Although the Voyager spacecraft have proved the long-term reliability of thermoelectric generators (TEGs) during their 40+ year missions, very little progress has been made in TEG technology in recent decades.
Boost converters perform the difficult job of upconverting small thermoelectric output voltages to usable levels. Commercial boost converters are inefficient for use with thermoelectrics, often wasting 70% or more of the power because they are not optimized for such low voltage inputs.
In 2014 when Matrix started the design of the PowerWatch, the smallest wireless wearable prototype from an advanced research lab had a volume of over 100cc, approximately the size of a bar of soap, with no display or user input. In order to reduce that to a size that a consumer would find attractive (less than 40cc)