Electronics Weekly Electronics Design & Components Tech News
Thin umbilical cables to the scull tend to be the answer.
These cables need to be very flexible as the brain is fixed within the skull, but rather ‘floats’ in liquid (cerebrospinal fluid – ‘CSF’) within the skull cavity.
The gap is a few millimeters and the cable needs cross this while allowing for relative movement between brain and skull.
How much better to cross this gap wirelessly – which is what researchers are trying to do.
Revealed at ISSCC, the International Solid-State Ccuits Converence this week was a step in this direction, where research lab Imec is beaming power to a neural implant through the CSF using ultrasound.
Why ultrasound?
Because it transfers power in this environment more efficiently than electromagnetic waves.
Power is at a premium here (see later), causing Imec to use beam-steering even at this short range, rather than broadcasting a wide spread of ultrasound, to allow for brain movement and the vagaries of surgery when the implant is installed.
This work reported in this ISSCC paper is not the first time ultrasound beam steering has been proposed for this application, but the research team has taken power efficiency to the next level by re-using the power that drives its piezoelectric transducer array many times before it is dissipated as heat – so called ‘adiabatic’ driving.
There are 16 piezoelectric elements in a line on the array (it steers in one dimension), and to form a beam these need to be activated in patterns. As piezoelectric transducers are essentially capacitors, this means charging and discharging 16 capacitances with accurately-controlled timing.
At any moment during operation, charge will need to be added to some of the transducers, while from others it will need to be withdrawn.
The trick here is, for that moment, to identify pairs of transducers that need to be at the same state of charge, but are currently equally far from that state of charge, but in opposite directions. If these are briefly shorted together, they will both attain the correct charge state, without drawing energy from the power rails.
Part of Imec’s development has been to create a beam-forming algorithm where these pairs occur very frequently, maximising energy-saving, and that also eliminates the external capacitors that some adiabatic schemes require – space is at a premium.
It is a quantised scheme, using only possible five charge levels, which theoretically allows up to 75% of energy can be recycled, said Imec, whose researchers also picked and ultrasonic frequency of 8MHz and 116μm pitch for the transducers to focus well across the short operating distance.
The 2 x 0.75mm prototype 65nm cmos chip (left) has 64 drivers, connected in fours to produce 16 outputs. The pitch of the drivers is 116 x 116μm to match the pitch of the piezo transducers on the ~5 x 5mm piezo IC (top photo). Steering can be as far as 53° without creating grating lobes.
To save even more energy, the drivers themselves use a similar form of charge-sharing.
Without stacking, the IC and the transducer array together occupy 8 x 5.3mm.
Why the need to save power and size?
This ultrasonic link is intended to be part of a larger scheme, with the ultrasonic power transmitter and all associated electronics installed within the thickness of the skull in a ~6mm hole – its communication to the outside world will be via a second (non-ultrasonic) wireless link though the scalp.
The small bone hole explains the need for miniaturisation, while the strict power limit is due to the need to keep local tissue heating below 1°C.
Imec Netherlands worked with Imec Belgium and Delft University of Technology.
ISSCC 2024 paper 6.2: ‘An ultrasound-powering Tx with a global charge-redistribution adiabatic drive achieving 69% power
reduction and 53° maximum beam steering angle for implantable applications’.
ISSCC
The annual International Solid-State Circuits Conference in San Francisco is the world’s shop window for circuit advances aimed at ICs – they are the state-of-the-art.
