2. Double-helix coil for wrap-around implants

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 possible failure of the connectors.

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

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

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

locations prohibitive to large implants.

Recent Wireless Power Transfer Technologies

Figure 1.

70

In addition to these practical difficulties, the usage of a solenoidal coil for wraparound implants suffers from another implementational problem. In most cases, it is desirable to use a flexible PSC for the power transmitter because this planar coil can be easily integrated with a garment, a blanket or a bedding sheet, providing a high convenience for unobtrusive recharging of the implant. In order to achieve the maximum coupling between the transmission and reception coils, the solenoidal coil of the implant is expected to be oriented perpendicularly to the body surface (Figure 2a). This requirement is highly problematic because, as indicated by human anatomy [17], many tubular organs within the body are oriented parallel to the body surface, which provides the worst orientation to the PSC for power transmission because of the weakest magnetic coupling (Figure 2b).

2.1 Structural design of DH coil

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

clockwise current direction.

wireless power delivery.

2.2 Inductance and mutual inductance of DH coil

LDH <sup>¼</sup> <sup>X</sup><sup>n</sup>

denotes the inductance of a single cell.

Figure 4.

73

i¼1

Xn j ¼ 1 j 6¼ i

Figure 4a shows a set of parallel sinusoidal wires printed on both sides of a flexible PCB. This PCB is made of polyimide film which is characterized by high strength, low RF energy loss, small thickness, and high flexibility. The wires on two sides are connected to each other in series via a column of through holes (one of them is shown), forming closed loops. The current paths are denoted by the black arrows in Figure 4a. In addition to the DH coil, sensors, actuators, microprocessor and electronic elements (not shown) can be installed on the same flexible PCB. During surgery, the hermetically sealed PCB (using a biocompatible polymer material) is wrapped around the tubular structure at the position of interest forming a double helix winding along with all electronic components, as shown in Figure 4b. It can be observed that, after the tubular structure is formed, the closed loops on the two sides of the PCB form opposite tilt angles but maintain the clockwise/counter-

Wireless Power Transfer for Miniature Implantable Biomedical Devices

The two-layer structure in Figure 4 can be extended to a multiple-layer structure using a PCB with more than two layers. Similarly, the 45° tilted angle in each layer can be modified to adapt to specific tubular organ orientation for optimal

As shown in Figure 5, for simplicity, the DH coil is separated into several cells

By superposition, the inductance of the DH coil is equal to the sum of the cell inductance and the mutual inductance between each cell (Figure 5a), namely:

> Li <sup>¼</sup> <sup>X</sup><sup>n</sup> i¼1

Xn j ¼ 1 j 6¼ i

Mi,j þ nLcell: (1)

and each cell consists of an inner loop (Loop 1) and an outer loop (Loop 2).

Mi,j <sup>þ</sup>X<sup>n</sup>

i¼1

where Mi,j denotes the mutual inductance between cell i and cell j, and Lcell

(a) Schematic representation of the DH coil and current directions indicated by arrows. Conductors are printed

on both sides of the flexible PCB; (b) the PCB is wrapped to form a DH coil.

To address these significant problems, we have developed an air-core DH coil for tubular implants [18, 19]. The new coil can be printed on a flat flexible printed circuit board (PCB) and installed on a tubular biostructure during surgery. As shown in Figure 3, when the DH coil is wrapped around a tubular organ parallel to the skin surface, the optimal coupling can be achieved with a PSC integrated with a garment or a bedding sheet.

Figure 2.

The implanted solenoid coil (a) perpendicular and (b) parallel to a planar transmitter (shown in blue).

Figure 3. A DH coil is wrapped around a biological structure (red curve) to serve as a power receiver.
