# Parallel DNA Synthesis to Produce Multi-Usage Two-Dimensional Barcodes
## 1 Introduction  
Data storage on DNA is an attractive alternative to storing digital information on conventional physical storage media [1; 2; 3]. The driving forces behind the advent of molecular data storage are better understood from the perspective of the hard limitations of current storage approaches. First, the extreme stability of deoxyribonucleic acids, with a purported t\({}_{2}\) half-life of 30 million years at 25 \({}^{\circ}\)C for a single phosphodiester bond [4], dwarfs that of any manufactured product for the storage of data. Second, reading data on DNA simply equates to sequencing, and any sequencing technology in the past, present or future will read data on DNA, which is in stark contrast to the often obsolete hardware necessary to retrieve data on older storage formats. Finally, the physical density of data on DNA is at the very least seven orders of magnitude greater than that of standard technologies [5; 6]. Current research in the field focuses on increasing writing (synthesis) throughput, decreasing production costs and expanding storage capacity [7]. The production of DNA for the purpose of data storage relies on synthetic DNA oligomers, most commonly obtained using solid-phase phosphoramidite chemistry. DNA phosphoramidites are reactive nucleotide building blocks that are sequentially added onto a growing oligonucleotide strand [8]. Concomitant to being able to produce larger amounts of DNA and thus larger file sizes, the question of accessing only a fraction of the available data arises as it quickly becomes cumbersome to sequence a complete DNA pool [2]. Approaches to alleviate this issue have aimed at designing specific sets of primers that would be able to amplify only the desired subset of the DNA library [9; 10; 11; 12; 13]. In doing so, a fraction of the data can be accessed by knowing its address, i.e., the primer binding region. This method for random access to data assumes that the end-user has been given the keys to subpool amplification, that is,the set of primers necessary to retrieve the requested information. While this information can eventually be collected without any PCR enrichment via highly sensitive, single-molecule, third-generation sequencing methodologies, this defeats the purpose of partial data access. Primer sequence information may be, at the simplest level, available as cleartext to the user. Sensitive information should, on the other hand, ideally be retrievable under controlled conditions and on a need-to-know basis. Data security at the molecular stage has received little attention so far, possibly due to the limited accessibility of nucleic acid sequencers and the high level of required know-how necessary to operate and manipulate sequencing readouts. This situation is inevitably going to change with the development of affordable portable sequencers [9; 14], with sequencing costs continuing to massively decrease and bioinformatics training becoming continuously more widespread. Attempts at providing secure information in DNA format have mostly focused on safeguarding data behind sequence or molecular locks [15; 16; 17] that need to be provided by the intended user. Messages hidden within DNA also constitute a relatively secure form of information transmission; an early example of such steganography approaches applied to biomolecules needed a known PCR primer to retrieve the concealed message [18]. Barcoding is another useful security measure as it provides a means of tracing and tracking the source of the material and therefore renders falsification more difficult. A DNA tag, i.e., the insertion of a known DNA sample within manufactured goods, is a simple way to create a molecular fingerprint that helps in controlling the production chain, makes any counterfeiting process cumbersome and expensive and, simultaneously, can add identity to the product [19; 20; 21; 22].