hairless mouse skin was used for the in vitro permeation study using Franz diffusion cells. Three intervention arms were tested: (1) Control (topical gabapentin only), (2) Phonophoresis with ultrasound (1 MHz and 3 MHz), and (3) TENS at varying frequencies (50 Hz, 100 Hz, 150 Hz). For Phonophoresis, ultrasound was applied in pulsed mode at intensities of 0.8 and 1.0 W/cm² for 5 minutes. For TENS, stimulation was provided using a trapezoidal sweep pattern for 10 minutes. Samples were collected at 0, 1, 2, and 3 hours and analyzed using HPLC. Results: The 1 MHz, 1.0 W/cm² Phonophoresis condition resulted in the highest permeation (35.0 ± 2.3 µg/cm² at 3 hours), followed by TENS at 150 Hz (30.0 ± 1.8 µg/cm²), and then control (12.0 ± 1.2 µg/cm²). Statistically significant differences (p < 0.05) were observed between all groups via ANOVA. Drug permeation was time-dependent and enhanced markedly by both active methods compared to passive application. Discussion: Ultrasound at 1 MHz penetrates deeper tissues, aligning well with the needs of neuromusculoskeletal pain management. The mechanical and thermal effects of ultrasound, such as cavitation and acoustic streaming, likely contributed to enhanced permeability. Although 3 MHz ultrasound is generally preferred for superficial applications, this study confirms the benefit of 1 MHz for deeper tissue targeting. TENS demonstrated moderate but meaningful enhancement in drug permeation, likely through improved local circulation and neural modulation. These findings reinforce the rationale for using 1 MHz Phonophoresis for pain control. Limitations include the use of hairless mouse skin instead of human samples, and the lack of long-term retention or penetration depth analysis. Future work should explore in vivo validation and examine therapeutic outcomes on pain scores and functional recovery. Conclusion: This study confirms that Phonophoresis using 1 MHz ultrasound significantly enhances transdermal drug delivery, outperforming both TENS and control groups. These findings support the integration of ultrasound-assisted transdermal therapy in physiotherapeutic pain management regimens. Further clinical research is warranted.