cleaning my bookmarks I found this retired Sparkfun product Miniature Solar Cell - CPC1822
somebody wrote that it generates a few uA … now with current node consumption getting under 1 uA that maybe interesting again, maybe in combination with a super capacitor to build a lowcost selfpowered thing.
found this alternative :
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https://www.hackster.io/bobricius/bpw34-solar-supercapacitor-powered-led-watch-1c725b
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https://hackaday.io/project/21342-bpw34-solar-powered-led-watch
Charging:
Charging is very simple because vinatech scap are 3V, just I use 3V
fixed voltage LDO MCP1700T3002ETT with 10R resistor to limit charging
current + shottky diode
for protection reverse current to LDO … charging voltage drop to
acceptable 2.95V
Discharging:
Heart is fixed 3V version of MAX1724 extremely low 1.5µA quiescent
supply current Step-Up DC-DC Converter which can work until 0.4V imput
voltage
I can use about 87% of stored energy. In all operation time I have
stable 3.001V for MCU.
I also have on board space for newest MAX17222 … 300nA Iqc bu I am
unable to solder 6-Pin μDFN
Average consumption is now about 7uA.
MCU:
I am using PICOPOWER Microchip/ATMEL ATMEGA328 standby consumption with
runing RTC is 0.9uA
if it works for a watch… why not for a node 
- instead of the time we need heartbeatrate / temperature / bloodpressure and oxygen level
