Walt Maclay: If battery technology advanced like semiconductors have, you would have a device the size of a pinhead that could power your car and it’d cost a penny. Unfortunately, we’re never going to have that. If you want to get much, much denser power than we have in a battery now, you could use nuclear. And I don’t think we’re going to see nuclear in a wearable device anytime in the future, that I can predict. So, you’re limited by the storage capacity of chemical energy. And, we’re making progress, every year it gets a little bit better. But it’s not going to get double in next year. Walt Maclay: So that limitation will remain indefinitely. So, you have to work around it. You have to make trade off between how long between charges and the battery size, which determines in many cases, the size of the whole device. The other thing you work on is, how far are you going to transmit? If you’ve got a wireless device, if you transmit to your cellphone in your pocket, it takes a lot less power than if you’re transmitting to a cell tower that’s several miles away. So, the type of transmission will determine the power that’s used. You can also operate something in short bursts. If you’ve got a small amount of data, you don’t have to have the transmitter on all the time. You collect it for a while and then, you turn it on, send a bunch of data and turn it off. Walt Maclay: And then, you can use devices that are lower power. For example, GPS is moderate power. If it’s on all the time, it’s going use quite a bit of battery during the whole day. An accelerometer uses a lot less power. In fact, if you combine an accelerometer with a gyroscope, you can keep track of where you are for a while. Eventually, it gets errors. So you could turn on the GPS every now and then and make a correction but in the meantime, you could use a lower power accelerometer and gyroscope. Audience question: There are watches powered by the movement of your arm. There are watches powered by light. I think they are probably are ways of generating power by heat coming off your body. Are there other ways besides batteries we can look at? Walt Maclay: The question is about energy harvesting from heat and movement and other areas. Energy harvesting generates power in the milliwatts and microwatts. And, most useful devices that are being sold today are in the tens of milliwatts and hundreds of milliwatts. So, energy harvesting isn’t useful for most of the wearable devices. I wish it were and there’s definitely work going on there. And, it depends upon what trade offs you’re willing to make. If you’re willing to have a simple device that’s not very powerful, doesn’t send very small data for short distances. Maybe you could use energy harvesting.