Note 9 - Summary of Stock Options (Tables)

6 Months Ended

Jun. 30, 2018

Notes Tables

Share-based Compensation, Stock Options, Activity [Table Text Block]

	Six Months Ended June 30, 2018
						Weighted
				Weighted		Average
	Number			Average		Remaining		Aggregate
	of			Exercise		Contractual		Intrinsic
	Shares			Price		Term (years)		Value
Options outstanding, beginning of period		2,694,224		$	5.80		8.6	$	249,154
Options granted		2,023,559		$	3.60
Options exercised		-		$	-
Options canceled		(508,428	)	$	5.66
Options outstanding, end of period		4,209,355		$	4.76		8.8	$	433,626
Vested and exercisable and expected to vest, end of period		3,935,007		$	4.80		8.7	$	392,290
Vested and exercisable, end of period		1,124,400		$	5.65		7.5	$	10,167

Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block]

			Options Outstanding						Options Exercisable
							Weighted
			Number		Weighted		Average		Number		Weighted
			Outstanding		Average		Remaining		Exercisable		Average
Range of			as of		Exercise		Contractual		as of		Exercise
Exercise Prices			June 30, 2018		Price		Term (Years)		June 30, 2018		Price
$1.75	-	$1.97		552,500	$	1.94		9.9		12,500	$	1.97
$2.64	-	$2.72		82,500	$	2.67		9.5		9,896	$	2.64
$3.11	-	$3.82		319,376	$	3.63		9.2		55,730	$	3.76
$4.30	-	$4.97		1,477,533	$	4.52		9.0		336,424	$	4.65
$5.01	-	$5.67		822,652	$	5.34		8.7		195,456	$	5.22
$6.00	-	$6.61		567,878	$	6.05		7.6		326,724	$	6.00
$7.00	-	$7.92		348,538	$	7.65		8.2		149,292	$	7.68
	$9.92			38,135	$	9.92		4.4		38,135	$	9.92
$59.60	-	$149.04		243	$	100.46		2.5		243	$	100.46
				4,209,355	$	4.76		8.8		1,124,400	$	5.65

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

	Three Months Ended		Six Months Ended
	June 30,		June 30,
	2018	2017	2018	2017
Expected term (in years)	5	5	5	5
Average volatility	69%	48%	75%	48%
Risk-free interest rate	2.82%	1.76%	2.63%	1.90%
Dividend yield	0%	0%	0%	0%

	Three Months Ended		Six Months Ended
	June 30,		June 30,
	2018	2017	2018	2017
Expected term (in years)	10	10	10	10
Average volatility	91%	55%	91%	55%
Risk-free interest rate	2.85%	2.00%	2.85%	2.29%
Dividend yield	0%	0%	0%	0%

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017
Cost of revenue	$	17	$	4	$	28	$	7
Research and development		65		57		187		108
Selling, general and administrative		694		394		1,314		712
Total	$	776	$	455	$	1,529	$	827

2017 Employee Stock Purchase Plan [Member]

Notes Tables

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

	Three Months Ended	Six Months Ended
	June 30, 2018	June 30, 2018
Expected term (in years)	0.25	0.25
Average volatility	74%	67%
Risk-free interest rate	1.73%	1.58%
Dividend yield	0%	0%