## 2 Hardware Selection--Commercial off the Shelf or Custom? As we will describe in more detail in Section 3, flexible hardware is inherently less efficient than hardware targeted for a specific application. Considering the broad range of requirements in Figure 2 combined with a strict energy constraint, there is little hope for a single system platform to support the full range of applications. General purpose sensing platforms like the motes of the WSN community are typically too large in size or consume too much power to meet application requirements in the BSN domain. The first implementation choice for a BSN application is whether to use commercial off the shelf (COTS) components or whether to design custom integrated circuits (ICs). We observe that both COTS and custom silicon solutions are optimal in certain parts of the BSN design space. Custom ICs can invariably provide a higher energy efficiency than COTS when they are tailored for a specific application. For this reason, BSN applications with lifetimes exceeding roughly one to two weeks (on the right in Figure 2) will most likely require custom hardware. Size also plays a role. For example, COTS systems can provide longer lifetimes if a larger solution (providing larger batteries) is acceptable for a given application. We illustrate this relationship in Figure 3. Users are likely to remove certain types of BSN sensors on a nightly basis, providing an excellent opportunity to recharge the battery and limiting the required lifetime to less than a day. For systems with short lifetime requirements between re-charges (e.g., a few days or less), COTS systems can provide reasonable size, performance, and lifetime for many BSN applications. These generalizations do not account for the fact that each application also has requirements based upon sensed data rates, communication and real-time requirements, sensor properties, and other factors that influence the boundary between using COTS _versus_ custom in Figure 2 and Figure 3.  
There are many applications for which there may be no clear line of demarcation in the decision process for selecting a COTS or custom ASIC (application specific integrated circuit) design. For example, ECG measurements require no transducer element other than a contact with the skin and may be implemented as a single purpose device with a clear and well defined scope of operation--all characteristics amenable to ASIC solutions. For long-term continuous ECG monitoring, for example to screen for cardiac arrhythmia, very low energy operation is essential, demanding an ASIC solution. However, Holter monitors are an example of a very successful and widely used 24-hour ECG recording system that can easily use COTS and have no need for an ASIC solution. Inertial measurements require the use of a MEMS device as the sensor and are not easily integrated into a  
Figure 3: Hardware design space for COTS _versus_ custom circuits. single chip ASIC design due to the fundamentally different manufacturing processes required for MEMS _versus_ electronics. BSN sensors that measure acceleration may need to incorporate multiple ASICs or a mixture of ASICs and COTS. The final decision between a custom design _versus_ a COTS design must account for the previous points combined with the economics of the intended application. Designing a COTS system is orders of magnitude faster and cheaper than building a custom IC based node, and COTS nodes provide excellent solutions in many lower lifetime BSN scenarios. For example, low volume research platforms or nodes intended for short term clinical monitoring applications may be more economically produced via a COTS design. In such applications, the final device operational characteristics are much less well defined, and engineering costs are ongoing. In this case the economies of reducing such costs via the employment of a flexible platform outweigh the benefits of extra efficiency that an ASIC solution would offer.