Note 6 - Warrants - Warrants Outstanding (Details) - $ / shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Balance, outstanding (in shares)	26,872,430	22,104,817
Balance, outstanding, price range (in dollars per share)	$ 1	$ 1
Issued (in shares)	3,861,041	2,611,513
Issued, price range (in dollars per share)	$ 0.25	$ 0.48
Balance, outstanding (in shares)	30,733,471	24,716,330
Balance, outstanding, price range (in dollars per share)	$ 1	$ 1
Balance, weighted average price per share (in dollars per share)
Minimum [Member]
Balance, outstanding, price range (in dollars per share)	0.25	0.125
Issued, price range (in dollars per share)	0.16	0.25
Balance, outstanding, price range (in dollars per share)	0.20	0.125
Maximum [Member]
Balance, outstanding, price range (in dollars per share)	0.42	0.45
Issued, price range (in dollars per share)	0.24	0.35
Balance, outstanding, price range (in dollars per share)	$ 0.40	$ 0.44