[
    {
        "id": 1,
        "chunk": "# A Universal Strategy for Growing a Tenacious Hydrogel Coating from a Sticky Initiation Layer  \n\nRongnian Xu, Yunlei Zhang, Shuanhong Ma,\\* Zhengfeng Ma, Bo Yu, Meirong Cai, and Feng Zhou\\*  \n\nControllably coating the surfaces of substrates/medical devices with hydrogels exhibits great application potential, but lacks universal techniques. Herein, a new method, namely ultraviolet-triggered surface catalytically initiated radical polymerization (UV-SCIRP) from a sticky initiation layer (SIL) (SIL@UV-SCIRP), is proposed for growing hydrogel coatings. The method involves three key steps: 1) depositing a sticky polydopamine/ $\\mathsf{\\Pi}_{\\mathsf{F e}^{3+}}$ coating on the surface of the substrates-SIL, 2) reducing $\\mathsf{F e}^{3+}$ ions to $\\mathsf{F e}^{2+}$ ions as active catalysts by UV illumination with the assistance of citric acid, and 3) conducting SCIRP in a monomer solution at room temperature for growing hydrogel coatings. In this manner, practically any substrate’s surface (natural or artificial materials) can be modified by hydrogel coatings with controllable thickness and diverse compositions. The hydrogel coatings exhibit good interface bonding with the substrates and enable easy changes in their wett­ability and lubrication performances. Importantly, this novel method facilitates the smooth growth of uniform hydrogel lubrication coatings on the surface of a range of medical devices with complex geometries. Finally, as a proof-of-concept, the slippery balls coated with hydrogel exhibited smooth movement within the catheter and esophagus. Hence, this method can prove to be a pioneering universal modification tool, especially in surface/interface science and engineering.",
        "category": " Introduction"
    },
    {
        "id": 2,
        "chunk": "# 1. Introduction  \n\nDue to good biocompatibility, stimulus responsiveness, and biochemical properties analogous to those of natural tissues, hydrogels, which are essentially water-containing hydrophilic polymers with 3D cross-linked network structures, have evoked a keen research interest in a wide range of areas, including tissue engineering,[1] actuation,[2] and biomedicine.[3] Over the past few decades, hydrogels with the latest and most comprehensive functionalities have been synthesized through various strategies.[4] Nevertheless, a myriad of efforts and attention have been devoted to the ontological design of hydrogels. Meanwhile, the controlled modification of hydrogels as functional coatings on the surfaces of substrates has emerged as both a promising and challenging topic,[5] particularly in the field of electronic and medical devices. In connection therewith, a number of transformative developments have been achieved pertaining to the strategies of coating substrates with hydrogels, including surface bridge method,[6] surface initiation method,[7] hydrogel painting method,[8] and surface catalytically initiated radical polymerization (SCIRP) method.[9,10]  \n\nConsidering the surface bridge method, it first requires to graft coupling molecules with specific functional groups, while the thickness of the hydrogel coating depends upon the amount of monomer solution dropped on the surface, thereby resulting in poor thickness controllability. The hydrogel painting method is based on the silane grafting of the polymer layer before undergoing the curing process. Suo et  al. formulated a family of hydrogel paints in the form of photoinitiatorgrafted polymer chains.[11] Furthermore, a concept of renatured hydrogel painting was proposed to decorate uniform hydrogel coatings on tangible surfaces.[12] The SCIRP method entails the iron catalyst to be doped into the substrates beforehand, while the inherent properties of the substrates would suffer some change.[9] The surface photo/thermal-initiation method proposed by Zhao et  al. is currently a trending research topic that enables the modification of hydrogel coatings on the surfaces of a wide range of medical devices and soft robots.[13,14] Nonetheless, such initiation method is based on long-term $(30{-}90~\\mathrm{\\min})$ UV irradiation or heating along with an accumulated thermal effect, which may induce cross-linking polymerization or viscosity increasement of the entire monomer solution that stays away from the reaction interface; in particular, it is only functional for polymer substrates that can swell in organic solvents containing benzophenone. Analogous to Zhao’s strategy, Gong et al. developed a universal method for decorating hydrogel coatings onto the surface of diverse substrates in two steps: first, forming a thin, physically bound primer layer containing radical initiators, then conducting photo-induced polymerization after applying presolution to the treated surface.[15] Hence, a more general strategy for controllably growing hydrogels on the surface of diverse solid materials regardless of material geometry and category under mild reaction conditions has remained a key challenge in this field. This work revolves around the objective of devising a wider-ranging and universal strategy to modify various substrates with desirable hydrogel layers.  \n\nMussel-inspired polydopamine (PDA) has emerged as a widely popular and preferred functional molecule for coating diverse substrates, including metals, metal oxides, polymers, and ceramics, which can be attributed to its conducive features, such as mild reaction conditions and easy one-step processing.[16] In particular, the profusely available functional catechol groups in PDA provide effective anchoring sites for transition metal ions to form polymer–metal ion complexes,[17] such as PDA- $\\mathrm{Fe}^{3+}$ . Based on two typical scientific mechanisms, $\\mathrm{Fe}^{2+}$ is capable of catalytically decomposing persulfate to generate free radicals that induce monomer polymerization at low temperatures[18] and $\\mathrm{Fe}^{3+}$ ions can be reduced to $\\mathrm{Fe}^{2+}$ ions when facilitated by citrate acid under the illumination of UV light,[19] an innovative method called ultraviolet-triggered surface catalytically initiated radical polymerization (UV-SCIRP) from a sticky initiation layer (SIL) (SIL $@$ UV-SCIRP) is proposed to grow hydrogel coatings universally on various substrate surfaces, which can efficiently compete with the abovementioned methods.",
        "category": " Introduction"
    },
    {
        "id": 3,
        "chunk": "# 2. Results and Discussion  \n\nDepositing the PDA adhesive coating on the surface of the substrates is a key step in this method. Commonly, the PDA coating contains adhesive catechol groups and reactive sites, helping it bond with substrates through numerous molecular interactions.[20] Citric acid (CA) is a compound with three carboxyl groups. The pH level of the mixed solution of $\\mathrm{FeCl}_{3}{\\cdot}6\\mathrm{H}_{2}\\mathrm{O}$ and CA is 4.0, and only one carboxyl group of CA experienced deprotonation in this circumstance, so the deprotonated carboxyl will coordinate with $\\mathrm{Fe}^{3+}$ ions to form an $\\mathrm{Fe}^{3+}.$ –CA complex. Then, the substrate deposited with PDA can partly coordinate with $\\mathrm{Fe}^{3+}$ ions (competitively associated with citric acid) to form a $\\mathrm{PDA}/\\mathrm{CA}{-}\\mathrm{Fe}^{3+}$ composite layer (Figure 1a). The conceptual mechanism of the method is shown in Figure  1b. The $\\mathrm{Fe}^{3+}$ ions within $\\mathrm{PDA}/\\mathrm{CA}{-}\\mathrm{Fe}^{3+}$ can then be reduced to $\\mathrm{Fe}^{2+}$ ions using UV illumination with the assistance of CA,[19,21] generating an active sticky catalyst initiation layer (SIL). Subsequently, the solid–liquid interface redox reaction is performed between $\\mathrm{Fe}^{2+}$ and $\\mathrm{S}_{2}\\mathrm{O}_{8}{}^{2-}$ ions in monomer solution to generate radical anion ${\\mathrm{SO}}_{4}^{-}{\\cdot}$ , along with the dramatic reduction of the decomposition activation energy,[18,22] to initiate monomer polymerization at room temperature. Intuitively, the poly(acrylic acid)–poly(acrylamide) (PAA-PAM) hydrogel coating can be successfully grown on the surface of the Ti substrate by the SIL $@$ UV-SCIRP method, according to the fluorescence optical photograph (Figure 1c).  \n\nThe detailed preparation process of the proposed SIL@UVSCIRP method is demonstrated in Figure  1d. Specifically, the substrates are immersed first into the trimethylaminomethane (Tris) solution to deposit the adhesive PDA coating and were subsequently subjected to the treatment in aqueous solution containing $\\mathrm{FeCl}_{3}{\\cdot}6\\mathrm{H}_{2}\\mathrm{O}$ and CA for $^3\\mathrm{~h~}$ to form a PDA/ ${\\mathrm{CA-Fe}}^{3+}$ composite layer (Figure  1d-i), in which $\\mathrm{Fe}^{3+}$ forms a competitive cross-linking coordination with PDA and CA. Next, the PDA/CA– $\\mathrm{\\cdot}\\mathrm{Fe}^{3+}$ -decorated substrates are irradiated by UV light to reduce $\\mathrm{Fe}^{3+}$ to $\\mathrm{Fe}^{2+}$ , which acts as an active surface catalyst (Figure  1d-ii). The $\\mathrm{Fe}^{2+}$ catalyst-loaded substrates are then immersed into the monomer solution to perform interface-dominated radical polymerization for in situ growth of hydrogel coatings on the surfaces of substrates at room temperature (Figure  1d-iii,iv). Among them, it is anticipated that the grown hydrogel network combines preferentially with a sticky initiation layer (SIL) by noncovalent interactions, such as ion coordination $(\\mathrm{COO^{-}{-}F e^{3+}{-}C O O^{-}})$ and hydrogen bonding $(\\mathrm{COOH-N-H})$ . At the same time, the radical anion $\\mathrm{SO}_{4}{}^{-\\cdot}$ may extract reactive hydrogen from the amine group $\\left(\\mathrm{-NH-}\\right)$ of PDA, resulting in the generation of an RN• radical to realize covalent coupling with the hydrogel network (Figure S1, Supporting Information).[23] The hydrogel coating is denoted as $\\mathrm{H}_{\\mathrm{m}}$ (H represents the hydrogel, and m represents the composition of the hydrogel). The exceptional advantages of the current method can be outlined as follows: the modification appears suitable for most of the surfaces, the entire polymerization process is conducted at room temperature, the monomer solution needs not be degassed, the coating process can be completed within a few minutes, the thickness of the hydrogel coating is highly controllable, and the monomer solution reacted upon is reusable up to several times (Figure 1e).  \n\nThe substrates of hydrophilic titanium $(\\mathrm{Ti})$ and hydrophobic polyethylene (PE) were employed to investigate the growth kinetics of the $\\mathrm{H_{PAA-PAM}}$ coating under numerous conditions. The thickness of the $\\mathrm{H_{PAA-PAM}}$ coating on the surfaces of Ti and PE substrates is highly dependent on the PDA deposition time and the coating growth time. As shown in Figure 2a, the thickness of the $\\mathrm{H_{PAA-PAM}}$ coating on $\\mathrm{\\DeltaTi}$ gradually increases from 76.4 to $385.1\\upmu\\mathrm{m}$ , while it progresses from 65.5 to $285.1\\upmu\\mathrm{m}$ on PE, along with extending the PDA deposition time from 12 to $72\\mathrm{~h~}$ . The most likely scenario is that a longer deposition time enables an ever-increasing number of PDA molecules on the substrate, facilitating coordination with a greater number of $\\mathrm{Fe}^{3+}$ ions to produce a high number of $\\mathrm{Fe}^{2+}$ catalysts after UV irradiation. Figure  2b demonstrates that extending the growth time from $30~\\mathrm{s}$ to 10 min in the monomer solution also tends to induce an evident increase in the thickness of the HPAA-PAM coating on Ti (from 48.1  to $303.5~{\\upmu\\mathrm{m}}_{,}^{}$ and PE (from 48.1  to $278.1~{\\upmu\\mathrm{m}})$ substrates. The inserted cross-sectional micro­scopy images (Figure  2a,b) intuitively demonstrate the growth process of the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating (colored layers). Meanwhile, the chemical component of the grown $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating can be confirmed using Fourier transform infrared spectroscopy (Figure  S2, Supporting Information). Taking the Ti substrate as an example, the peaks at 3335  and $3186~\\mathrm{cm}^{-1}$ are assigned to the acylamino group $(\\mathrm{NH}_{2}\\mathrm{-}\\mathrm{C=}\\mathrm{O-}$ ), while 2930  and $1651~\\mathrm{cm}^{-1}$ are the characteristic peaks of alkyl $(\\mathrm{-CH}_{3},\\mathrm{-CH}_{2}\\mathrm{-})$ and carbonyl $(-\\mathrm{COOH})$ groups, signifying the successful modification of the $\\mathrm{H_{PAA-PAM}}$ coating. Significantly, the $\\mathrm{SIL}@$ UV-SCIRP method is suitable for a series of monomers, and hydrogel coatings with diverse components can be successfully modified on the Ti substrates, including poly(vinyl alcohol)– poly(2-hydroxyethyl methacrylate), poly(ethylene glycol) methyl ether methacrylate–poly(acrylic acid), PAA-PAM, poly(ethylene glycol) methyl ether methacrylate–poly(acrylic acid)–poly(2- hydroxyethyl methacrylate)), poly(vinyl alcohol)–poly(acrylic acid)–poly(acrylamide), and poly(2-hydroxyethyl methacrylate)– sodium alginate–calcium chloride (Figure  2c). However, even more importantly, we found that the monomer solution is reusable up to several times for growing the hydrogel coating on the PDA/CA–Fe3+-decorated substrate, which can be regarded as a great improvement compared to UV-initiated technology.[13] As illustrated in Figure S3a, Supporting Information, the growth thickness of the $\\mathrm{H_{PAA-PAM}}$ coating on the Ti substrate amounts to ${\\approx}150~\\ensuremath{\\upmu\\mathrm{m}}$ every time. Finally, the bottom monomer solution polymerized due to the accumulative diffusion of the generated $\\mathrm{Fe}^{2+}$ ion catalyst at the interface, while the top phase remained unpolymerized after growing ${\\approx}15$ times (Figure S3b, Supporting Information).  \n\n![](images/ed80c78c27956f11ddc34630f67571dad70c7f0c4870cc713add936936198d82.jpg)  \nFigure 1.  Mechanism of SIL@UV-SCIRP method. a) Schematic diagram showing anchoring of the adhesive PDA/CA–Fe3+ coating on the substrate surface through certain interactions. b) The interface polymerization mechanism involved in the SIL $@$ UV-SCIRP method to form a uniform hydrogel coating on the substrate surface. c) Fluorescence optical photograph of Ti substrate half-grown with $H_{P A A-P A M}$ coating containing rhodamine 6G. The universal substrates demonstrated in the paper are Ti, alumina (Al), gold (Au), polyethylene (PE), polyurethane (PU), poly(tetrafluoroethylene) (PTFE), poly(ethylene terephthalate) (PET), polypropylene (PP), nylon, polyimide (PI), Si, glass, ceramic, pine, and hair surfaces. d) Schematic diagram illustrating the detailed preparation process of the SIL $@$ UV-SCIRP method. (i) Formation of PDA/CA– ${\\cdot}\\mathsf{F e}^{3+}$ coating layer on the substrate surface; (ii) in situ generation of $\\mathsf{F e}^{2+}$ ions catalyst with UV irradiation with the assistance of CA; (iii) immersing the catalyst-loaded substrate into the monomer solution for achieving interface radical polymerization; (iv) formation of uniform hydrogel coating (orange network) on the substrate surface. e) Exceptional benefits of the current SIL $@$ UV-SCIRP method for growing hydrogel coatings.  \n\nOwing to the assembly advantage of PDA coatings on diverse substrates,[16] hydrogel coatings can be successfully grown on nearly all substrates (natural or artificial), including metals, polymers, inorganics, and organisms, by the SIL $@$ UV-SCIRP method. As shown in Figure 2d, the thickness of the HPAA-PAM coating on Ti, alumina (Al), gold (Au), PE, polyurethane (PU), poly(tetrafluoroethylene) (PTFE), poly(ethylene terephthalate) (PET), polypropylene (PP), nylon, polyimide (PI), Si, glass, ceramic, pine, and hair surfaces was found to be $\\approx151.9$ , 150.6, 181.9, 122.9, 128.7, 108.4, 145.1, 150.0, 146.8, 142.9, 150.3, 143.2, 155.5, 135.3, and $43.0~{\\upmu\\mathrm{m}}$ , respectively (deposition time: $24\\mathrm{~h~}$ , growth time: $2\\ \\mathrm{min}$ ). The presence of the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating enables an easy change of the wettability of the substrates. The contact angles for bare Ti, Al, Au, PE, PU, PTFE, PET, PP, nylon, PI, Si, and glass substrates are ${\\approx}51.7{^{\\circ}}$ , $89.4^{\\circ}$ , $54.1^{\\circ}$ , $99.0^{\\circ}$ , $105.4^{\\circ}$ , $103.5^{\\circ}$ , $55.9^{\\circ}$ , $86.4^{\\circ}$ , $51.5^{\\circ}$ , $64.1^{\\circ}$ , $82.5^{\\circ}$ , $19.9^{\\circ}$ , and $82.8^{\\circ}$ (Figure S4, Supporting Information), respectively. As a comparison, all of them suffer a reduction and fall below $15^{\\circ}$ after being modified by the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating. Intuitively, uniform coating on substrates with diverse geometric shapes/sizes can be observed and verified clearly through fluorescence imaging after dyeing the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating with rhodamine 6G on the outer surface of bone/pine/hair (Figure 2e).  \n\n![](images/0410d5d0e3ad1ffb7f61f492cf4f3b9f6e4299b20a96c902c6f4ab0888a75452.jpg)  \nFigure 2.  Controllable modification of hydrogel coating on various substrates by the SIL $@$ UV-SCIRP method. a,b) The thickness change of the $\\mathsf{H}_{\\mathsf{P A A-P A M}}$ coating on the Ti/PE substrate surface with PDA deposition time and polymerization growth time (the inserted pictures are the crosssectional optical microscopy images of Ti/PE substrates and hydrogel coating layer). c) The thickness of hydrogel coating with different components on the Ti substrate (deposition time: 24 h). d) The thickness of the $H_{P A A-P A M}$ coating and contact angle of various substrates with the $H_{P A A-P A M}$ coating (deposition time: $24\\ h$ , growth time: 2  min). e) Optical images (left) of various substrates and fluorescence images (right) of various substrates (e1: bone, $\\boldsymbol{\\mathrm{e}}_{2}$ : pine.) after modifying the HPAA-PAM coating with rhodamine 6G $(\\mathsf{I}\\mathsf{m g}\\mathsf{m}\\mathsf{L}^{-1})$ , and $\\left(\\mathsf{e}_{3}\\right)$ optical (left) and fluorescence microscopy images (right) of hair after growing the $H_{P A A\\cdot P A M}$ coating with rhodamine 6G $(1\\mathrm{mg}\\mathrm{m}\\mathrm{L}^{-1}.$ ) (deposition time: $24\\mathsf{h}$ , growth time: $2\\mathsf{m i n}$ ).  \n\nThe interface combination force between the substrate and the hydrogel coating was analyzed by administering a $90^{\\circ}$ peeling test with PET as stiff backing (Figure 3a). Figure  3b portrays three typical snapshots from the peeling test process, where the partial HPAA-PAM coating still remains on the Ti substrate after testing (yellow dotted box), signifying a strong bonding force between the grown hydrogel and the Ti substrate. More intuitively, the PE substrate decorated with hydrogel coating after the $90^{\\circ}$ peeling test was dyed with rhodamine B $(1\\ \\mathrm{mg\\mL^{-1}})$ to obtain the corresponding fluorescence images. As shown in Figure S5, Supporting Information, there is apparent residual on the PE substrate surface. By and large, the peeling force per unit width of the HPAA-PAM coating for the Ti substrate is found to be ${\\approx}70\\ \\mathrm{~N~m^{-1}}$ , while it is approximately $35~\\mathrm{N~m^{-1}}$ for the PE substrate (Figure 3c). Attributable to the presence of the adhesive PDA layer, the bonding strength of the HPAA-PAM coating is considerably better than that of the previously reported method.[9] Nevertheless, due to the weak mechanical strength of the classic PAA-PAM hydrogel network (elastic modulus $\\approx~39~\\mathrm{\\kPa})$  \n\n![](images/c2c8c763c2beaad45abde62651990da9619fe08a8dc8f96f0ad0f4d9adff876f.jpg)  \nFigure 3.  a) Schematic illustration of the $90^{\\circ}$ peeling test for analyzing the interface combination strength between the substrate and the hydrogel coating, and stiff backing (PET) is introduced to prevent elongation of the HPAA-PAM coating along the peeling direction. b) Snapshots of the $90^{\\circ}$ peeling test on Ti substrate with the $H_{P A A-P A M}$ coating and residual hydrogel (yellow dotted box). The scale bar is 2 cm. c) The $90^{\\circ}$ peeling force per width versus displacement curve of the $\\mathsf{H}_{\\mathsf{P A A-P A M}}$ coating on $\\bar{\\mathsf{T i}}$ and PE substrates. d) Cross-sectional SEM morphology exhibiting the interface between $\\bar{\\mathsf{T i}}$ substrate and $H_{P A A\\cdot P A M}$ coating, and e) corresponding chemical component analysis obtained by EDS mapping, carbon (C, red), oxygen (O, yellow), nitrogen (N, turquoise), and titanium (Ti, blue). f) The friction coefficient curves of Ti and PE substrates before/after growing the $\\mathsf{H}_{\\mathsf{P A A-P A M}}$ coating. g) The friction coefficients of various substrates before and after growing the $\\mathsf{H}_{\\mathsf{P A A-P A M}}$ coating. h) The variation in friction coefficient of Ti substrate upon circularly growing HPAA-PAM coating by SIL $@$ UV-SCIRP method. In all cases, the PDA deposition time is $24\\mathsf{h}$ , and the polymerization growth time is $2\\min$ .  \n\n(Figure S6, Supporting Information) and slim thickness of the hydrogel coating, no valuable network dissipation is achieved to obtain higher interface strength. However, due to the universality of the SIL $@$ UV-SCIRP method for different kinds of monomer solutions, it would still be suitable for obtaining tough hydrogel coatings.  \n\nThe good combination between the hydrogel coating and substrates is attributable to the possible formation of a covalent couple between the SIL and hydrogel coating.[23] Subsequently, the cross-sectional scanning electron microscopy morphology (Figure  3d) and energy dispersive spectroscopy mapping (Figure 3e) further validate the good combination of the HPAA-PAM coating and the Ti/PE substrates (Figure S7, Supporting Information). The resulting HPAA-PAM coating effectuates an evident reduction in the surface friction force of the substrates, which could be characterized by the friction coefficients measured from a ball-on-disk reciprocating tribometer with water as a lubricant. The friction coefficients decrease significantly from 1.287  to 0.075 $_{(\\approx17}$ -fold) for the Ti substrate and 1.822  to 0.040 $(\\approx43$ -fold) for the PE substrate over the entire 300  cycles (Figure  3f). Moreover, to prove the durability and lifetime of the $\\mathrm{H_{PAA-PAM}}$ coating, the reciprocating tests were extended up to 7200  cycles (Figure S8, Supporting Information). The friction coefficient of the PE substrate with the $\\mathrm{H_{PAA-PAM}}$ coating was stable at 0.032  during the entire 7200 cycles, demonstrating the good robustness of the hydrogel coating. Figure $3\\mathrm{g}$ demonstrates that all the bare substrates, including Ti, Al, Au, PE, and nylon, exhibit high friction coefficients in water $(>1.25)$ , while their surfaces reveal a very low friction coefficient $_{(<0.08)}$ after growing the HPAA-PAM coating, clearly highlighting the versatility of the lubrication enhancement brought by the SIL $@$ UV-SCIRP method. More importantly, the proposed method can be used to achieve constant lubrication modification for the substrates continuously, and even more so, without damaging the inherent surface feature of the substrate itself (Figure 3h).  \n\nThe patterned coating of hydrogels with controllable shape and size is also feasible through our SIL $@$ UV-SCIRP method, assisted by the UV grayscale exposure technique.[24] Not only regular patterns such as round (left) and grid (right) of the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coatings can be easily constructed (Figure 4a), but also sophisticated patterns such as butterfly can be successfully generated on the substrate surface (Figure  4b). Intriguingly, the temperature-responsive HPNIPAM-PAA coating pattern was constructed on the Ti substrate satisfactorily to accomplish the challenge of the “Sichuan opera face” (Figure  4c). In a $20~^{\\circ}\\mathrm{C}$ water bath, the “face” looks blurry due to the network swelling of the hydrogel coating, while it becomes clear and white in a $40~^{\\circ}\\mathrm{C}$ water bath. Furthermore, controllable hydrogel coating could be performed on porous hydrophobic substrates to prepare functional membrane materials. As shown in Figure  4d, the porous PE substrate with nine channels tends to be hydrophobic in air, and water drops tend to suspend on its surface, while it changes to be hydrophilic after growing the $\\mathrm{H_{PAA-PAM}}$ coating to allow the water to easily drop down from the channels. Such an evident wettability transition can also be realized from a hydrophobic state to a hydrophilic state for the PTFE filter membrane after growing the $\\mathrm{H_{PAA-PAM}}$ coating (Figure 4e), inferring its potential applicability in the field of liquid separation. Furthermore, the SIL $@$ UV-SCIRP method is highly conducive to developing hydrogel-based composite materials. As shown in Figure  4f, the hydrogel fiber composite material can be prepared easily by growing the HPAA-PAM coating in situ on nonwoven fabrics and comes with a significant enhancement in tensile strength (from ${\\approx}2.65$ to $8.80~\\mathrm{\\MPa}$ ) as well as elastic modulus (from 4.58  to 19.13  MPa) (Figure S9a,b, Supporting Information).  \n\n![](images/d20be7dabdd6e2fca5f89abf6620a4e6b93e4aa021281d4c2104bca1c773ac26.jpg)  \nFigure 4.  The patterned coating of hydrogels on a flat substrate and uniform coating of hydrogels on structured substrates. a) Fluorescence images of the substrate after growing regular patterned $H_{P A A-P A M}$ coating with rhodamine 6G $(\\bar{1}\\mathsf{m g}\\mathsf{m}\\mathsf{L}^{-\\bar{1}})$ round (left), and grid (right). b) Schematic illustration of the butterfly model (left) and (right) fluorescence photograph of the Ti substrate after growing the patterned $H_{P A A\\cdot P A M}$ coating with rhodamine 6G $(\\mathsf{l}\\mathsf{m}\\mathsf{g}\\mathsf{m}\\mathsf{L}^{-\\mathsf{l}})$ . c) The dynamic color change of the intricate HPNIPAM-PAA coating patter te to realize the art of a “Sichuan opera face” in a 20 and $40~^{\\circ}\\mathsf{C}$ water bath. d) Photographs showing the transition from the hydrophobic (left) to hydrophilic (right) state of the PE substrate with nine channels before/after growing the $H_{P A A-P A M}$ coating. e) Optical and fluoresce es of the PTFE filter ne before/after growing the HPAA-PAM coating with rhodamine 6G $(7~\\mathrm{mg}~\\mathrm{mL^{-1}})$ . f) Preparation of fiber-reinforced PAA-PAM composite hydrogel materials by applying the SIL $@$ UV-SCIRP method. In all cases, the PDA deposition time is $24\\ h$ , and the polymerization growth time is $30~\\mathsf{s}$ .  \n\nAlthough the modification of hydrogel coating on medical device surfaces is deemed as an effective approach to change their inherent lubrication property,[13] it is highly demanding to simultaneously fulfill two key requirements of applicability for substrates with different components and precisely controllable thickness of the coating. The $\\mathrm{SIL}@$ UV-SCIRP method enables the growth of $\\mathrm{H_{PAA-PAM}}$ coating with a controllable thickness on diverse medical devices possessing various shapes and sizes, including hollow throat (a), suction head (b), biliary stent (c), drainage tube (d), polyvinyl chloride (PVC) catheter (e), stomach tube (f), latex catheter (g), and titanium alloy bulb of artificial hip joint (h) (Figure 5, left). Noticeably, the uniformity of the $\\mathrm{H_{PAA-PAM}}$ coating can be observed under fluorescence imaging after being dyed with rhodamine 6G (Figure  5a–h, right). Meanwhile, the cross-sectional fluorescence microscopy images of the modified PVC catheter (Figure S10a, Supporting Information) and latex catheter (Figure S10b, Supporting Information) show that the thickness of the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating is 84.6 and $53.0\\upmu\\mathrm{m}.$ , respectively. Additionally, water was used as a lubricant to analyze the lubrication performance of the two kinds of hydrogel-modified catheters. The average friction coefficients of the bare PVC catheter (Figure 5i) and latex catheter (Figure 5j) without hydrogel coating is 3.504 and 1.879, respectively. In contrast, the average friction coefficients of the PVC catheter and latex catheter decrease to ${\\approx}0.0437$ and $0.0467$ after modifying the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating, validating their remarkable lubrication capability. Predictably, our SIL $@$ UV-SCIRP method provides the capability to modify the hydrophilic hydrogel coating on the surfaces of various medical devices to reduce the interface friction force during the process of implantation/intervention.  \n\nFinally, to validate the potential applicability of the SIL $@$ UV-SCIRP method in implanted/interventional processes under a 3D cavity tissue environment, hydrogel-coated spherical solids with slippery surface features were employed to conduct the demonstration experiments. As shown in Figure 6a and Movie S1, Supporting Information, the alumina ball (diameter: $2.5~\\mathrm{\\mm})$ with lubricated HPAA-PAM coating can smoothly migrate along the water flow within the cavity of the “S” PVC catheter tube under the $50~\\mathrm{rpm}$ flow rate (Figure S11, Supporting Information). In contrast, the bare alumina ball without hydrogel coating gets stuck in the corner due to the high friction force at the contact interface. Correspondingly, Figure 6b clearly illustrates the motion path curves of the bare alumina ball and lubricated alumina ball. The bare alumina ball can migrate for only ${\\approx}4.5~\\mathrm{cm}$ during the first $4\\mathrm{~s~}$ , but then it remains stagnant for the following $4{-}10\\mathrm{~s~}$ . Conversely, the alumina ball with the $\\mathrm{H}_{\\mathrm{PAA-PAM}}$ coating can move easily along the $\\ensuremath{^\\circ}\\mathrm{S}^{\\prime\\prime}$ track for ${\\approx}27$ cm within 10 s (Figure 6b). To quantitatively examine the effect of lubrication on the movement of balls, the interface friction forces between the balls and the inner surface of the channels were measured by pulling the constrained balls from one end of the catheter tube to the other end. As shown in Figure 6c, the friction force between the bare alumina ball and the channel could reach to $0.2~\\mathrm{N}$ , while it is only ${\\approx}0.03\\ \\mathrm{N}$ after decorating with the $\\mathrm{H_{PAA-PAM}}$ coating. To simulate the shearing process more meticulously in implanted/interventional scenes, natural cattle esophagus, regarded as a typical tissue transportation scenario,[25] and a large steel ball (diameter: $5~\\mathrm{cm}$ was employed to inspect the friction force at the interface (Figure S12, Supporting Information). A schematic illustration of the cattle esophagus and real optical images of the sample are shown in Figure  6d. The friction force between the bare steel ball and the internal surface of the cattle esophagus is found to be ${\\approx}6\\ \\mathrm{N}$ , while it is only ${\\approx}1\\mathrm{\\DeltaN}$ for the modified ball (Figure 6e). These results mentioned above are a strong inference for the potential applicability of the SIL $@$ UV-SCIRP method for the lubrication modification of biomedical equipment.",
        "category": " Results and discussion"
    },
    {
        "id": 4,
        "chunk": "# 3. Conclusion  \n\nAn innovative modification method called SIL@UV-SCIRP is proposed to grow hydrogel coatings with good interface combination for universal substrate surfaces, including metals, polymers, inorganics, and organisms. Compared to the reported techniques, the remarkable benefits of the current method are obvious: it can theoretically be applied on any substrate, the reaction is performed mildly at room temperature, the monomer solution need not be degassed, the coating can be completed effectively within a few minutes, the thickness of the hydrogel layer is highly controllable either through the PDA deposition time or via the monomer polymerization time, and the reacted monomer solution is reusable up to several times. The hydrogel coatings exhibit decent interface combination strength with substrates and is highly feasible for varying the inherent wetting and lubrication performances of the relevant substrate surfaces. Numerous regular and intricate patterned hydrogel coatings can be generated readily by fixed-site reduction of $\\mathrm{Fe}^{2+}$ catalysts with the assistance of the UV grayscale exposure technique. Significantly, this method is a universal tool to modify hydrogel coatings on the outer surface of diverse medical devices, such as hollow throat, suction head, biliary stent, drainage tube, PVC catheter, stomach tube, latex catheter, and titanium alloy bulb of an artificial hip joint. Finally, to prove the potential applicability of this method in implanted/interventional processes, lubricated spherical solids with hydrogel coatings were employed to simulate the directional migration behavior of specific devices within 3D constrained channels. Both alumina and steel balls with hydrophilic hydrogel lubrication coatings exhibit a smooth movement ability within the channel of the artificial PVC catheter tube or even natural cattle esophagus. We believe that this innovative method could become a universal modification tool in surface/interface science and engineering.  \n\n![](images/fc9d65a56816b23d782da393414a10c1695c03f3b75804b22122b9f188bb02cc.jpg)  \nFigure 5.  Growing hydrogel coatings on the surface of various medical devices. Optical photos of medical devices (left) and corresponding fluorescence photos after partly coating $H_{P A A\\cdot P A M}$ with rhodamine 6G $(\\mathsf{I}\\mathsf{m g}\\mathsf{m}\\mathsf{L}^{-1})$ (PDA deposition time: $24\\mathsf{h}$ , polymerization growth time: 30 s). a) Hollow throat; b) Suction head; c) biliary stent; d) drainage tube; e) PVC catheter; f) stomach tube; g) latex catheter; h) titanium alloy bulb of artificial hip joint. The scale bar is $2c m$ . The friction coefficient curves of bare catheters and catheters modified with $\\mathsf{H}_{\\mathsf{P A A-P A M}}$ coating (load: $0.2~\\mathsf{N}$ , sliding frequency: $1H z$ , lubricant: $H_{2}O)$ : i) PVC catheter, j) latex catheter.  \n\n![](images/4a9ce161245df7921f9e75fd50d8fef480a15dc1114502ae9d6923689ea859e4.jpg)  \nFigure 6.  Schematic illustrations of the conceptual demonstration for the potential application of hydrogel coatings. a) Snapshots of demonstrating the movement state of the controlled alumina ball (above) and modified-alumina ball with $H_{P A A-P A M}$ coating (below) within the ${}^{u}{\\sf S}^{\\prime\\prime}$ PVC catheter tube under a 50-rpm flow rate (PDA deposition ti $24\\mathsf{h}$ rowth time: $30~\\mathsf{s}$ , diameter of alumina ball: $2.5~\\mathsf{m m}$ ). b) The migration path curves of the controlled alumina ball (above) ball with $\\mathsf{H}_{\\mathsf{P A A-P A M}}$ coating (below) within the channel of the PVC catheter tube. c) The friction force versus displaceme a ball and the urfac e of the PVC catheter tube under a tensile speed of $5\\min\\min^{-1}$ . d) Schematic illustration of the it eal photos. e) The friction fo place curves between the steel ball and the inner surface of natura ttl before/after growing $\\mathsf{H}_{\\mathsf{P A A-P A M}}$ coating (tensile speed: $50\\min\\operatorname*{min}{}$ ; PDA deposition time: $24\\mathsf{h}$ , polymerization growth time: $30~\\mathsf{s}$ , diameter of steel ball: $5c m$ ).",
        "category": " Conclusions"
    },
    {
        "id": 5,
        "chunk": "# Supporting Information  \n\nSupporting Information is available from the Wiley Online Library or from the author.",
        "category": " References"
    },
    {
        "id": 6,
        "chunk": "# Acknowledgements  \n\nThe authors are grateful for financial support from National Natural Science Foundation of China (22032006, 52075522, and 22072169), Key Research Program of the Chinese Academy of Sciences (XDPB24), Outstanding Youth Fund of Gansu Province (21JR7RA095), Key Research Project of Shandong Provincial Natural Science Foundation (ZR2021ZD27), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2019411).",
        "category": " Acknowledgements"
    },
    {
        "id": 7,
        "chunk": "# Conflict of Interest  \n\nThe authors declare no conflict of interest.",
        "category": " Conclusions"
    },
    {
        "id": 8,
        "chunk": "# Author Contributions  \n\nR.X. and Y.Z. contributed equally to this work. S.M. and F.Z. conceived the idea and supervised the entire research. R.X. and Y.Z. performed the experiments and completed the whole characterizations. R.X. and S.M. drafted the manuscript. F.Z. revised and finalized the manuscript. All the authors discussed the results and provided technical suggestions.",
        "category": " Author Contributions"
    },
    {
        "id": 9,
        "chunk": "# Data Availability Statement  \n\nResearch data are not shared.",
        "category": " Conclusions"
    },
    {
        "id": 10,
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