Note 9 - Options and Warrants to Purchase Common Stock - Summary of the Status of the Warrants Outstanding (Details) - $ / shares	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Warrants Outstanding and Exercisable Shares Issuable upon Exercise of Outstanding Warrants (in shares)	24,813,373
Warrants Outstanding and Exercisable Weighted Average Remaining Contractual Life (Year)	5 years 73 days
Warrants Outstanding and Exercisable Weighted Average Exercise Price (in dollars per share)	$ 0.15	$ 0.14
Exercise Price 1 [Member]
Warrants Outstanding and Exercisable Shares Issuable upon Exercise of Outstanding Warrants (in shares)	461,340
Warrants Outstanding and Exercisable Weighted Average Remaining Contractual Life (Year)	2 years 292 days
Warrants Outstanding and Exercisable Weighted Average Exercise Price (in dollars per share)	$ 1.15
Exercise Price 2 [Member]
Warrants Outstanding and Exercisable Shares Issuable upon Exercise of Outstanding Warrants (in shares)	3,283,283
Warrants Outstanding and Exercisable Weighted Average Remaining Contractual Life (Year)	3 years
Warrants Outstanding and Exercisable Weighted Average Exercise Price (in dollars per share)	$ 0.25
Exercise Price 3 [Member]
Warrants Outstanding and Exercisable Shares Issuable upon Exercise of Outstanding Warrants (in shares)	2,068,750
Warrants Outstanding and Exercisable Weighted Average Remaining Contractual Life (Year)	4 years 109 days
Warrants Outstanding and Exercisable Weighted Average Exercise Price (in dollars per share)	$ 0.04
Exercise Price 4 [Member]
Warrants Outstanding and Exercisable Shares Issuable upon Exercise of Outstanding Warrants (in shares)	11,000,000
Warrants Outstanding and Exercisable Weighted Average Remaining Contractual Life (Year)	4 years 182 days
Warrants Outstanding and Exercisable Weighted Average Exercise Price (in dollars per share)	$ 0.10
Exercise Price 5 [Member]
Warrants Outstanding and Exercisable Shares Issuable upon Exercise of Outstanding Warrants (in shares)	8,000,000
Warrants Outstanding and Exercisable Weighted Average Remaining Contractual Life (Year)	3 years
Warrants Outstanding and Exercisable Weighted Average Exercise Price (in dollars per share)	$ 0.15