When it comes to making implantable medical devices smaller and smarter, advanced circuitry is one challenge. The other is encapsulation. While titanium and titanium alloys have long been the materials of choice for cardiac pacemakers and defibrillators, glass encapsulation is making possible smaller implants that can be used throughout the body. These include neurostimulators for the treatment of epilepsy, implantable drug pumps, cochlear implants, and physiological sensors.

The main reason glass encapsulation can be used in smaller implantables has to do with the cold sealing process. Sealing titanium implants against moisture requires the use of a high-temperature laser welding technique, so the implants must be substantially larger than they would otherwise be in order to minimize the risk of damaging the circuitry during the welding process. That is not the case with cold welding used in glass encapsulation. This technique doesn’t require high temperatures for sealing, thus enabling smaller, less invasive implants.

Glass encapsulation also offers effective transmission of the radio frequency (RF) signals used to send bio data to an external reader or for recharging of an on-board battery. The reason for its effectiveness is that glass encapsulation is largely transparent to radio signals; therefore, much less power is needed to transmit a signal. This allows for smaller antennae and, in turn, smaller implants.

Next generation of smart implants

Extending the life of an implant’s power supply is a constant concern among designers as replacing a defibrillator’s or pacemaker’s battery – which typically lasts five to 15 years – is a surgical procedure.

Some researchers are exploring ways to extend the battery’s life, perhaps by recharging remotely from outside the body, via an external RF link. Others are considering the use of various body energy-harvesting techniques, drawing on sources such as the patient’s heartbeat, blood flow inside the vessels, movement of the body parts, and changes in the body temperature for converting to electrical energy. There’s even been discussion of finding ways to convert the body’s own natural salts and sugars into bio-fuel for powering implants.

The use of glass encapsulation, with its high transparency to RF energy and light energy, will help simplify and advance this research.

CGEs and PGEs

Currently, researchers are investigating two main glass encapsulation technologies for use in smart implants: cylindrical glass encapsulation and planar glass encapsulation. Both offer longer lifespans – one to 10 years longer.

A cylindrical glass encapsulation (CGE) implant typically consists of only one tubular glass component. Simple cylindrical glass encapsulated microchips have long been used in veterinary medicine to serve as passive identifiers (for pets, livestock, and racehorses). These battery-free devices are implanted under the animal’s skin using a large-bore hypodermic needle. Once implanted, the microchip is activated by a low-power RF signal emitted by a scanner, causing the microchip to transmit a unique, preprogrammed identification number.

For humans, typical applications of injectable CGE implants include those for various biological sensing tasks, such as continuous monitoring of glucose in a diabetic patient’s interstitial fluid. With outside diameters from 4 mm to 8 mm, wall thicknesses of just 0.4 to 0.6 mm, and lengths from 12 mm to 50 mm, (CGE) implants (Figure 1) can be small enough to be injected in a patient with a hypodermic needle.

Future biological sensing applications employing electrical feed-throughs (conductors used to connect two sides of a part, such as a circuit board) may include implants designed to monitor a patient’s blood pressure or cholesterol levels. The technology could even be used one day for lab-on-a-chip (LOC) devices that integrate several laboratory functions.

Planar glass encapsulated (PGE) devices (Figure 2) are likely to include dense circuitry and high lead counts (up to 100) and are used in deep-brain stimulation for the treatment of epilepsy or Parkinson’s disease, and the monitoring of intracranial pressure. In the future, CGEs could act as  piezoelectric elements to harvest mechanical energy produced by the patient’s movements as a source of power for the implant.

The PGE assembly process (Figure 3) provides for high feature (leads) density in extremely small implants and high hermeticity. Feed-throughs are implanted in the bottom glass shell of the encapsulation “sandwich.” Next, tungsten vias are gold plated, followed by application of gold contacts to the inner and outer surfaces of the glass. The top glass shell is added and welded to the bottom glass shell after the internal electronics are connected. The last step is the addition of lead interconnections for integration of the implant into the body.

Laser welding is cool

A room-temperature pulsed laser welding process by Valtronic is for sealing PGE implants under special environmental conditions. The temperature inside the implant’s cavity remains at less than 80°C throughout the sealing process, thereby eliminating the potential for heat damage to the encapsulated circuitry. No auxiliary materials such as intermediate layers or adhesives are involved in the encapsulation process, which helps to maintain glass’s high biocompatibility. The process also ensures the implant’s high hermeticity, preserving the integrity of embedded elements by protecting them from moisture.

The process’s ability to ensure high hermeticity – confirmed through extensive tritium leakage tests, steam tests in autoclaves, and direct helium leakage testing – protects patients from the effects of the breakdown of components and material inside the implant. Shear, pressure, tensile vibration, and de-bonding energy tests are used to characterize the mechanical stability of the complete implant package.

Tomorrow’s implants

Researchers at Valtronic’s company headquarters in Switzerland are currently refining the three-line process for planar implant encapsulation involving provision of the cover glass and metallization (inside/outside); inspection of the electronic device; and joining by laser welding while applying innovative design approaches (Figures 4 and 5) to accelerate the development of the next generation of smart implants.

The design-engineering effort involves wrapping the implant in a 600-μm-thick silicone jacket to protect the glass and internal electronics from mechanical stresses. Other components of the design include the two-part planar glass encapsulation, ASIC/MEMS/System-in-Package (SiP) electronic component, wire-bonded/flip-chip interconnections, gold stud bumps, and electrode leads. At just 2.55-mm thick, device implantation throughout the body is now possible.

In other words, there appears to be a really big future ahead for really small implantables. 

About the author . . . Frédéric Mauron is an experienced medical device professional. He joined the logistics department of Valtronic in 1994, where he became Logistics Manager until 2004. He then joined the sales team, gaining experience in active implant development and manufacturing.