Note 3 - Stockholders' Equity, Stock Options and Warrants - Summary of Status of Options and Warrants Outstanding (Details)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Shares, options (in shares)	3,120,255
Warrant exercises, exercise price per share (in dollars per share) | $ / shares
Shares, warrants (in shares)	2,556,382
Weighted average remaining life, warrants (Year)
Warrant One [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 1
Shares, warrants (in shares)	1,372,828
Weighted average remaining life, warrants (Year)	4 years 131 days
Warrant Two [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 1.07
Shares, warrants (in shares)	697,946
Weighted average remaining life, warrants (Year)	4 years 127 days
Warrant Three [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 2.25
Shares, warrants (in shares)	385,000
Weighted average remaining life, warrants (Year)	3 years 208 days
Warrant Four [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 123.75
Shares, warrants (in shares)	94,084
Weighted average remaining life, warrants (Year)	2 years 62 days
Warrant Five [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 243.75
Shares, warrants (in shares)	2,529
Weighted average remaining life, warrants (Year)	1 year 36 days
Warrant Six [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 309.375
Shares, warrants (in shares)	2,850
Weighted average remaining life, warrants (Year)	1 year 40 days
Warrant Seven [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 309.50
Shares, warrants (in shares)	222
Weighted average remaining life, warrants (Year)	1 year 131 days
Warrant Eight [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 506.25
Shares, warrants (in shares)	59
Weighted average remaining life, warrants (Year)	229 days
Warrant Nine [Member]
Warrant exercises, exercise price per share (in dollars per share) | $ / shares	$ 609.375
Shares, warrants (in shares)	862
Weighted average remaining life, warrants (Year)	219 days
Stock Options One [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 0.91
Shares, options (in shares)	10,000
Weighted average remaining life, options (Year)	9 years 292 days
Stock Options Two [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 0.965
Shares, options (in shares)	3,000
Weighted average remaining life, options (Year)	9 years 321 days
Stock Options Three [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 0.97
Shares, options (in shares)	191,753
Weighted average remaining life, options (Year)	9 years 189 days
Stock Options Four [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 1.01
Shares, options (in shares)	124,358
Weighted average remaining life, options (Year)	9 years 186 days
Stock Options Five [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 1.10
Shares, options (in shares)	22,730
Weighted average remaining life, options (Year)	9 years 277 days
Stock Options Six [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 1.13
Shares, options (in shares)	143,807
Weighted average remaining life, options (Year)	10 years
Stock Options Seven [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 1.35
Shares, options (in shares)	111,112
Weighted average remaining life, options (Year)	9 years 259 days
Stock Options Eight [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 1.454
Shares, options (in shares)	17,200
Weighted average remaining life, options (Year)	9 years 94 days
Stock Options Nine [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 1.47
Shares, options (in shares)	2,373,226
Weighted average remaining life, options (Year)	8 years 361 days
Stock Options Ten [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 2.10
Shares, options (in shares)	14,286
Weighted average remaining life, options (Year)	8 years 277 days
Stock Options Eleven [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 2.25
Shares, options (in shares)	293
Weighted average remaining life, options (Year)	8 years 58 days
Stock Options Twelve [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 2.42
Shares, options (in shares)	20,640
Weighted average remaining life, options (Year)	8 years 51 days
Stock Options Thirteen [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 2.80
Shares, options (in shares)	57,145
Weighted average remaining life, options (Year)	8 years 186 days
Stock Options Fourteen [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 3.75
Shares, options (in shares)	3,998
Weighted average remaining life, options (Year)	8 years 3 days
Stock Options Fifteen [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 4.125
Shares, options (in shares)	3,636
Weighted average remaining life, options (Year)	8 years 94 days
Stock Options Sixteen [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 4.1975
Shares, options (in shares)	7,147
Weighted average remaining life, options (Year)	8 years 80 days
Stock Options Seventeen [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 4.25
Shares, options (in shares)	3,529
Weighted average remaining life, options (Year)	7 years 277 days
Stock Options Eighteen [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 5.125
Shares, options (in shares)	3,902
Weighted average remaining life, options (Year)	8 years 69 days
Stock Options Nineteen [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 65.75
Shares, options (in shares)	190
Weighted average remaining life, options (Year)	7 years 116 days
Stock Options Twenty [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 73.50
Shares, options (in shares)	1,157
Weighted average remaining life, options (Year)	7 years 186 days
Stock Options Twenty One [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 77.50
Shares, options (in shares)	2,323
Weighted average remaining life, options (Year)	7 years 3 days
Stock Options Twenty Two [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 80.25
Shares, options (in shares)	187
Weighted average remaining life, options (Year)	7 years 94 days
Stock Options Twenty Three [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 86.25
Shares, options (in shares)	232
Weighted average remaining life, options (Year)	6 years 277 days
Stock Options Twenty Four [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 131.25
Shares, options (in shares)	81
Weighted average remaining life, options (Year)	4 years 69 days
Stock Options Twenty Five [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 148.125
Shares, options (in shares)	928
Weighted average remaining life, options (Year)	4 years 262 days
Stock Options Twenty Six [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 150
Shares, options (in shares)	1,760
Weighted average remaining life, options (Year)	4 years 47 days
Stock Options Twenty Seven [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 162.50
Shares, options (in shares)	123
Weighted average remaining life, options (Year)	6 years 186 days
Stock Options Twenty Eight [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 206.25
Shares, options (in shares)	121
Weighted average remaining life, options (Year)	6 years 94 days
Stock Options Twenty Nine [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 248.4375
Shares, options (in shares)	121
Weighted average remaining life, options (Year)	5 years 14 days
Stock Options Thirty [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 262.50
Shares, options (in shares)	130
Weighted average remaining life, options (Year)	5 years 14 days
Stock Options Thirty-One [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 281.25
Shares, options (in shares)	529
Weighted average remaining life, options (Year)	4 years 200 days
Stock Options Thirty-Two [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 318.75
Shares, options (in shares)	3
Weighted average remaining life, options (Year)	4 years 313 days
Stock Options Thirty-Three [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 346.875
Shares, options (in shares)	72
Weighted average remaining life, options (Year)	5 years 277 days
Stock Options Thirty-Four [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 431.25
Shares, options (in shares)	306
Weighted average remaining life, options (Year)	5 years 251 days
Stock Options Thirty-Five [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 506.25
Shares, options (in shares)	188
Weighted average remaining life, options (Year)	5 years 186 days
Stock Options Thirty-Six [Member]
Range of exercise prices, options (in dollars per share) | $ / shares	$ 596.25
Shares, options (in shares)	42
Weighted average remaining life, options (Year)	5 years 91 days