Abstract

Miniature implantable electronic devices play increasing roles in modern medicine. In order to implement these devices successfully, the wireless power transfer (WPT) technology is often utilized because it provides an alternative to the battery as the energy source; reduces the size of implant substantially; allows the implant to be placed in a restricted space within the body; reduces both medical cost and chances of complications; and eliminates repeated surgeries for battery replacements. In this work, we present our recent studies on WPT for miniature implants. First, a new implantable coil with a double helix winding is developed which adapts to tubularly shaped organs within the human body, such as blood vessels and nerves. This coil can be made in the planar form and then wrapped around the tubular organ, greatly simplifying the surgical procedure for device implantation. Second, in order to support a variety of experiments (e.g., drug evaluation) using a rodent animal model, we present a special WPT transceiver system with a relatively large power transmitter and a miniature implantable power receiver. We present a multi-coil design that allows steady power transfer from the floor of an animal cage to the bodies of a group of free-moving laboratory rodents.

Keywords: implantable device, wireless power transfer, animal experiment, power transmitter, power receiver, antenna, double-helix coil, blood vessel, nervous system, resonance, even magnetic field, miniaturization

## 1. Introduction

Diagnosis and treatment of human diseases using an implantable electronic device represent a new trend in modern medicine. While the developments of biosensors, bioelectric stimulators and drug release mechanisms are important in the designs of medical implants, these developments are application specific. Therefore, they cannot be studied in a unified fashion. On the other hand, essentially all implantable devices require a common component: a power supply, which is usually a battery. Recent advances in wireless power transfer (WPT) provide an alternative method to power implantable electronic devices [1–3]. The WPT technology not only eliminates the needs of repeated surgical replacements of a depleted battery

within the human body, but also reduces the size of the implant, simplifies the implantation procedure, and enables the device to be placed in restricted anatomic locations prohibitive to large implants.

during surgery. In addition, the optimal orientation of the implant may not comply with the orientation of the power transmitter coil outside the human body. In order to solve these problems, we will present a new form of coil, called double-helix (DH) coil (Figure 1c), to be applied to tubular organs within the human body. Implantable devices are used not only for diagnosis and treatment of human diseases, but also for developments of new drugs and therapeutic mechanisms (e.g., electric stimulation). In the early stage of these developments, an animal model (e.g., the rodent model) is often utilized to study both treatment efficacies and side effects. In these studies, specially designed microsensors are often implanted within the body of a laboratory rodent to measure certain variables of interest [9]. Frequently, animal behaviors are also monitored by videotaping and other means [15]. This approach often encounters a significant problem of lacking a suitable power supply because the use of either a battery or a wire connection to the implant inside the body seriously interferes with animal's mobility and behavior. In this case, the application of the WPT technology is essential because it allows much reduced weight and size of the system being carried by, or implanted within, the rodent [16]. In order to provide the animal with a sufficient space for free movements, a special WPT system with a large stationary transmitter (in which the coil is embedded under the floor of an animal cage) and a miniature receiver (implanted within or carried by the animal) is

Wireless Power Transfer for Miniature Implantable Biomedical Devices

DOI: http://dx.doi.org/10.5772/intechopen.89120

required. In order for the WPT system to perform properly regardless of the animal's location within the cage, the transmitter must produce an even radio-frequency (RF) magnetic field throughout the floor of the animal cage. As a result, the wirelessly delivered energy is relatively even everywhere over the entire floor. This chapter studies this problem and presents a seven-coil design with several desirable proper-

This chapter is organized as follows. In Section 2, we describe the DH coil that can be applied to tubular organs within the human body. The coupling factor and power transfer efficiency (PTE) were analyzed. To further evaluate the performance of the DH coil, both simulations and experiments were conducted and presented. In Section 3, we present a power mat consisting of an array of planar transmitter coils. This mat produces a nearly even magnetic field distribution over the entire animal cage floor. For clarity, we present our evaluation, simulation and/ or experimental results at the end of each methodological section. Finally, we

The human body contains networks of tubular organs, such as nerves, lymphatic channels and blood vessels [17]. In order to monitor the functions or pathologic states of these organs (e.g., clogging of a certain major blood vessel) or provide therapeutic functions (e.g., stimulating a peripheral nerve), a wirelessly powered miniature implant wrapped around a tubular or rod-like biological structure is highly desirable. Although an ordinary solenoidal coil can support this wrap-around implant, an intact tubular organ usually cannot be cut and rejoined to allow a solenoidal coil to be threaded into the desired implanting position. Alternatively, one may wind the coil wire around the tubular organs manually during surgery. This method is practically unacceptable due to the restricted time of surgery and difficulties in quality control manual winding. Another method is to wrap the tubular organ by a coil that has been cut longitudinally. To reform an intact coil, a surgeon needs to reconnect the wires by soldering or using special connectors. This approach is also unrealistic due to the high risk of infection involved and the

ties, including the theoretical optimality and ease of implementation.

conclude this chapter in Section 4.

possible failure of the connectors.

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2. Double-helix coil for wrap-around implants

Due to the importance of WPT in the next-generation medical implants, there have been extensive studies over recent years [4–7]. One of the major limiting factors in battery-less implants is the low power output at the receiving end due to the weak coupling of the wireless power link. Many existing WPT components in biomedical implants operate in the low-MHz frequency range, e.g., those utilizing the widely accepted 13.56 MHz industrial, scientific, and medical (ISM) band. Although the WPT component in this frequency range is easy to design and robust, a relatively large receiver antenna is required, which limits its application to implantable devices in millimeter scales [8]. In recent years, it has been demonstrated that this challenge can be overcome by increasing the operating frequency and producing spatially focused regions within the biological tissue [9]. This approach effectively makes the WPT component smaller; however, the design of the power receiver coils that both operate in a high frequency and adapt to anatomical features of biological organs or tissues has not be well studied. In essentially all magnetic resonance based WPT systems reported, the power receiver within the implant utilizes a spiral coil in either a planar or a solenoidal form, as shown in Figure 1a and b, respectively [4–6]. In general, the planar spiral coil (PSC) [4, 5, 7] has been used in implants having a relatively larger surface, such as certain cardiac pacemaker [10]. In other cases, medical implants are often designed in a cylindrical or capsular shape such as the Bion® microstimulator [11]. For these cases, the use of a solenoidal coil is more common [12–14]. Despite successful designs exist, these two forms of coils cannot meet the requirements for all medical implants. For example, the human body has many tubular- shaped organs, such as nerves, lymphatic channels and blood vessels. An implant that wraps around such a biological structure to perform sensing and therapeutic functions is often desirable. In these cases, serious problems are encountered, with rare exceptions, because the structure is not allowed to be cut, and it is difficult to cut and rejoin a solenoidal coil

Figure 1. Power receiver coils within the body: (a) planar spiral coil, (b) solenoidal coil, and (c) DH coil.

#### Wireless Power Transfer for Miniature Implantable Biomedical Devices DOI: http://dx.doi.org/10.5772/intechopen.89120

during surgery. In addition, the optimal orientation of the implant may not comply with the orientation of the power transmitter coil outside the human body. In order to solve these problems, we will present a new form of coil, called double-helix (DH) coil (Figure 1c), to be applied to tubular organs within the human body.

Implantable devices are used not only for diagnosis and treatment of human diseases, but also for developments of new drugs and therapeutic mechanisms (e.g., electric stimulation). In the early stage of these developments, an animal model (e.g., the rodent model) is often utilized to study both treatment efficacies and side effects. In these studies, specially designed microsensors are often implanted within the body of a laboratory rodent to measure certain variables of interest [9]. Frequently, animal behaviors are also monitored by videotaping and other means [15]. This approach often encounters a significant problem of lacking a suitable power supply because the use of either a battery or a wire connection to the implant inside the body seriously interferes with animal's mobility and behavior. In this case, the application of the WPT technology is essential because it allows much reduced weight and size of the system being carried by, or implanted within, the rodent [16]. In order to provide the animal with a sufficient space for free movements, a special WPT system with a large stationary transmitter (in which the coil is embedded under the floor of an animal cage) and a miniature receiver (implanted within or carried by the animal) is required. In order for the WPT system to perform properly regardless of the animal's location within the cage, the transmitter must produce an even radio-frequency (RF) magnetic field throughout the floor of the animal cage. As a result, the wirelessly delivered energy is relatively even everywhere over the entire floor. This chapter studies this problem and presents a seven-coil design with several desirable properties, including the theoretical optimality and ease of implementation.

This chapter is organized as follows. In Section 2, we describe the DH coil that can be applied to tubular organs within the human body. The coupling factor and power transfer efficiency (PTE) were analyzed. To further evaluate the performance of the DH coil, both simulations and experiments were conducted and presented. In Section 3, we present a power mat consisting of an array of planar transmitter coils. This mat produces a nearly even magnetic field distribution over the entire animal cage floor. For clarity, we present our evaluation, simulation and/ or experimental results at the end of each methodological section. Finally, we conclude this chapter in Section 4.
