Iontophoresis-driven microneedle patch for the active transdermal delivery of vaccine macromolecules

Coronavirus disease 2019 (COVID-19) has evolved into a pandemic and poses a great threat to public health. Vaccination provides an effective way to prevent pathogen infection. The development of effective vaccines is key to preventing epidemic transmission and achieving herd immunity. To date, preventive vaccines have eliminated many diseases, such as smallpox and poliomyelitis, and have the potential to curb the occurrence of many other common infectious diseases and halt the global spread of the emerging COVID-19 pandemic. Most vaccines are given by intramuscular (IM) injection, but this method has many inherent limitations: fear of needles in children and adolescents, procedural pain, and infections caused by the repeated use of needles. Due to the rich network of immune cells in the skin, transdermal immunization is becoming an attractive alternative. However, owing to the stratum corneum (SC) barrier, only small molecule vaccines (~500 Da) can be effectively administered through the skin and enter the systemic circulation, so macromolecular vaccines have poor skin permeability and low bioavailability. To break through the barrier of the SC and improve the transdermal permeation efficiency of vaccines, transdermal vaccination strategies have been developed to enhance the transdermal immune effect. Microneedles (MNs) provide a new solution in the field of transdermal vaccine delivery due to their unique advantages of painless minimally invasive delivery, self-administration, and improved permeability. The length of the MN is specially designed to give it the ability to penetrate the SC without stimulating nerve endings. The epidermis and dermis have a dense network of immune cells, such as Langerhans cells (LCs) and dermal dendritic cells (DDCs), whose anatomical sites can be reached by the MN during puncture. MNs provide microchannels through the SC by skin penetration, allowing the vaccine macromolecules to permeate the anatomical sites of the immune cells. However, transdermal immunity using MNs is promising. However, transdermal vaccine delivery using MNs usually relies on passive diffusion via poked microchannels, which severely limits the transport speed and efficiency into the skin. On the other hand, the iontophoresis technique can actively drive ionized and hydrophilic small molecules through the SC layer using a mild current (usually <0.5 mA/cm²), and therefore, this strategy has been applied in transdermal vaccine delivery and dermatological treatment. Iontophoresis can control the transport process of vaccine molecules via the applied electricity because of electromigration and electro-osmosis. Iontophoresis has been successful in promoting the transdermal delivery of small hydrophilic molecules, but it usually fails in the transdermal delivery of macromolecules (e.g., proteins with a molecular weight >13 kDa) due to the barrier of SC. However, most vaccines, such as nucleic acid vaccines, are macromolecules, which are difficult to administer transdermally into the skin using iontophoresis. Therefore, it is promising to utilize the synergistic advantages of macromolecule delivery using MNs and active delivery using iontophoresis to overcome the limitations of the low delivery efficiency of MNs and the failure of macromolecule delivery of iontophoresis. In this work, we developed a wearable iontophoresis-driven MN system for the efficient and active transdermal delivery of macromolecular vaccines. This system mainly consists of an iontophoresis-driven MN patch and an iontophoresis-driven device. The transdermal vaccine delivery strategy of the iontophoresis-driven MN patch is “press and poke,” iontophoresis-driven delivery, and immune response in which solid MNs are first pressed onto the skin to create microchannels through the SC and then automatically retracted. The vaccine macromolecules are then delivered through these created microchannels via passive diffusion and iontophoresis and captured by antigen-presenting cells in the epidermis and dermis, finally activating the cells to exert immune effects. The iontophoresis-driven MN patch combines the advantages of the MN and iontophoresis techniques and significantly promotes the transdermal delivery efficiency of vaccine macromolecules. Moreover, a flexible polyacrylamide/chitosan hydrogel with high loading ability and good electrical conductivity was selected as the vaccine storage chamber, which not only addressed the limitation for low vaccine loading of dissolvable MNs but also guaranteed stable conductivity between the electrodes and skin during iontophoresis. In vivo transdermal immunization demonstrated that the iontophoresis-driven MN patch could achieve an effective immune response that was even stronger than that using traditional intramuscular injection. Therefore, our iontophoresis-driven MN system is low-cost and universally, showing promise as an alternative for vaccine self-administration.

System design and components: - Developed a wearable iontophoresis-driven microneedle (MN) delivery system composed of (i) an iontophoresis-driven MN patch and (ii) a portable iontophoresis device supplying constant current. - The MN patch integrates Ag/AgCl electrodes on a flexible substrate, stainless steel 316L solid microneedles, circular polyacrylamide/chitosan conductive hydrogel blocks (serving as vaccine reservoirs and ionic conductors), and a double-sided adhesive impermeable elastic gasket that conforms to skin and prevents leakage/infection. Patch and device form factor: - Patch dimensions: 28 × 18 × 2 mm³; designed to adhere to the wrist and curved skin. - Device dimensions: 53 × 19 × 10 mm³ in a 3D-printed insulating shell; wearable via custom watchband; total system mass ~18 g. - Electrical connection via a flexible PCB connector delivering the iontophoresis current to the patch. Electronics and current control: - Battery-powered with rechargeable lithium cell (micro-USB charging). - Circuit comprises charging module, boost module (sets supply voltage for the constant-current stage), and a constant-current output module based on LM334. - Output behavior: voltage increases linearly with load resistance; constant current maintained across 5–20 kΩ loads (typical skin impedance ~10 kΩ). - Demonstrated stable iontophoretic current of 0.5 mA for 1 h with minimal fluctuation. Microneedle array fabrication and geometry: - Solid MNs micromachined from stainless steel 316L for high mechanical strength and biocompatibility. - Array: 69 needles uniformly arranged on a 12.4 mm cylindrical substrate. - Geometry: average height ~800 μm; tip radius ~20 μm; base diameter ~400 μm; inter-needle spacing ~1200 μm (center-to-center). - Sharp conical tips enable reliable penetration of the stratum corneum (SC). Hydrogel vaccine reservoir: - Polyacrylamide/chitosan hydrogel disks are elastic, conductive, biocompatible, and possess high vaccine loading capacity. - Hydrogels are positioned so MNs penetrate them and then the skin during pressing, ensuring intimate electrical contact and low resistance pathways during iontophoresis. Operational protocol (transdermal vaccine delivery strategy): 1) Press and poke: The adhered patch is pressed manually to drive solid MNs through the hydrogel and SC, forming transient microchannels. Upon release, elastic rebound of gasket and hydrogels retracts MNs from skin; microchannels gradually self-heal. The press can be repeated to reopen channels for additional delivery cycles. 2) Iontophoresis-driven delivery: Vaccine diffuses from hydrogel into skin via created microchannels (Fickian diffusion). A mild current is applied between the working electrode (on MN base) and a counter electrode, producing electromigration and electroosmosis, with most electroosmotic flow favoring the low-resistance MN-created pathways. Delivery rate depends on vaccine properties, microchannel characteristics, current amplitude, and duration. Passive diffusion and iontophoresis act synergistically. 3) Immune response: Antigens entering epidermis/dermis are captured by APCs (LCs, DDCs), which migrate to draining lymph nodes to activate T helper cells, initiating adaptive immune responses. In vitro and in vivo evaluation overview: - In vitro: Used ovalbumin (OVA) as a model macromolecular antigen; demonstrated that transdermal delivery amount is tunable via applied iontophoresis current. - In vivo: BALB/c mice immunization showed that transdermal OVA delivery using the iontophoresis-driven MN patch elicited effective immune responses exceeding those from conventional intramuscular injection.

- Synergistic enhancement: Combining MN-created microchannels with iontophoresis significantly increases transdermal delivery efficiency of macromolecular vaccines relative to MN passive diffusion alone or iontophoresis alone. - Dose control: In vitro tests with ovalbumin showed delivered amount can be modulated by adjusting iontophoresis current (current-dependent delivery). - Immunogenicity: In BALB/c mice, transdermal OVA delivery via the iontophoresis-driven MN patch induced an immune response stronger than traditional intramuscular injection. - Device performance: Portable device maintained a stable constant current of 0.5 mA for 1 h across skin-like impedances (~10 kΩ), with current output independent of load from 5–20 kΩ. - Practicality: The system is compact (patch 28×18×2 mm³; device 53×19×10 mm³), lightweight (~18 g), low-cost, and suitable for self-administration.

The study addresses the challenge that macromolecular vaccines poorly traverse the stratum corneum (SC) and that iontophoresis alone is ineffective for large biomolecules. By mechanically creating microchannels with stainless steel microneedles, the principal SC barrier is bypassed. Superimposing a controlled iontophoretic current leverages electromigration and electroosmosis along the low-resistance MN-formed pathways, augmenting and regulating transport beyond passive diffusion. The observed current-dependent in vitro delivery of ovalbumin validates controllability of dosing, while the stronger immune responses in BALB/c mice compared to intramuscular injection demonstrate that antigen deposition into skin-resident immune cell-rich layers enhances immunogenicity. The hydrogel reservoir increases antigen loading and ensures stable electrical contact, enabling reproducible iontophoresis. Collectively, the results substantiate that the MN–iontophoresis synergy effectively enables active, efficient, and controllable transdermal delivery of vaccine macromolecules with practical, wearable hardware suitable for self-administration.

This work presents a wearable iontophoresis-driven microneedle patch and companion constant-current device that together enable active, efficient, and controllable transdermal delivery of macromolecular vaccines. Key contributions include: (i) a press-and-poke strategy using retractable solid stainless steel MNs to open transient skin microchannels; (ii) a conductive, high-capacity polyacrylamide/chitosan hydrogel reservoir that supports both antigen storage and stable iontophoresis; (iii) a compact, low-power device delivering stable constant current; and (iv) validation that delivery can be tuned by current and that skin-targeted vaccination elicits immune responses surpassing intramuscular injection in mice. The platform’s low cost, usability, and form factor make it promising for at-home vaccine self-administration. Future work could optimize current waveforms and dosing schedules, broaden antigen types (including nucleic acids and adjuvants), investigate long-term safety and skin tolerability with repeated use, and translate to human-scale clinical evaluation.