December 31

2017

2016

2015

Deferred tax assets:

 Net operating loss carryforwards

$

27,578,200

$

38,299,400

$

33,424,100

 Charitable contributions

300

500

500

 Inventory reserves

205,800

361,300

267,700

 Amortization

22,800

89,500

51,400

 Allowance for doubtful accounts

116,900

102,300

72,300

 Stock-based compensation

519,900

341,400

260,600

Total deferred tax assets

28,443,900

39,194,400

34,076,600

Deferred tax liabilities:

 Depreciation

(80,500)

(83,300)

(62,400)

 Deferred revenue

(6,400)

 -

 -

Net deferred tax assets

28,357,000

39,111,100

34,014,200

Valuation allowance

$

(28,357,000)

$

(39,111,100)

$

(34,014,200)