Intangible Assets
 12 Months Ended
Dec. 31, 2013
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangible Assets
Intangible Assets
Intangible assets consist of the following (in thousands):
 
As of
 
As of
 
December 31, 2013
 
December 31, 2012
Patents issued
$
4,893

 
$
3,932

Patents pending
4,224

 
3,386

Trademarks
40

 
40

Total intangible assets
9,157

 
7,358

Less patent amortization
(1,302
)
 
(687
)
Intangible assets, net
$
7,855

 
$
6,671



Amortization commences upon patent issuance. The useful life of the patents, once approved, will not exceed 20 years, and will depend on the nature of the patent. The average estimated amortization period of the Company's current portfolio is approximately 17 years from the date of patent issuance. The average estimated remaining amortization period of patents acquired as part of the Symyx asset purchase transaction is approximately 4 years.
The following table presents patent amortization expense included in the Consolidated Statement of Operations (in thousands):
 
Years Ended December 31,
 
2013
 
2012
 
2011
Amortization expense
$
615

 
$
588

 
$
81



Estimated future aggregate annual amortization expense for both issued and pending intangible assets is as follows (in thousands):
As of December 31:
 
2014
$
730

2015
699

2016
627

2017
580

2018
500

Thereafter
4,679

Total
$
7,815