## 1 Introduction  
Microfluidics is increasingly influential to not only academia but also industry in diverse fields including biology, chemistry, medicine, foods, energy, environment, etc. because of the advantageously small footprint sizes, high sensitivity, feature diversity, and compartmentalization [1]. In particular, the medical applications of microfluidics are rapidly growing because of two aspects. Feature diversity and compartmentalization allow the fabrication of so-called organ- or body-on-a-chip devices, which recreate organ or bodily functions mimicking appropriate bodily responses on microchips for drug screening [2]. Small footprint sizes allow efficient and complex detection in diagnostics by so-calledmicro total analysis systems (u-TAS) [3]. For all of these purposes, microfluidic devices commonly created in glass or polymer are building blocks, and desired functions are integrated to further functionalize them. Recently, our group developed the ship-in-a-bottle integration technique to integrate functional microcomponents or create biomimetic structures by constructing three-dimensional (3D) polymer micro- and nanocomponents inside closed glass microchannels [4]. In this technique, femtosecond laser direct writing is employed to polymerize microcomponents from polymer solution inside transparent microchannels, based on multi-photon absorption processes, allowing the fabrication of nearly arbitrary and well-resolved 3D structures, as explained later. For the efficient multi-photon absorption processes, ultrashort pulses shorter than several tens of a picosecond is necessary. However, picosecond laser polymerization requires more light exposure to encounter the loss of diffusing radicals between pulses [5]. Thus, femtosecond laser polymerization combines an attractive trade-off between pulse delivery and radical generation. Using this advantage of femtosecond laser polymerization, 3D micro- and nanochannel array scaffolds were recently integrated to study cancer cell migration in biomimetic structures [6]. However, glass microfluidics and the integrated polymers are inherently inert for biological cells. We envision that the 3D integration of biomaterials such as proteins will increase biochip versatility as well as biomimetic environment due to the diversity of native and synthetic proteins available in terms of function and biocompatibility. Protein molecules in aqueous solution can be cross-linked to construct 3D proteinaceous microstructures by the exposure of a focused femtosecond laser along its trajectory in well-resolved and controlled manners [7; 8; 9]. Such microstructures made of protein have been applied for cell culture scaffolds [10; 11], pH-actuators [9; 12; 13] and optical elements [13; 14]. 3D microstructures made of collagen have been shown to promote cell adhesion [10; 11]. pH-change has been shown to cause the expansion or shrinking of microstructures made of bovine serum albumin (BSA) and other proteins [9; 12]. This pH-actuation of BSA was applied to create pH-tunable lenses [13]. Optical waveguides made of BSA are considered environmentally friendly for device disposal [14]. These recent advances hint at enhancing the versatility of microfluidics due to specific protein functions such as cell adhesion, molecule detection, and microstructures designed for pH-actuation. Proteinaceous pad structures integrated into microfluidics were recently reported for the detection and capture of living cells [15]. Because pads were fabricated from a protein solution on a glass surface prior to aligning and bonding a polydimethylsiloxane (PDMS) channel to the glass, this fabrication method is restricted by the PDMS bonding process. The bonding process requires dry conditions and the proteinaceous components only attached to the glass substrate, so it appears to be difficult to fill an entire cross-section of a PDMS channel with them. The required accuracy of alignment and bonding strength also would restrict design choices.