Pulsed Electrodeposition for Preparing Hollow, Conical Needles with Sub-Micron Dimensions
Authors: Durney Andrew , Hotvedt Elizabeth Claire , Mukaibo Hitomi 
Published: February 2020

Arrays of vertically-aligned nano/micro-needles are widely studied for various applications related to energy, health and the environment. In particular, there is an increasing interest in using these materials as a gene delivery device, where delivery is typically done by piercing cells with DNA-coated needles. Conically-shaped nano/micro-needles offer a unique advantage for this application: the broad base provides mechanical stability, while the sharp tip results in less cell trauma. On the other hand, hollow-structured needles can allow direct fluidic intracellular access and can inject genetic material without the need for complex surface modification.
Despite these benefits, preparation of hollow, conical needle arrays (hcNAs) for gene delivery has remained a challenge due to the small dimensions required to pierce individual cells, which are typically sized between 10 and 100 μm. Here, we describe a template synthesis method using pulsed current electrodeposition that results in hcNAs that are less than 2 μm in diameter.
Poly(ethylene terephthalate) (PET) membranes with conically-shaped pores were prepared and served as the template for the hcNA synthesis (1). Following previous work, an electroless deposition method was used to obtain solid gold needles that replicate the entire structure of the template pore (Figure 1a) (2). We confirm vertical conical structures with heights of 6.27 ± 0.28 μm and base and tip diameters of 1.21 ± 0.05 and 0.17 ± 0.04 μm, respectively.
Hollow, conical needle arrays were prepared using an established nickel plating bath and pulsed current (PC) electrodeposition method (3). A typical voltage/current vs. time waveform for the deposition steps is shown in Figure 2. It is well-known that PC electrodeposition enhances the metal ion concentration at the electrode surface by allowing diffusion during the “off” pulse. This is expected to be beneficial for electrodeposition of nickel into conical pores with high aspect ratios and sub-micron diameters. Conventional galvanostatic instrumentation was used with a nickel wire counter electrode and Ag/AgCl reference electrode. Prior to electrodeposition, a thin layer of gold was sputtered onto the template surface for conductivity. The deposition parameters (i.e.current density, “on” pulse duration, duty cycle and number of cycles) were varied to study the effect of mass transport within the template conical pores. After electrodeposition, the PET template was removed by chemical dissolution to expose the needle array.
Microneedles were imaged with field-emission scanning electron microscopy (FE-SEM). Figure 1b shows the Ni needle array that was prepared using PC electrodeposition. The height of the Ni needles are 2.45 ± 0.23 mm and their base and tip diameters are 1.07 ± 0.06 and 0.68 ± 0.01 μm, respectively. These values indicate that nickel has deposited partially into the pores, but not all the way to the tip. The Figure 1 insets on the lower right show needles imaged from above. In contrast to the inset in Figure 1a, where the contrast is brightest at the center (i.e.where the needle tip is located), the inset in Figure 1b shows a darker gray region in the center. This indicates that the needle has a concave feature, and that we were successfully able to prepare hcNAs by combining PC electrodeposition with template synthesis. In our presentation, we will discuss in further detail the effect of pulse deposition parameters on the mass transport of Ni-ions into the template pores, and how these parameters may be used to control the dimensions of hcNAs.
References
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2. V. P. Menon and C. R. Martin, Anal. Chem., 67, 1920 (1995).
3. F. Nasirpouri, M. R. Sanaeian, A. S. Samardak, E. V. Sukovatitsina, A. V. Ognev, L. A. Chebotkevich, M. G. Hosseini and M. Abdolmaleki, Appl. Surf. Sci., 292, 795 (2014).