Note 11 - Equity Incentive Plans - Summary of Stock Option Transactions (Details)	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Shares available for grant, beginning balance (in shares)	200,650
Number of options outstanding, beginning balance (in shares)	865,163
Weighted average exercise price, beginning balance (in dollars per share) | $ / shares	$ 8.13
Shares available for grant, authorized under 2019 Plan (in shares)	1,600,000
Shares available for grant, removed from 2016 Plan (in shares)	(223,389)
Shares available for grant, granted (in shares)	(1,413,590)
Number of options outstanding, granted (in shares)	1,413,590
Weighted average exercise price, granted (in dollars per share) | $ / shares	$ 0.76
Shares available for grant, forfeited (in shares)
Number of options outstanding, forfeited (in shares)	(7,399)
Weighted average exercise price, forfeited (in dollars per share) | $ / shares	$ 5.35
Shares available for grant, canceled (in shares)	22,739
Number of options outstanding, canceled (in shares)	(104,554)
Weighted average exercise price, canceled (in dollars per share) | $ / shares	$ 11
Shares available for grant, ending balance (in shares)	186,410
Number of options outstanding, ending balance (in shares)	2,166,800
Weighted average exercise price, ending balance (in dollars per share) | $ / shares	$ 3.19