A flexible implant could lead to better hearing

Materials World magazine
27 Nov 2019

Making hearing implants soft and more flexible could help people with dysfunctional inner ears to hear again. Idha Valeur reports.

A new softer and malleable hearing implant could change the lives of people with damaged inner ears or dysfunctional auditory nerves.

Researchers from École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, found that by manipulating known materials such as silicone and platinum, they could make an implant that functions better than current auditory brainstem implants (ABI). People with inner ear or auditory nerve problems who use an implanted hearing aid are dependent on the device sending direct electrical signals to the auditory stem in the brain. Current ABIs are quite stiff and do not fully mould to the curvature of the brainstem, whereas a conformable implant will be able to deliver more targeted electrical pulses to the tissue, which results in better hearing.

EPFL has, therefore, worked with Harvard Medical School clinicians and Massachusetts Eye and Ear, USA, to develop a better performing device. ‘This is a surface electrode implant designed to deliver electrical pulses to the very surface of the auditory brainstem,’ EPFL Laboratory for Soft BioElectronic Interface (LSBI) EPFL, Professor Stéphanie Lacour told Materials World. ‘Its function is similar to that of clinical implants but its form-factor and compliance are such that the implant can smoothly conform to the curvilinear surface of the brainstem. Conforming to the very surface of the brainstem provides a better interface for the electrodes to deliver more selectively electrical pulses to the neural tissue.’

What is it made of?

The implant is made of platinum electrodes wrapped in silicone. Lacour explained that silicone is a standard carrier and material used for encapsulation of medical implants. ‘It is an elastomer that can be manufactured in thin membranes, sub-millimetre, and is very compliant. This is a visco-elastic polymer that can reversibly stretch and relax. We can, therefore, design an implant that can conform to the anatomy of the auditory brainstem,’ she said.

‘Platinum (Pt) is also a standard electrode material. We use it for its good charge injection properties, which refers to the efficacy of the electrical stimulation. To ensure the electrodes are also soft, we use particles of Pt mixed with silicone to form a composite that has both mechanical compliance and good electrochemical properties’.

The platinum problem

Although platinum is a common and effective electrode material, it is also rigid, which often causes problems when trying to make a conformable and soft implant for specific purposes.

To overcome this, the researchers found they could manipulate the rigidity using Kirigami – a papercutting technique originating from Japan. ‘We have observed a spontaneous pattern with micro-cracks in a thin gold film deposited on silicone. This microstructure enables the gold film to reversibly stretch,’ Lacour said. ‘We studied this built-in structure and explored how to replicate and control precisely with the position of the micro-cracks to optimise both electrical conductivity and mechanical compliance. The generic pattern allows for the engineering of elasticity in any film material.’

Before human trials

After a successful trial on mice, the team has created an implant in a human size and that works with current surgical techniques. Lacour stressed that several aspects require further adjustment before they are fully ready to progress to human trials, such as the aim to pursue the translation of the neurotechnology. ‘This means scaling and validating the soft implant in larger animal models and tailoring the stimulation patterns with the soft implant to improve the outcomes of the auditory brainstem stimulation. Ultimately, we aim to translate the devices to humans,’ she said.

Further, Lacour said the pathway to human trials is long and that some of the upcoming challenges require that the technology and manufacturing approach meet the regulations for medical devices. They will also need to develop a complete implantable system consisting of ‘implant and stimulator, and we need to demonstrate the efficacy of the new system and ultimately its superior auditory outcome compared to current ABI systems,’ she said.

Lacour believes this technology would be applicable in other parts of the nervous system, as ‘the technology is quite generic and the layout of the surface electrode array can be adjusted to various surfaces of the nervous system – the cortex, the spinal cord and peripheral nerves’.