Note 12 - Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2021

Notes Tables

Share-based Payment Arrangement, Option, Activity [Table Text Block]

							Weighted
	Shares			Number of			Average
	Available			Options			Exercise
	For Grant			Outstanding			Price
Balance at January 1, 2019		200,650			865,163		$	8.13
Authorized under 2019 Plan on Adoption		1,600,000			—		$	—
Removed from 2016 Plan		(223,389	)		—		$	—
Granted		(1,791,030	)		1,791,030		$	0.67
Forfeited		483,849			(592,528	)	$	1.57
Canceled		25,100			(252,013	)	$	9.25
Balance at December 31, 2019		295,180			1,811,652		$	2.74
Authorized under 2019 Plan Amendment		6,400,000			—		$	—
Granted		(5,579,800	)		5,579,800		$	2.66
Exercised		—			(414,676	)	$	1.45
Forfeited		105,910			(105,992	)	$	1.65
Canceled		9,573			(57,751	)	$	10.57
Balance at December 31, 2020		1,230,863			6,813,033		$	2.70
Authorized under 2019 Plan Amendment		8,900,000			—		$	—
Granted		(5,178,438	)		5,178,438		$	7.09
Exercised		—			(1,070,921	)	$	1.63
Forfeited		630,317			(650,054	)	$	3.16
Canceled		—			(54,390	)	$	11.98
Balance at December 31, 2021		5,582,742			10,216,106		$	4.96

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Year Ended December 31,
	2021			2020			2019
Risk-free interest rate		0.91% - 1.35%			0.40% - 0.88%			1.68% - 2.31%
Expected term (in years)		5.44 - 6.08			5.22 - 10.00			5.39 - 10.00
Expected volatility		122% - 131%			94% - 111%			83% - 85%
Dividend yield		—	%		—	%		—	%

Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]

	Year Ended December 31,
	2021		2020		2019
Research and development	$	4,304	$	1,563	$	253
General and administrative		4,563		2,795		374
Total stock-based compensation	$	8,867	$	4,358	$	627