Note 9 - Common Stock - Summary of Outstanding Warrants (Details) - $ / shares	6 Months Ended
	Jun. 30, 2019	May 22, 2017
Shares Outstanding Under Warrants (in shares)	642,622
September 2014 Issuance Through Private Offering [Member]
Expiration Date	Sep. 23, 2019
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	86,831
Vendors and Contractor Warrants, October 2014 Issuance [Member]
Expiration Date	Oct. 13, 2019
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	29,000
Vendors and Contractor Warrants, November 2014 Issuance [Member]
Expiration Date	Nov. 12, 2019
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	12,500
Employee Performance Bonus Warrants, February 2015 Issuance [Member]
Expiration Date	Feb. 17, 2025
Exercise Price (in dollars per share)	$ 4
Shares Outstanding Under Warrants (in shares)	75,697
Contractor Warrants, March 2015 Issuance [Member]
Expiration Date	Mar. 26, 2025
Exercise Price (in dollars per share)	$ 2.72
Shares Outstanding Under Warrants (in shares)	1,454
Contractor Warrants, May 2015 Issuance [Member]
Expiration Date	May 12, 2025
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	36,229
May 2015 Issuance Second Contractor [Member]
Expiration Date	May 17, 2020
Exercise Price (in dollars per share)	$ 4.24
Shares Outstanding Under Warrants (in shares)	21,585
Performance Bonus Warrants, December 2015 Issuance [Member]
Expiration Date	Dec. 16, 2025
Exercise Price (in dollars per share)	$ 5.60
Shares Outstanding Under Warrants (in shares)	26,875
Distributor Warrants, April 2016 Issuance [Member]
Expiration Date	Apr. 01, 2026
Exercise Price (in dollars per share)	$ 6.08
Shares Outstanding Under Warrants (in shares)	25,000
Contractor Warrants, May 2016 Issuance [Member]
Expiration Date	May 11, 2021
Exercise Price (in dollars per share)	$ 7.74
Shares Outstanding Under Warrants (in shares)	5,000
June 2016 Issuance Related to 2016 Loan Agreement [Member]
Expiration Date	Jun. 20, 2026
Exercise Price (in dollars per share)	$ 4.98
Shares Outstanding Under Warrants (in shares)	100,402
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Expiration Date	May 25, 2027
Exercise Price (in dollars per share)	$ 9.50	$ 9.50
Shares Outstanding Under Warrants (in shares)	222,049	222,049