Note 10 - Options and Warrants to Purchase Common Stock - Summary of the Status and Changes of the Warrants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Outstanding at beginning of year, warrant (in shares)	3,744,623	461,340
Outstanding at beginning of year, warrant, weighted average price (in dollars per share)	$ 0.36	$ 1.15
Issued, warrant (in shares)	18,193,750	3,283,283
Issued, warrant, weighted average price (in dollars per share)	$ 0.07	$ 0.25
Exercised, warrant (in shares)	(3,125,000)
Exercised, warrant, weighted average price (in dollars per share)	$ 0.04
Forfeited, warrant (in shares)
Forfeited, warrant, weighted average price (in dollars per share)
Outstanding at end of year, warrant (in shares)	18,813,373	3,744,623
Outstanding at end of year, warrant, weighted average price (in dollars per share)	$ 0.14	$ 0.36