Note 5 - Equity - Common Stock Purchase Warrant Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Outstanding, number of warrants (in shares)	3,864,552	748,060
Outstanding, weighted average exercise price (in dollars per share)	$ 2.36	$ 5.17
Issued, number of warrants (in shares)	3,611,504	3,166,492
Issued, weighted average exercise price (in dollars per share)	$ 0.47	$ 1.74
Cancelled or expired, number of warrants (in shares)		(15,385)
Cancelled or expired, weighted average exercise price (in dollars per share)
Exercised, number of warrants (in shares)		(34,615)
Exercised, weighted average exercise price (in dollars per share)
Outstanding, number of warrants (in shares)	7,476,056	3,864,552
Outstanding, weighted average exercise price (in dollars per share)	$ 1.45	$ 2.36