In the past decade, significant progress has been made in the field of biomaterials, for potential applications in tissue engineering or drug delivery. We have recently developed a new class of thermoplastic elastomers, based on ureidopyrimidinone (UPy) quadruple hydrogen bonding motifs. These supramolecular polymers form nanofiber-like aggregates initially via the dimerization of the UPy units followed by lateral urea-hydrogen bonding. Combined kinetic and thermodynamic studies unravel the pathway complexity in the formation of these polymorphic nanofibers and the subtlety of the polymer's design, while these morphologies are so critically important when these materials are used in combination with cells. We also show that the cell behavio...