Note 5 - Share-based Compensation - Stock Options (Details) - USD ($)		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020
2018 Equity Incentive Plan [Member]
Options outstanding, balance (in shares)		18,865,525	9,214,356
Weighted average exercise price per share, balance (in dollars per share)		$ 0.19	$ 0.25
Options granted (in shares)		2,483,691	8,610,689
Weighted average exercise price per share, granted (in dollars per share)		$ 0.19	$ 0.19
Options expired (in shares)		0	(200,000)
Weighted average exercise price per share, Expired (in dollars per share)		$ 0	$ 0.26
Options Non-vested (in shares)		(4,218,833)
Weighted average exercise price per share, Non-vested (in dollars per share)		$ 0.23
Options Vested (in shares)		17,130,383
Vested, June 30, 2021 (in dollars per share)		$ 0.18
Vested, aggregate intrinsic value	[1]	$ 250,000
Options outstanding, balance (in shares)		21,349,216	17,625,045
Weighted average exercise price per share, balance (in dollars per share)		$ 0.19	$ 0.20
2018 Equity Incentive Plan [Member] | Minimum [Member]
Exercise price per share, balance (in dollars per share)		0.16	0.16
Exercise price per share, granted (in dollars per share)		0.13	0.17
Exercise price per share, Non-vested (in dollars per share)		0.12
Exercise price per share, Vested (in dollars per share)		0.12
Exercise price per share, Expired (in dollars per share)			0.18
Exercise price per share, balance (in dollars per share)		0.12	0.16
2018 Equity Incentive Plan [Member] | Maximum [Member]
Exercise price per share, balance (in dollars per share)		0.40	0.43
Exercise price per share, granted (in dollars per share)		0.23	0.22
Exercise price per share, Non-vested (in dollars per share)		0.40
Exercise price per share, Vested (in dollars per share)		0.43
Exercise price per share, Expired (in dollars per share)			0.34
Exercise price per share, balance (in dollars per share)		$ 0.43	$ 0.43
The 2007 Equity Incentive Plan [Member]
Options outstanding, balance (in shares)		5,689,363	8,769,451
Weighted average exercise price per share, balance (in dollars per share)		$ 0.44	$ 0.42
Options expired (in shares)		(1,453,855)	(930,000)
Weighted average exercise price per share, Expired (in dollars per share)		$ 0.40	$ 0.56
Options outstanding, balance (in shares)		4,235,508	7,839,451
Weighted average exercise price per share, balance (in dollars per share)		$ 0.45	$ 0.41
Outstanding, aggregate intrinsic value	[1]
The 2007 Equity Incentive Plan [Member] | Minimum [Member]
Exercise price per share, balance (in dollars per share)		0.23	$ 0.23
Exercise price per share, Expired (in dollars per share)		0.39	0.39
Exercise price per share, balance (in dollars per share)		0.23	0.23
The 2007 Equity Incentive Plan [Member] | Maximum [Member]
Exercise price per share, balance (in dollars per share)		0.94	0.94
Exercise price per share, Expired (in dollars per share)		0.51	0.58
Exercise price per share, balance (in dollars per share)		$ 1.65	$ 1.65
Non Plan [Member]
Options outstanding, balance (in shares)		20,749,583	19,604,107
Weighted average exercise price per share, balance (in dollars per share)		$ 0.41	$ 0.41
Options granted (in shares)		43,956	292,437
Exercise price per share, granted (in dollars per share)		$ 0.23
Weighted average exercise price per share, granted (in dollars per share)		$ 0.23	$ 0.18
Options Non-vested (in shares)		(1,996,576)
Weighted average exercise price per share, Non-vested (in dollars per share)		$ 0.45
Options Vested (in shares)		18,796,963
Vested, June 30, 2021 (in dollars per share)		$ 0.41
Vested, aggregate intrinsic value	[1]	$ 28,000
Options outstanding, balance (in shares)		20,793,539	19,896,544
Weighted average exercise price per share, balance (in dollars per share)		$ 0.41	$ 0.41
Non Plan [Member] | Minimum [Member]
Exercise price per share, balance (in dollars per share)		0.17	0.23
Exercise price per share, granted (in dollars per share)			0.17
Exercise price per share, Non-vested (in dollars per share)		0.17
Exercise price per share, Vested (in dollars per share)		0.17
Exercise price per share, balance (in dollars per share)		0.17	0.17
Non Plan [Member] | Maximum [Member]
Exercise price per share, balance (in dollars per share)		1.00	1.00
Exercise price per share, granted (in dollars per share)			0.21
Exercise price per share, Non-vested (in dollars per share)		0.45
Exercise price per share, Vested (in dollars per share)		1.00
Exercise price per share, balance (in dollars per share)		$ 1.00	$ 1.00

[1] Aggregate intrinsic value based on closing common stock price of $0.17 at June 30, 2021