Electrochemical Corrosion of STS304 Acupuncture Needles by Electrical Stimulation

1. Introduction 

Electroacupuncture (EA) is a widely used form of acupuncture. Compared with manual needling, it has the benefit of a stronger therapeutic effect and is more cost effective for the patient [1]. However, the proper application of EA requires appropriate methodology for electrical stimulation at the acupoints of the body. Most metal acupuncture needles are made of Type 304 (STS304) or Type 316 stainless steel. Generally, stainless steels are widely employed in medical implants, such as stents and orthopedic replacements, because of their relatively low cost, ease of fabrication, reasonable chemical stability, and good resistance to corrosion. However, it is unavoidable that all metals experience some extent of electrochemical dissolution in the implant environment, which comprised ample body fluids, minerals, amino acids, and proteins [2].

Acupuncture needles composed of STS304 are increasingly being used as implanted electrodes for functional electrical stimulation of acupuncture points, or acupoints, clinically and in experimental research, even though their biocompatibility is poorly understood. The notion of biocompatibility has evolved from the previous concept of an inert material to a more recent model based on a material's ability to perform with an appropriate host response in a specific environment [3]. The detection of apoptosis is widely used in biocompatibility studies of biomaterials.

Although several studies have assessed the in vitro behavior of these materials [2, 3, 4, 5], no in vivo study has been performed to assess the biocompatibility of stainless steel corrosion products for electrical stimulation. The aim of this study was to examine the stability of STS304 acupuncture needles during in vivo electrical stimulation. Thus we investigated the morphological changes in acupuncture needles and acupoints after electrical stimulation.

2. Materials and Methods 

3. Results 

3.1. Acupuncture needle surface characterization 

We used an SEM to image a representative surface of an acupuncture needle to evaluate the severity of corrosion. As shown in Figure 1, the damage to the acupuncture needles was severe, considering the level of applied current. No significant differences in the tip shape of the acupuncture needles were observed in needles that had been used for electrical stimulation with a small amount of current. By contrast, the penetrated margin of the STS304 acupuncture needle surface was degraded by atmospheric oxygen after electrical stimulation (2-Hz frequency, 0.05-mA intensity, 0.25-ms pulse width, 30-minute duration; Figure 1C). Electrical stimulation with currents in excess of the capacity of STS304 acupuncture needles resulted in severe corrosion of the tip and the penetrated margin from atmospheric oxygen (10-Hz frequency, 1-mA intensity, 10-ms pulse width, 30-minute duration; Figures 1B and 1D).

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Figure 1. Scanning electron micrograph of STS304 acupuncture needles. (A) and (C) were treated with an electrical current of 2 Hz, 0.05 mA, 0.25-ms pulse width and 30-minute duration using a PG-306 pulse generator. (B) and (D) were treated with an electrical current of 10 Hz, 1 mA, 10-ms pulse width and 30-minute duration using a Grass S88 stimulator. (C) and (D) show the morphology of the acupuncture needle surface inserted into the tibial muscle.

The electrical capacity of thin STS304 acupuncture needles in vivo depends on the thickness of the needles and the amount of oxygen in the atmosphere. The surface composition of the acupuncture needles was assessed by energy dispersive x-ray spectrometry. The amount of carbon, nitrogen and oxide in the tissue increased following electrical stimulation, but the amount of iron decreased due to degradation of the acupuncture needle surface (Table 1). The first step of acupuncture needle degradation following electrical stimulation was an increase in the composition of silica at the penetrated margin with air (Figure 1C, Table 1).

Table 1. Relative chemical composition (%) of stainless steel 304 (STS304) acupuncture needles at the rectangular boxes of Figure 1 using scanning electron microscope and energy dispersive x-ray spectrometry

		C	N	O	Na	Al	Si	S	Cl	K	Sn	Ca	Cr	Fe	Ni	Cu	Mn	P/Mo
	STS304	0.08	−	−	−	−	0.94	0.03	−	−	−	−	18.7	71	9.2	−	2.1	0.04
	A	3.25	1.93	3.31	0.42	2.29	0.78	0.21	0.21	0.15	−	0.13	18.27	61.16	7.25	0.65	−	−
	B	8.21	2.3	9.5	1.37	3.01	0.95	0.41	0.49	0.18	0.33	−	15.96	51.82	5.47	−	−	−
	C	10.83	1.99	15.56	0.85	0.42	2.39	0.24	5.96	0.27	0.28	2.37	12.93	39.78	4.7	1.42	−	−
	D	12.48	2.63	28.94	0.6	18.13	0.58	−	19.08	0.25	−	−	3.04	11.71	2.56	−	−	−

3.2. Detection of fragmentation of nuclear DNA at the EA-stimulated point 

We examined apoptotic cell death due to the corrosion products of acupuncture needles that developed following 30 minutes of electrical stimulation with an electrical current of 10 Hz, 1-ms width, and 1-mA intensity (Figures 2B, 2D, and 2F). A normal region (Figures 2A, 2C and 2E) of the same tibial muscle insertion with an acupuncture needle was used as a control. On the fifth day after electrical stimulation in the tibial muscle and skin, corrosion products of STS304 acupuncture needles were found to remain in the skin (Figure 2B), connective junction of skin and muscle (Figure 2D), and deep muscle (Figure 2F), while no significant change was seen in normal regions (Figures 2A, 2C and 2E). Furthermore, apoptotic cell death in the skin and muscle layer appeared in the vicinity of acupuncture needle corrosion products on the fifth day after electrical stimulation.

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Figure 2. Apoptotic nuclei are present (brown stained TUNEL reaction) on the fifth day following electrical stimulations. (A, B) Skin. (C, D) Connective junction of skin and muscle. (E, F) Deep muscle. The arrow shows the corrosion product of acupuncture needles. All of the tissues were stained with methyl green counterstaining solution. (A, C, E) Acupuncture needles inserted without electrical stimulus (B, D, F) Treated with an electrical current of 10 Hz, 1-ms width and 30-minute duration with the Grass S88 stimulator.

4. Discussion 

The aim of this study was to examine the stability of STS304 acupuncture needles by in vivo electrical stimulation. We found that electrical stimulation changed the morphology of the acupuncture needles and stimulated acupoints.

Little information is available to guide the acupuncturist in the placement of intradermal electrodes for EA. Generally, the needles are inserted as deeply as possible into the target acupoints, with or without concomitant electrical stimulation. We demonstrated the stability of implanted acupuncture needles used for electrical stimulation. We also studied the effects of STS304 acupuncture needle corrosion products, which formed following electrical stimulation, on the morphology of intradermal acupoints.

In the electrochemical study of STS304 in simulated body fluids over a one-week period, STS304 became more susceptible to pitting corrosion when it was covered with cells [2]. The corrosion resistance of stainless steel 316 decreased in the presence of cultured murine fibroblast L929 cells, and a decrease in diffusivity at the interface was indicated by a decrease in the cathodic current density. In the cytotoxicity test of stainless steel 316 corrosion products on vascular smooth muscle cells, growth inhibition, altered cell morphology, and induced cell necrosis correlated well with an increased level of nickel ions in the corrosion products [3]. It is well known that the presence of microorganisms on a metal surface can alter its local physical and chemical properties and lead to microbiologically influenced corrosion [6]. Although these in vitro studies of corrosion products demonstrated the possibility of corrosion in cell cultures, in vivo studies that have assessed corrosion-related changes of STS304 acupuncture needles for electrical stimulation are limited.

A previous study found that techniques for the selective activation of small axons require long-duration stimulation pulses (> 500 μs) and large stimulation amplitudes, which may lead to electrode corrosion [7]. Popovic et al [8] reported that bipolar intramuscular electrodes are more efficient and more selective than monopolar electrodes, but only if they are placed as far apart within the muscle as possible. They also noted that anodic corrosion may rule out the bipolar configuration for electrode materials such as stainless steel [8]. With a balanced bipolar current, it is assumed that the chemical products of electrolysis are neutralized with each polarity reversal. However, this may not be the case at low frequencies, or if the pulse duration or interphase interval is long [1]. Further more, Cargo et al [9] reported that the optimal pulse duration for reducing the possibility of tissue damage is less than or equal to 0.01 ms for intramuscular stimulation.

In this study, two acupuncture needles were inserted within specific acupoints, as close as possible, to determine the effects of bipolar currents on specific acupoints. Electrode acupuncture needles for electrical stimulation were inserted percutaneously via needles, even into deep muscles. The risk of tissue damage and acupuncture needle corrosion was studied as a function of the conditions of electrical stimulations being used in clinical and animal studies.

The penetrated margin of the STS304 acupuncture needle surface was particularly easily degraded by atmospheric oxygen after electrical stimulation (2-Hz frequency, 0.2-ms pulse width, 30-minute duration). The duration of EA stimulation is typically 15 to 30 minutes, but the duration may be longer in cases of dental and surgical analgesia, withdrawal or paralysis [10]. The intensity of electrical stimulation in animal studies is generally stronger than in clinical studies because of the use of anesthesia. This means that the electrical conditions of acupuncture in animal studies make the STS304 acupuncture needles more susceptible to corrosive change than in a clinical setting.

Noninsulated STS304 acupuncture needles are being used as electrodes for electrical stimulation in Oriental medicine, without any safety guidelines for electrical stimulation. Therefore, we recommend that the possibility of STS304 acupuncture needle corrosion should be announced on the packaging of acupuncture needles to protect both patients and operators from acupuncture needle corrosion, which may occur during electrical stimulation. This recommendation is particularly important when discussing the standards of acupuncture needles due to the broad use of stainless steel acupuncture needles as electrodes for electrical stimulation in Oriental medicine.

In conclusion, the biocompatibility of corrosion products of STS304 acupuncture needles might affect the tissue response following electrical stimulation. Further study is needed to protect both patients and practitioners from the detrimental effects of acupuncture needle corrosion during electrical stimulation.