  The rising demand for sustainable IoT has promoted the adoption of
battery-free devices intermittently powered by ambient energy for sensing.
However, the intermittency poses significant challenges in sensing data
collection. Despite recent efforts to enable one-to-one communication, routing
data across multiple intermittently-powered battery-free devices, a crucial
requirement for a sensing system, remains a formidable challenge.
  This paper fills this gap by introducing Swift, which enables seamless data
routing in intermittently-powered battery-free sensing systems. Swift overcomes
the challenges posed by device intermittency and heterogeneous energy
conditions through three major innovative designs. First, Swift incorporates a
reliable node synchronization protocol backed by number theory, ensuring
successful synchronization regardless of energy conditions. Second, Swift
adopts a low-latency message forwarding protocol, allowing continuous message
forwarding without repeated synchronization. Finally, Swift features a simple
yet effective mechanism for routing path construction, enabling nodes to obtain
the optimal path to the sink node with minimum hops. We implement Swift and
perform large-scale experiments representing diverse realworld scenarios. The
results demonstrate that Swift achieves an order of magnitude reduction in
end-to-end message delivery time compared with the state-of-the-art approaches
for intermittentlypowered battery-free sensing systems.
