Sonia Norouzi Esfahany pressed a small, flexible patch onto the surface of a pig liver. The tissue was wet, as living tissue tends to be, and the patch stuck. A pulse of ultraviolet light locked it in place. Then something less expected happened: as the patch warmed to body temperature, it gently contracted, pulling the edges of a simulated wound closer together - no sutures, no staples, no additional intervention.
The material Esfahany helped create at the University of Missouri is a 44D-printed bioadhesive, a class of smart materials that combines three properties engineers have long struggled to deliver in a single platform: strong adhesion to wet biological tissue, rapid solidification on command, and shape memory. The work, published in Chemical Engineering Journal, was led by Jian Lin of the Department of Mechanical and Aerospace Engineering, with collaborators spanning biomedical engineering, chemical engineering, and the School of Medicine.
Bioadhesives are not new. Surgeons already use cyanoacrylate-based tissue glues and fibrin sealants. But most existing adhesives fail on at least one count. Cyanoacrylate is rigid once set and generates heat during curing, which can damage surrounding tissue. Fibrin sealants are flexible but weak. Neither can change shape after application.
Lin’s material sidesteps all three limitations. It adheres to wet tissue through a combination of chemical bonding and mechanical interlocking at the microscopic level. UV light triggers rapid cross-linking of the polymer chains, solidifying the material in seconds without the exothermic reaction that makes cyanoacrylate problematic. And the shape-memory behaviour, encoded during the 3D printing process, allows the material to be deformed, stretched, bent, twisted, and then recover its original programmed shape when heated. Body temperature is sufficient to trigger the transition.
Engineering Deep Dive: How 4D Printing Adds a Time Dimension
The “4D” in 4D printing refers to the fourth dimension of time. A 3D printed object holds a static shape. A 4D printed object changes shape after fabrication in response to an external stimulus, in this case, heat. The distinction matters because it allows engineers to design devices that adapt to their environment after deployment. This property is particularly valuable inside the human body, where conditions vary from patient to patient.
The shape-memory effect in Lin’s bioadhesive relies on the polymer’s glass transition temperature, a concept familiar to A-level Chemistry students as the point at which an amorphous polymer shifts from a rigid, glassy state to a flexible, rubbery one. The team tuned the polymer formulation so that its glass transition temperature sits just below human body temperature, around 32 to 35 degrees Celsius. At room temperature, the material is firm enough to handle and position. Once applied to the body, the patient’s warmth raises the polymer above its glass transition temperature, allowing the stored elastic energy to drive shape recovery.
The printing process itself is where the “programming” happens. During fabrication, the material is printed in its permanent, memorised shape, the shape it will return to when heated. After printing, it is cooled and mechanically deformed into a temporary shape that is easier to handle or apply. This two-step process, print then deform, is analogous to compressing a spring: the energy stored during deformation is released when the constraint (in this case, low temperature) is removed.
Adhesion to wet tissue is the second engineering challenge. Wet surfaces are notoriously difficult to bond because a layer of water prevents direct contact between the adhesive and the substrate. The Mizzou team’s material addresses this through two mechanisms. First, the polymer formulation includes hydrophilic functional groups that displace surface water through a process called interfacial spreading. Second, the UV-cured material develops microscopic surface roughness that interlocks with the tissue’s own irregular topography, creating mechanical adhesion that supplements the chemical bond. The combination delivers adhesion strengths significantly higher than commercial fibrin sealants on wet tissue.
For students considering biomedical engineering, the cross-disciplinary nature of this work is instructive. The polymer chemistry draws on chemical engineering. The 3D printing process is a branch of mechanical engineering. The biological testing, conducted on ex vivo pig liver tissue with in vivo studies planned, is biomedical engineering. And the shape-memory design requires materials science. No single discipline could have produced this material alone.
The UV curing mechanism also deserves attention. The material uses a photoinitiator, a molecule that absorbs UV photons and generates free radicals, which then initiate chain reactions linking polymer molecules together. The process is fast, typically complete within 10 to 30 seconds of UV exposure, and spatially precise: only the illuminated region solidifies, allowing surgeons to cure the adhesive exactly where needed. This is the same photopolymerisation chemistry used in dental fillings and stereolithography 3D printing, applied here to a material that must function inside a living body.
One limitation the team acknowledges is the depth of cure. UV light penetrates tissue poorly, so the adhesive works best on surface wounds and external applications. For deep surgical sites, alternative curing mechanisms, perhaps visible light or two-component chemical systems, would be needed. The current formulation is a proof of concept, not a finished product, and the path from lab bench to operating theatre will require biocompatibility studies, regulatory approval, and clinical trials that could take years.
The Mizzou team is now planning in vivo studies in animal models. If the material performs as well in living systems as it does on excised pig liver, the applications extend well beyond wound closure. Wearable sensors that conform to skin when warm and stiffen when cool, self-adjusting orthopaedic patches, and temporary closure devices that reposition themselves using the patient’s own body heat. Each is a variation on the same theme: a material that does not merely sit where it is placed, but actively responds to its surroundings.
